備戰-Java 容器

備戰-Java 容器

 

　　　　玉階生白露，夜久侵羅襪。

 

簡介：備戰-Java 容器

一、概述

容器主要包括 Collection 和 Map 兩種，Collection 存儲着對象的集合，而 Map 存儲着key-value 鍵值對（兩個對象）的映射表。

Collection

1. Set

• TreeSet：基於紅黑樹實現，支持有序性操作，例如根據一個范圍查找元素的操作。但是查找效率不如 HashSet，HashSet 查找的時間復雜度為 O(1)，TreeSet 則為 O(logN)。

• HashSet：基於哈希表實現，支持快速查找，但不支持有序性操作。並且失去了元素的插入順序信息，也就是說使用 Iterator 遍歷 HashSet 得到的結果是不確定的。

• LinkedHashSet：具有 HashSet 的查找效率，並且內部使用雙向鏈表維護元素的插入順序。

2. List

• ArrayList：基於動態數組實現，支持隨機訪問。

• Vector：和 ArrayList 類似，但它是線程安全的。

• LinkedList：基於雙向鏈表實現，只能順序訪問，但是可以快速地在鏈表中間插入和刪除元素。不僅如此，LinkedList 還可以用作棧、隊列和雙向隊列。

3. Queue

• LinkedList：可以用它來實現雙向隊列。

• PriorityQueue：基於堆結構實現，可以用它來實現優先隊列。

Map

• TreeMap：基於紅黑樹實現。

• HashMap：基於哈希表實現。

• HashTable：和 HashMap 類似，但它是線程安全的，這意味着同一時刻多個線程同時寫入 HashTable 不會導致數據不一致。它是遺留類，不應該去使用它，而是使用 ConcurrentHashMap 來支持線程安全，ConcurrentHashMap 的效率會更高，因為 ConcurrentHashMap 引入了分段鎖。

• LinkedHashMap：使用雙向鏈表來維護元素的順序，順序為插入順序或者最近最少使用（LRU）順序。(LRU算法是Least Recently Used的縮寫，即最近最少使用)

二、容器中的設計模式

迭代器模式

 Collection 繼承了 Iterable 接口，其中的 iterator() 方法能夠產生一個 Iterator 對象，通過這個對象就可以迭代遍歷 Collection 中的元素。

從 JDK 1.5 之后可以使用 foreach 方法來遍歷實現了 Iterable 接口的聚合對象。

List<String> list = new ArrayList<>(); 2 list.add("a"); 3 list.add("b"); 4 for (String item : list) { 5  System.out.println(item); 6 }

適配器模式

java.util.Arrays.asList() 可以把數組類型轉換為 List 類型。

@SafeVarargs 2 public static <T> List<T> asList(T... a)

值得注意的是 asList() 的參數為泛型的變長參數，不能使用基本類型數組作為參數，只能使用相應的包裝類型數組。

Integer[] arr = {1, 2, 3}; 2 List list = Arrays.asList(arr);

也可以使用以下方式調用 asList()：

List list = Arrays.asList(1, 2, 3);

三、源碼分析

如果沒有特別說明，以下源碼分析基於 JDK 1.8。

在 IDEA 中 雙擊 shift 鍵調出 Search EveryWhere，查找源碼文件，找到之后就可以閱讀源碼。

ArrayList

1. 概覽

因為 ArrayList 是基於數組實現的，所以支持快速隨機訪問。RandomAccess 接口標識着該類支持快速隨機訪問，其默認數組大小為10

/*
 * Copyright (c) 1997, 2017, Oracle and/or its affiliates. All rights reserved.  3  * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.  4  *  5  *  6  *  7  *  8  *  9  *  10  *  11  *  12  *  13  *  14  *  15  *  16  *  17  *  18  *  19  *  20  *  21  *  22  *  23  *  24  */

package java.util;  27 
import java.util.function.Consumer;  29 import java.util.function.Predicate;  30 import java.util.function.UnaryOperator;  31 import sun.misc.SharedSecrets;  32 
/**
 * Resizable-array implementation of the <tt>List</tt> interface. Implements  35  * all optional list operations, and permits all elements, including  36  * <tt>null</tt>. In addition to implementing the <tt>List</tt> interface,  37  * this class provides methods to manipulate the size of the array that is  38  * used internally to store the list. (This class is roughly equivalent to  39  * <tt>Vector</tt>, except that it is unsynchronized.)  40  *  41  * <p>The <tt>size</tt>, <tt>isEmpty</tt>, <tt>get</tt>, <tt>set</tt>,  42  * <tt>iterator</tt>, and <tt>listIterator</tt> operations run in constant  43  * time. The <tt>add</tt> operation runs in <i>amortized constant time</i>,  44  * that is, adding n elements requires O(n) time. All of the other operations  45  * run in linear time (roughly speaking). The constant factor is low compared  46  * to that for the <tt>LinkedList</tt> implementation.  47  *  48  * <p>Each <tt>ArrayList</tt> instance has a <i>capacity</i>. The capacity is  49  * the size of the array used to store the elements in the list. It is always  50  * at least as large as the list size. As elements are added to an ArrayList,  51  * its capacity grows automatically. The details of the growth policy are not  52  * specified beyond the fact that adding an element has constant amortized  53  * time cost.  54  *  55  * <p>An application can increase the capacity of an <tt>ArrayList</tt> instance  56  * before adding a large number of elements using the <tt>ensureCapacity</tt>  57  * operation. This may reduce the amount of incremental reallocation.  58  *  59  * <p><strong>Note that this implementation is not synchronized.</strong>  60  * If multiple threads access an <tt>ArrayList</tt> instance concurrently,  61  * and at least one of the threads modifies the list structurally, it  62  * <i>must</i> be synchronized externally. (A structural modification is  63  * any operation that adds or deletes one or more elements, or explicitly  64  * resizes the backing array; merely setting the value of an element is not  65  * a structural modification.) This is typically accomplished by  66  * synchronizing on some object that naturally encapsulates the list.  67  *  68  * If no such object exists, the list should be "wrapped" using the  69  * {@link Collections#synchronizedList Collections.synchronizedList}  70  * method. This is best done at creation time, to prevent accidental  71  * unsynchronized access to the list:<pre>  72  * List list = Collections.synchronizedList(new ArrayList(...));</pre>  73  *  74  * <p><a name="fail-fast">  75  * The iterators returned by this class's {@link #iterator() iterator} and  76  * {@link #listIterator(int) listIterator} methods are <em>fail-fast</em>:</a>  77  * if the list is structurally modified at any time after the iterator is  78  * created, in any way except through the iterator's own  79  * {@link ListIterator#remove() remove} or  80  * {@link ListIterator#add(Object) add} methods, the iterator will throw a  81  * {@link ConcurrentModificationException}. Thus, in the face of  82  * concurrent modification, the iterator fails quickly and cleanly, rather  83  * than risking arbitrary, non-deterministic behavior at an undetermined  84  * time in the future.  85  *  86  * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed  87  * as it is, generally speaking, impossible to make any hard guarantees in the  88  * presence of unsynchronized concurrent modification. Fail-fast iterators  89  * throw {@code ConcurrentModificationException} on a best-effort basis.  90  * Therefore, it would be wrong to write a program that depended on this  91  * exception for its correctness: <i>the fail-fast behavior of iterators  92  * should be used only to detect bugs.</i>  93  *  94  * <p>This class is a member of the  95  * <a href="{@docRoot}/../technotes/guides/collections/index.html">  96  * Java Collections Framework</a>.  97  *  98  * @author Josh Bloch  99  * @author Neal Gafter  100  * @see Collection  101  * @see List  102  * @see LinkedList  103  * @see Vector  104  * @since 1.2  105  */

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable  109 {  110     private static final long serialVersionUID = 8683452581122892189L;  111 
    /**
 * Default initial capacity.  114      */
    private static final int DEFAULT_CAPACITY = 10;  116 
    /**
 * Shared empty array instance used for empty instances.  119      */
    private static final Object[] EMPTY_ELEMENTDATA = {};  121 
    /**
 * Shared empty array instance used for default sized empty instances. We  124  * distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when  125  * first element is added.  126      */
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};  128 
    /**
 * The array buffer into which the elements of the ArrayList are stored.  131  * The capacity of the ArrayList is the length of this array buffer. Any  132  * empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA  133  * will be expanded to DEFAULT_CAPACITY when the first element is added.  134      */
    transient Object[] elementData; // non-private to simplify nested class access

    /**
 * The size of the ArrayList (the number of elements it contains).  139  *  140  * @serial
     */
    private int size;  143 
    /**
 * Constructs an empty list with the specified initial capacity.  146  *  147  * @param initialCapacity the initial capacity of the list  148  * @throws IllegalArgumentException if the specified initial capacity  149  * is negative  150      */
    public ArrayList(int initialCapacity) {  152         if (initialCapacity > 0) {  153             this.elementData = new Object[initialCapacity];  154         } else if (initialCapacity == 0) {  155             this.elementData = EMPTY_ELEMENTDATA;  156         } else {  157             throw new IllegalArgumentException("Illegal Capacity: "+
 initialCapacity);  159  }  160  }  161 
    /**
 * Constructs an empty list with an initial capacity of ten.  164      */
    public ArrayList() {  166         this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;  167  }  168 
    /**
 * Constructs a list containing the elements of the specified  171  * collection, in the order they are returned by the collection's  172  * iterator.  173  *  174  * @param c the collection whose elements are to be placed into this list  175  * @throws NullPointerException if the specified collection is null  176      */
    public ArrayList(Collection<? extends E> c) {  178         elementData = c.toArray();  179         if ((size = elementData.length) != 0) {  180             // c.toArray might (incorrectly) not return Object[] (see 6260652)
            if (elementData.getClass() != Object[].class)  182                 elementData = Arrays.copyOf(elementData, size, Object[].class);  183         } else {  184             // replace with empty array.
            this.elementData = EMPTY_ELEMENTDATA;  186  }  187  }  188 
    /**
 * Trims the capacity of this <tt>ArrayList</tt> instance to be the  191  * list's current size. An application can use this operation to minimize  192  * the storage of an <tt>ArrayList</tt> instance.  193      */
    public void trimToSize() {  195         modCount++;  196         if (size < elementData.length) {  197             elementData = (size == 0)  198               ? EMPTY_ELEMENTDATA  199  : Arrays.copyOf(elementData, size);  200  }  201  }  202 
    /**
 * Increases the capacity of this <tt>ArrayList</tt> instance, if  205  * necessary, to ensure that it can hold at least the number of elements  206  * specified by the minimum capacity argument.  207  *  208  * @param minCapacity the desired minimum capacity  209      */
    public void ensureCapacity(int minCapacity) {  211         int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)  212             // any size if not default element table
            ? 0
            // larger than default for default empty table. It's already  215             // supposed to be at default size.
 : DEFAULT_CAPACITY;  217 
        if (minCapacity > minExpand) {  219  ensureExplicitCapacity(minCapacity);  220  }  221  }  222 
    private static int calculateCapacity(Object[] elementData, int minCapacity) {  224         if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {  225             return Math.max(DEFAULT_CAPACITY, minCapacity);  226  }  227         return minCapacity;  228  }  229 
    private void ensureCapacityInternal(int minCapacity) {  231  ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));  232  }  233 
    private void ensureExplicitCapacity(int minCapacity) {  235         modCount++;  236 
        // overflow-conscious code
        if (minCapacity - elementData.length > 0)  239  grow(minCapacity);  240  }  241 
    /**
 * The maximum size of array to allocate.  244  * Some VMs reserve some header words in an array.  245  * Attempts to allocate larger arrays may result in  246  * OutOfMemoryError: Requested array size exceeds VM limit  247      */
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;  249 
    /**
 * Increases the capacity to ensure that it can hold at least the  252  * number of elements specified by the minimum capacity argument.  253  *  254  * @param minCapacity the desired minimum capacity  255      */
    private void grow(int minCapacity) {  257         // overflow-conscious code
        int oldCapacity = elementData.length;  259         int newCapacity = oldCapacity + (oldCapacity >> 1);  260         if (newCapacity - minCapacity < 0)  261             newCapacity = minCapacity;  262         if (newCapacity - MAX_ARRAY_SIZE > 0)  263             newCapacity = hugeCapacity(minCapacity);  264         // minCapacity is usually close to size, so this is a win:
        elementData = Arrays.copyOf(elementData, newCapacity);  266  }  267 
    private static int hugeCapacity(int minCapacity) {  269         if (minCapacity < 0) // overflow
            throw new OutOfMemoryError();  271         return (minCapacity > MAX_ARRAY_SIZE) ?
 Integer.MAX_VALUE :  273  MAX_ARRAY_SIZE;  274  }  275 
    /**
 * Returns the number of elements in this list.  278  *  279  * @return the number of elements in this list  280      */
    public int size() {  282         return size;  283  }  284 
    /**
 * Returns <tt>true</tt> if this list contains no elements.  287  *  288  * @return <tt>true</tt> if this list contains no elements  289      */
    public boolean isEmpty() {  291         return size == 0;  292  }  293 
    /**
 * Returns <tt>true</tt> if this list contains the specified element.  296  * More formally, returns <tt>true</tt> if and only if this list contains  297  * at least one element <tt>e</tt> such that  298  * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.  299  *  300  * @param o element whose presence in this list is to be tested  301  * @return <tt>true</tt> if this list contains the specified element  302      */
    public boolean contains(Object o) {  304         return indexOf(o) >= 0;  305  }  306 
    /**
 * Returns the index of the first occurrence of the specified element  309  * in this list, or -1 if this list does not contain the element.  310  * More formally, returns the lowest index <tt>i</tt> such that  311  * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,  312  * or -1 if there is no such index.  313      */
    public int indexOf(Object o) {  315         if (o == null) {  316             for (int i = 0; i < size; i++)  317                 if (elementData[i]==null)  318                     return i;  319         } else {  320             for (int i = 0; i < size; i++)  321                 if (o.equals(elementData[i]))  322                     return i;  323  }  324         return -1;  325  }  326 
    /**
 * Returns the index of the last occurrence of the specified element  329  * in this list, or -1 if this list does not contain the element.  330  * More formally, returns the highest index <tt>i</tt> such that  331  * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,  332  * or -1 if there is no such index.  333      */
    public int lastIndexOf(Object o) {  335         if (o == null) {  336             for (int i = size-1; i >= 0; i--)  337                 if (elementData[i]==null)  338                     return i;  339         } else {  340             for (int i = size-1; i >= 0; i--)  341                 if (o.equals(elementData[i]))  342                     return i;  343  }  344         return -1;  345  }  346 
    /**
 * Returns a shallow copy of this <tt>ArrayList</tt> instance. (The  349  * elements themselves are not copied.)  350  *  351  * @return a clone of this <tt>ArrayList</tt> instance  352      */
    public Object clone() {  354         try {  355             ArrayList<?> v = (ArrayList<?>) super.clone();  356             v.elementData = Arrays.copyOf(elementData, size);  357             v.modCount = 0;  358             return v;  359         } catch (CloneNotSupportedException e) {  360             // this shouldn't happen, since we are Cloneable
            throw new InternalError(e);  362  }  363  }  364 
    /**
 * Returns an array containing all of the elements in this list  367  * in proper sequence (from first to last element).  368  *  369  * <p>The returned array will be "safe" in that no references to it are  370  * maintained by this list. (In other words, this method must allocate  371  * a new array). The caller is thus free to modify the returned array.  372  *  373  * <p>This method acts as bridge between array-based and collection-based  374  * APIs.  375  *  376  * @return an array containing all of the elements in this list in  377  * proper sequence  378      */
    public Object[] toArray() {  380         return Arrays.copyOf(elementData, size);  381  }  382 
    /**
 * Returns an array containing all of the elements in this list in proper  385  * sequence (from first to last element); the runtime type of the returned  386  * array is that of the specified array. If the list fits in the  387  * specified array, it is returned therein. Otherwise, a new array is  388  * allocated with the runtime type of the specified array and the size of  389  * this list.  390  *  391  * <p>If the list fits in the specified array with room to spare  392  * (i.e., the array has more elements than the list), the element in  393  * the array immediately following the end of the collection is set to  394  * <tt>null</tt>. (This is useful in determining the length of the  395  * list <i>only</i> if the caller knows that the list does not contain  396  * any null elements.)  397  *  398  * @param a the array into which the elements of the list are to  399  * be stored, if it is big enough; otherwise, a new array of the  400  * same runtime type is allocated for this purpose.  401  * @return an array containing the elements of the list  402  * @throws ArrayStoreException if the runtime type of the specified array  403  * is not a supertype of the runtime type of every element in  404  * this list  405  * @throws NullPointerException if the specified array is null  406      */
    @SuppressWarnings("unchecked")  408     public <T> T[] toArray(T[] a) {  409         if (a.length < size)  410             // Make a new array of a's runtime type, but my contents:
            return (T[]) Arrays.copyOf(elementData, size, a.getClass());  412         System.arraycopy(elementData, 0, a, 0, size);  413         if (a.length > size)  414             a[size] = null;  415         return a;  416  }  417 
    // Positional Access Operations

    @SuppressWarnings("unchecked")  421     E elementData(int index) {  422         return (E) elementData[index];  423  }  424 
    /**
 * Returns the element at the specified position in this list.  427  *  428  * @param index index of the element to return  429  * @return the element at the specified position in this list  430  * @throws IndexOutOfBoundsException {@inheritDoc}  431      */
    public E get(int index) {  433  rangeCheck(index);  434 
        return elementData(index);  436  }  437 
    /**
 * Replaces the element at the specified position in this list with  440  * the specified element.  441  *  442  * @param index index of the element to replace  443  * @param element element to be stored at the specified position  444  * @return the element previously at the specified position  445  * @throws IndexOutOfBoundsException {@inheritDoc}  446      */
    public E set(int index, E element) {  448  rangeCheck(index);  449 
        E oldValue = elementData(index);  451         elementData[index] = element;  452         return oldValue;  453  }  454 
    /**
 * Appends the specified element to the end of this list.  457  *  458  * @param e element to be appended to this list  459  * @return <tt>true</tt> (as specified by {@link Collection#add})  460      */
    public boolean add(E e) {  462         ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;  464         return true;  465  }  466 
    /**
 * Inserts the specified element at the specified position in this  469  * list. Shifts the element currently at that position (if any) and  470  * any subsequent elements to the right (adds one to their indices).  471  *  472  * @param index index at which the specified element is to be inserted  473  * @param element element to be inserted  474  * @throws IndexOutOfBoundsException {@inheritDoc}  475      */
    public void add(int index, E element) {  477  rangeCheckForAdd(index);  478 
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        System.arraycopy(elementData, index, elementData, index + 1,  481                          size - index);  482         elementData[index] = element;  483         size++;  484  }  485 
    /**
 * Removes the element at the specified position in this list.  488  * Shifts any subsequent elements to the left (subtracts one from their  489  * indices).  490  *  491  * @param index the index of the element to be removed  492  * @return the element that was removed from the list  493  * @throws IndexOutOfBoundsException {@inheritDoc}  494      */
    public E remove(int index) {  496  rangeCheck(index);  497 
        modCount++;  499         E oldValue = elementData(index);  500 
        int numMoved = size - index - 1;  502         if (numMoved > 0)  503             System.arraycopy(elementData, index+1, elementData, index,  504  numMoved);  505         elementData[--size] = null; // clear to let GC do its work

        return oldValue;  508  }  509 
    /**
 * Removes the first occurrence of the specified element from this list,  512  * if it is present. If the list does not contain the element, it is  513  * unchanged. More formally, removes the element with the lowest index  514  * <tt>i</tt> such that  515  * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>  516  * (if such an element exists). Returns <tt>true</tt> if this list  517  * contained the specified element (or equivalently, if this list  518  * changed as a result of the call).  519  *  520  * @param o element to be removed from this list, if present  521  * @return <tt>true</tt> if this list contained the specified element  522      */
    public boolean remove(Object o) {  524         if (o == null) {  525             for (int index = 0; index < size; index++)  526                 if (elementData[index] == null) {  527  fastRemove(index);  528                     return true;  529  }  530         } else {  531             for (int index = 0; index < size; index++)  532                 if (o.equals(elementData[index])) {  533  fastRemove(index);  534                     return true;  535  }  536  }  537         return false;  538  }  539 
    /*
 * Private remove method that skips bounds checking and does not  542  * return the value removed.  543      */
    private void fastRemove(int index) {  545         modCount++;  546         int numMoved = size - index - 1;  547         if (numMoved > 0)  548             System.arraycopy(elementData, index+1, elementData, index,  549  numMoved);  550         elementData[--size] = null; // clear to let GC do its work
 }  552 
    /**
 * Removes all of the elements from this list. The list will  555  * be empty after this call returns.  556      */
    public void clear() {  558         modCount++;  559 
        // clear to let GC do its work
        for (int i = 0; i < size; i++)  562             elementData[i] = null;  563 
        size = 0;  565  }  566 
    /**
 * Appends all of the elements in the specified collection to the end of  569  * this list, in the order that they are returned by the  570  * specified collection's Iterator. The behavior of this operation is  571  * undefined if the specified collection is modified while the operation  572  * is in progress. (This implies that the behavior of this call is  573  * undefined if the specified collection is this list, and this  574  * list is nonempty.)  575  *  576  * @param c collection containing elements to be added to this list  577  * @return <tt>true</tt> if this list changed as a result of the call  578  * @throws NullPointerException if the specified collection is null  579      */
    public boolean addAll(Collection<? extends E> c) {  581         Object[] a = c.toArray();  582         int numNew = a.length;  583         ensureCapacityInternal(size + numNew);  // Increments modCount
        System.arraycopy(a, 0, elementData, size, numNew);  585         size += numNew;  586         return numNew != 0;  587  }  588 
    /**
 * Inserts all of the elements in the specified collection into this  591  * list, starting at the specified position. Shifts the element  592  * currently at that position (if any) and any subsequent elements to  593  * the right (increases their indices). The new elements will appear  594  * in the list in the order that they are returned by the  595  * specified collection's iterator.  596  *  597  * @param index index at which to insert the first element from the  598  * specified collection  599  * @param c collection containing elements to be added to this list  600  * @return <tt>true</tt> if this list changed as a result of the call  601  * @throws IndexOutOfBoundsException {@inheritDoc}  602  * @throws NullPointerException if the specified collection is null  603      */
    public boolean addAll(int index, Collection<? extends E> c) {  605  rangeCheckForAdd(index);  606 
        Object[] a = c.toArray();  608         int numNew = a.length;  609         ensureCapacityInternal(size + numNew);  // Increments modCount

        int numMoved = size - index;  612         if (numMoved > 0)  613             System.arraycopy(elementData, index, elementData, index + numNew,  614  numMoved);  615 
        System.arraycopy(a, 0, elementData, index, numNew);  617         size += numNew;  618         return numNew != 0;  619  }  620 
    /**
 * Removes from this list all of the elements whose index is between  623  * {@code fromIndex}, inclusive, and {@code toIndex}, exclusive.  624  * Shifts any succeeding elements to the left (reduces their index).  625  * This call shortens the list by {@code (toIndex - fromIndex)} elements.  626  * (If {@code toIndex==fromIndex}, this operation has no effect.)  627  *  628  * @throws IndexOutOfBoundsException if {@code fromIndex} or  629  * {@code toIndex} is out of range  630  * ({@code fromIndex < 0 ||  631  * fromIndex >= size() ||  632  * toIndex > size() ||  633  * toIndex < fromIndex})  634      */
    protected void removeRange(int fromIndex, int toIndex) {  636         modCount++;  637         int numMoved = size - toIndex;  638  System.arraycopy(elementData, toIndex, elementData, fromIndex,  639  numMoved);  640 
        // clear to let GC do its work
        int newSize = size - (toIndex-fromIndex);  643         for (int i = newSize; i < size; i++) {  644             elementData[i] = null;  645  }  646         size = newSize;  647  }  648 
    /**
 * Checks if the given index is in range. If not, throws an appropriate  651  * runtime exception. This method does *not* check if the index is  652  * negative: It is always used immediately prior to an array access,  653  * which throws an ArrayIndexOutOfBoundsException if index is negative.  654      */
    private void rangeCheck(int index) {  656         if (index >= size)  657             throw new IndexOutOfBoundsException(outOfBoundsMsg(index));  658  }  659 
    /**
 * A version of rangeCheck used by add and addAll.  662      */
    private void rangeCheckForAdd(int index) {  664         if (index > size || index < 0)  665             throw new IndexOutOfBoundsException(outOfBoundsMsg(index));  666  }  667 
    /**
 * Constructs an IndexOutOfBoundsException detail message.  670  * Of the many possible refactorings of the error handling code,  671  * this "outlining" performs best with both server and client VMs.  672      */
    private String outOfBoundsMsg(int index) {  674         return "Index: "+index+", Size: "+size;  675  }  676 
    /**
 * Removes from this list all of its elements that are contained in the  679  * specified collection.  680  *  681  * @param c collection containing elements to be removed from this list  682  * @return {@code true} if this list changed as a result of the call  683  * @throws ClassCastException if the class of an element of this list  684  * is incompatible with the specified collection  685  * (<a href="Collection.html#optional-restrictions">optional</a>)  686  * @throws NullPointerException if this list contains a null element and the  687  * specified collection does not permit null elements  688  * (<a href="Collection.html#optional-restrictions">optional</a>),  689  * or if the specified collection is null  690  * @see Collection#contains(Object)  691      */
    public boolean removeAll(Collection<?> c) {  693  Objects.requireNonNull(c);  694         return batchRemove(c, false);  695  }  696 
    /**
 * Retains only the elements in this list that are contained in the  699  * specified collection. In other words, removes from this list all  700  * of its elements that are not contained in the specified collection.  701  *  702  * @param c collection containing elements to be retained in this list  703  * @return {@code true} if this list changed as a result of the call  704  * @throws ClassCastException if the class of an element of this list  705  * is incompatible with the specified collection  706  * (<a href="Collection.html#optional-restrictions">optional</a>)  707  * @throws NullPointerException if this list contains a null element and the  708  * specified collection does not permit null elements  709  * (<a href="Collection.html#optional-restrictions">optional</a>),  710  * or if the specified collection is null  711  * @see Collection#contains(Object)  712      */
    public boolean retainAll(Collection<?> c) {  714  Objects.requireNonNull(c);  715         return batchRemove(c, true);  716  }  717 
    private boolean batchRemove(Collection<?> c, boolean complement) {  719         final Object[] elementData = this.elementData;  720         int r = 0, w = 0;  721         boolean modified = false;  722         try {  723             for (; r < size; r++)  724                 if (c.contains(elementData[r]) == complement)  725                     elementData[w++] = elementData[r];  726         } finally {  727             // Preserve behavioral compatibility with AbstractCollection,  728             // even if c.contains() throws.
            if (r != size) {  730  System.arraycopy(elementData, r,  731  elementData, w,  732                                  size - r);  733                 w += size - r;  734  }  735             if (w != size) {  736                 // clear to let GC do its work
                for (int i = w; i < size; i++)  738                     elementData[i] = null;  739                 modCount += size - w;  740                 size = w;  741                 modified = true;  742  }  743  }  744         return modified;  745  }  746 
    /**
 * Save the state of the <tt>ArrayList</tt> instance to a stream (that  749  * is, serialize it).  750  *  751  * @serialData The length of the array backing the <tt>ArrayList</tt>  752  * instance is emitted (int), followed by all of its elements  753  * (each an <tt>Object</tt>) in the proper order.  754      */
    private void writeObject(java.io.ObjectOutputStream s)  756         throws java.io.IOException{  757         // Write out element count, and any hidden stuff
        int expectedModCount = modCount;  759  s.defaultWriteObject();  760 
        // Write out size as capacity for behavioural compatibility with clone()
 s.writeInt(size);  763 
        // Write out all elements in the proper order.
        for (int i=0; i<size; i++) {  766  s.writeObject(elementData[i]);  767  }  768 
        if (modCount != expectedModCount) {  770             throw new ConcurrentModificationException();  771  }  772  }  773 
    /**
 * Reconstitute the <tt>ArrayList</tt> instance from a stream (that is,  776  * deserialize it).  777      */
    private void readObject(java.io.ObjectInputStream s)  779         throws java.io.IOException, ClassNotFoundException {  780         elementData = EMPTY_ELEMENTDATA;  781 
        // Read in size, and any hidden stuff
 s.defaultReadObject();  784 
        // Read in capacity
        s.readInt(); // ignored

        if (size > 0) {  789             // be like clone(), allocate array based upon size not capacity
            int capacity = calculateCapacity(elementData, size);  791             SharedSecrets.getJavaOISAccess().checkArray(s, Object[].class, capacity);  792  ensureCapacityInternal(size);  793 
            Object[] a = elementData;  795             // Read in all elements in the proper order.
            for (int i=0; i<size; i++) {  797                 a[i] = s.readObject();  798  }  799  }  800  }  801 
    /**
 * Returns a list iterator over the elements in this list (in proper  804  * sequence), starting at the specified position in the list.  805  * The specified index indicates the first element that would be  806  * returned by an initial call to {@link ListIterator#next next}.  807  * An initial call to {@link ListIterator#previous previous} would  808  * return the element with the specified index minus one.  809  *  810  * <p>The returned list iterator is <a href="#fail-fast"><i>fail-fast</i></a>.  811  *  812  * @throws IndexOutOfBoundsException {@inheritDoc}  813      */
    public ListIterator<E> listIterator(int index) {  815         if (index < 0 || index > size)  816             throw new IndexOutOfBoundsException("Index: "+index);  817         return new ListItr(index);  818  }  819 
    /**
 * Returns a list iterator over the elements in this list (in proper  822  * sequence).  823  *  824  * <p>The returned list iterator is <a href="#fail-fast"><i>fail-fast</i></a>.  825  *  826  * @see #listIterator(int)  827      */
    public ListIterator<E> listIterator() {  829         return new ListItr(0);  830  }  831 
    /**
 * Returns an iterator over the elements in this list in proper sequence.  834  *  835  * <p>The returned iterator is <a href="#fail-fast"><i>fail-fast</i></a>.  836  *  837  * @return an iterator over the elements in this list in proper sequence  838      */
    public Iterator<E> iterator() {  840         return new Itr();  841  }  842 
    /**
 * An optimized version of AbstractList.Itr  845      */
    private class Itr implements Iterator<E> {  847         int cursor;       // index of next element to return
        int lastRet = -1; // index of last element returned; -1 if no such
        int expectedModCount = modCount;  850 
 Itr() {}  852 
        public boolean hasNext() {  854             return cursor != size;  855  }  856 
        @SuppressWarnings("unchecked")  858         public E next() {  859  checkForComodification();  860             int i = cursor;  861             if (i >= size)  862                 throw new NoSuchElementException();  863             Object[] elementData = ArrayList.this.elementData;  864             if (i >= elementData.length)  865                 throw new ConcurrentModificationException();  866             cursor = i + 1;  867             return (E) elementData[lastRet = i];  868  }  869 
        public void remove() {  871             if (lastRet < 0)  872                 throw new IllegalStateException();  873  checkForComodification();  874 
            try {  876                 ArrayList.this.remove(lastRet);  877                 cursor = lastRet;  878                 lastRet = -1;  879                 expectedModCount = modCount;  880             } catch (IndexOutOfBoundsException ex) {  881                 throw new ConcurrentModificationException();  882  }  883  }  884 
 @Override  886         @SuppressWarnings("unchecked")  887         public void forEachRemaining(Consumer<? super E> consumer) {  888  Objects.requireNonNull(consumer);  889             final int size = ArrayList.this.size;  890             int i = cursor;  891             if (i >= size) {  892                 return;  893  }  894             final Object[] elementData = ArrayList.this.elementData;  895             if (i >= elementData.length) {  896                 throw new ConcurrentModificationException();  897  }  898             while (i != size && modCount == expectedModCount) {  899                 consumer.accept((E) elementData[i++]);  900  }  901             // update once at end of iteration to reduce heap write traffic
            cursor = i;  903             lastRet = i - 1;  904  checkForComodification();  905  }  906 
        final void checkForComodification() {  908             if (modCount != expectedModCount)  909                 throw new ConcurrentModificationException();  910  }  911  }  912 
    /**
 * An optimized version of AbstractList.ListItr  915      */
    private class ListItr extends Itr implements ListIterator<E> {  917         ListItr(int index) {  918             super();  919             cursor = index;  920  }  921 
        public boolean hasPrevious() {  923             return cursor != 0;  924  }  925 
        public int nextIndex() {  927             return cursor;  928  }  929 
        public int previousIndex() {  931             return cursor - 1;  932  }  933 
        @SuppressWarnings("unchecked")  935         public E previous() {  936  checkForComodification();  937             int i = cursor - 1;  938             if (i < 0)  939                 throw new NoSuchElementException();  940             Object[] elementData = ArrayList.this.elementData;  941             if (i >= elementData.length)  942                 throw new ConcurrentModificationException();  943             cursor = i;  944             return (E) elementData[lastRet = i];  945  }  946 
        public void set(E e) {  948             if (lastRet < 0)  949                 throw new IllegalStateException();  950  checkForComodification();  951 
            try {  953                 ArrayList.this.set(lastRet, e);  954             } catch (IndexOutOfBoundsException ex) {  955                 throw new ConcurrentModificationException();  956  }  957  }  958 
        public void add(E e) {  960  checkForComodification();  961 
            try {  963                 int i = cursor;  964                 ArrayList.this.add(i, e);  965                 cursor = i + 1;  966                 lastRet = -1;  967                 expectedModCount = modCount;  968             } catch (IndexOutOfBoundsException ex) {  969                 throw new ConcurrentModificationException();  970  }  971  }  972  }  973 
    /**
 * Returns a view of the portion of this list between the specified  976  * {@code fromIndex}, inclusive, and {@code toIndex}, exclusive. (If  977  * {@code fromIndex} and {@code toIndex} are equal, the returned list is  978  * empty.) The returned list is backed by this list, so non-structural  979  * changes in the returned list are reflected in this list, and vice-versa.  980  * The returned list supports all of the optional list operations.  981  *  982  * <p>This method eliminates the need for explicit range operations (of  983  * the sort that commonly exist for arrays). Any operation that expects  984  * a list can be used as a range operation by passing a subList view  985  * instead of a whole list. For example, the following idiom  986  * removes a range of elements from a list:  987  * <pre>  988  * list.subList(from, to).clear();  989  * </pre>  990  * Similar idioms may be constructed for {@link #indexOf(Object)} and  991  * {@link #lastIndexOf(Object)}, and all of the algorithms in the  992  * {@link Collections} class can be applied to a subList.  993  *  994  * <p>The semantics of the list returned by this method become undefined if  995  * the backing list (i.e., this list) is <i>structurally modified</i> in  996  * any way other than via the returned list. (Structural modifications are  997  * those that change the size of this list, or otherwise perturb it in such  998  * a fashion that iterations in progress may yield incorrect results.)  999  * 1000  * @throws IndexOutOfBoundsException {@inheritDoc} 1001  * @throws IllegalArgumentException {@inheritDoc} 1002      */
    public List<E> subList(int fromIndex, int toIndex) { 1004  subListRangeCheck(fromIndex, toIndex, size); 1005         return new SubList(this, 0, fromIndex, toIndex); 1006  } 1007 
    static void subListRangeCheck(int fromIndex, int toIndex, int size) { 1009         if (fromIndex < 0) 1010             throw new IndexOutOfBoundsException("fromIndex = " + fromIndex); 1011         if (toIndex > size) 1012             throw new IndexOutOfBoundsException("toIndex = " + toIndex); 1013         if (fromIndex > toIndex) 1014             throw new IllegalArgumentException("fromIndex(" + fromIndex +
                                               ") > toIndex(" + toIndex + ")"); 1016  } 1017 
    private class SubList extends AbstractList<E> implements RandomAccess { 1019         private final AbstractList<E> parent; 1020         private final int parentOffset; 1021         private final int offset; 1022         int size; 1023 
        SubList(AbstractList<E> parent, 1025                 int offset, int fromIndex, int toIndex) { 1026             this.parent = parent; 1027             this.parentOffset = fromIndex; 1028             this.offset = offset + fromIndex; 1029             this.size = toIndex - fromIndex; 1030             this.modCount = ArrayList.this.modCount; 1031  } 1032 
        public E set(int index, E e) { 1034  rangeCheck(index); 1035  checkForComodification(); 1036             E oldValue = ArrayList.this.elementData(offset + index); 1037             ArrayList.this.elementData[offset + index] = e; 1038             return oldValue; 1039  } 1040 
        public E get(int index) { 1042  rangeCheck(index); 1043  checkForComodification(); 1044             return ArrayList.this.elementData(offset + index); 1045  } 1046 
        public int size() { 1048  checkForComodification(); 1049             return this.size; 1050  } 1051 
        public void add(int index, E e) { 1053  rangeCheckForAdd(index); 1054  checkForComodification(); 1055             parent.add(parentOffset + index, e); 1056             this.modCount = parent.modCount; 1057             this.size++; 1058  } 1059 
        public E remove(int index) { 1061  rangeCheck(index); 1062  checkForComodification(); 1063             E result = parent.remove(parentOffset + index); 1064             this.modCount = parent.modCount; 1065             this.size--; 1066             return result; 1067  } 1068 
        protected void removeRange(int fromIndex, int toIndex) { 1070  checkForComodification(); 1071             parent.removeRange(parentOffset + fromIndex, 1072                                parentOffset + toIndex); 1073             this.modCount = parent.modCount; 1074             this.size -= toIndex - fromIndex; 1075  } 1076 
        public boolean addAll(Collection<? extends E> c) { 1078             return addAll(this.size, c); 1079  } 1080 
        public boolean addAll(int index, Collection<? extends E> c) { 1082  rangeCheckForAdd(index); 1083             int cSize = c.size(); 1084             if (cSize==0) 1085                 return false; 1086 
 checkForComodification(); 1088             parent.addAll(parentOffset + index, c); 1089             this.modCount = parent.modCount; 1090             this.size += cSize; 1091             return true; 1092  } 1093 
        public Iterator<E> iterator() { 1095             return listIterator(); 1096  } 1097 
        public ListIterator<E> listIterator(final int index) { 1099  checkForComodification(); 1100  rangeCheckForAdd(index); 1101             final int offset = this.offset; 1102 
            return new ListIterator<E>() { 1104                 int cursor = index; 1105                 int lastRet = -1; 1106                 int expectedModCount = ArrayList.this.modCount; 1107 
                public boolean hasNext() { 1109                     return cursor != SubList.this.size; 1110  } 1111 
                @SuppressWarnings("unchecked") 1113                 public E next() { 1114  checkForComodification(); 1115                     int i = cursor; 1116                     if (i >= SubList.this.size) 1117                         throw new NoSuchElementException(); 1118                     Object[] elementData = ArrayList.this.elementData; 1119                     if (offset + i >= elementData.length) 1120                         throw new ConcurrentModificationException(); 1121                     cursor = i + 1; 1122                     return (E) elementData[offset + (lastRet = i)]; 1123  } 1124 
                public boolean hasPrevious() { 1126                     return cursor != 0; 1127  } 1128 
                @SuppressWarnings("unchecked") 1130                 public E previous() { 1131  checkForComodification(); 1132                     int i = cursor - 1; 1133                     if (i < 0) 1134                         throw new NoSuchElementException(); 1135                     Object[] elementData = ArrayList.this.elementData; 1136                     if (offset + i >= elementData.length) 1137                         throw new ConcurrentModificationException(); 1138                     cursor = i; 1139                     return (E) elementData[offset + (lastRet = i)]; 1140  } 1141 
                @SuppressWarnings("unchecked") 1143                 public void forEachRemaining(Consumer<? super E> consumer) { 1144  Objects.requireNonNull(consumer); 1145                     final int size = SubList.this.size; 1146                     int i = cursor; 1147                     if (i >= size) { 1148                         return; 1149  } 1150                     final Object[] elementData = ArrayList.this.elementData; 1151                     if (offset + i >= elementData.length) { 1152                         throw new ConcurrentModificationException(); 1153  } 1154                     while (i != size && modCount == expectedModCount) { 1155                         consumer.accept((E) elementData[offset + (i++)]); 1156  } 1157                     // update once at end of iteration to reduce heap write traffic
                    lastRet = cursor = i; 1159  checkForComodification(); 1160  } 1161 
                public int nextIndex() { 1163                     return cursor; 1164  } 1165 
                public int previousIndex() { 1167                     return cursor - 1; 1168  } 1169 
                public void remove() { 1171                     if (lastRet < 0) 1172                         throw new IllegalStateException(); 1173  checkForComodification(); 1174 
                    try { 1176                         SubList.this.remove(lastRet); 1177                         cursor = lastRet; 1178                         lastRet = -1; 1179                         expectedModCount = ArrayList.this.modCount; 1180                     } catch (IndexOutOfBoundsException ex) { 1181                         throw new ConcurrentModificationException(); 1182  } 1183  } 1184 
                public void set(E e) { 1186                     if (lastRet < 0) 1187                         throw new IllegalStateException(); 1188  checkForComodification(); 1189 
                    try { 1191                         ArrayList.this.set(offset + lastRet, e); 1192                     } catch (IndexOutOfBoundsException ex) { 1193                         throw new ConcurrentModificationException(); 1194  } 1195  } 1196 
                public void add(E e) { 1198  checkForComodification(); 1199 
                    try { 1201                         int i = cursor; 1202                         SubList.this.add(i, e); 1203                         cursor = i + 1; 1204                         lastRet = -1; 1205                         expectedModCount = ArrayList.this.modCount; 1206                     } catch (IndexOutOfBoundsException ex) { 1207                         throw new ConcurrentModificationException(); 1208  } 1209  } 1210 
                final void checkForComodification() { 1212                     if (expectedModCount != ArrayList.this.modCount) 1213                         throw new ConcurrentModificationException(); 1214  } 1215  }; 1216  } 1217 
        public List<E> subList(int fromIndex, int toIndex) { 1219  subListRangeCheck(fromIndex, toIndex, size); 1220             return new SubList(this, offset, fromIndex, toIndex); 1221  } 1222 
        private void rangeCheck(int index) { 1224             if (index < 0 || index >= this.size) 1225                 throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); 1226  } 1227 
        private void rangeCheckForAdd(int index) { 1229             if (index < 0 || index > this.size) 1230                 throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); 1231  } 1232 
        private String outOfBoundsMsg(int index) { 1234             return "Index: "+index+", Size: "+this.size; 1235  } 1236 
        private void checkForComodification() { 1238             if (ArrayList.this.modCount != this.modCount) 1239                 throw new ConcurrentModificationException(); 1240  } 1241 
        public Spliterator<E> spliterator() { 1243  checkForComodification(); 1244             return new ArrayListSpliterator<E>(ArrayList.this, offset, 1245                                                offset + this.size, this.modCount); 1246  } 1247  } 1248 
 @Override 1250     public void forEach(Consumer<? super E> action) { 1251  Objects.requireNonNull(action); 1252         final int expectedModCount = modCount; 1253         @SuppressWarnings("unchecked") 1254         final E[] elementData = (E[]) this.elementData; 1255         final int size = this.size; 1256         for (int i=0; modCount == expectedModCount && i < size; i++) { 1257  action.accept(elementData[i]); 1258  } 1259         if (modCount != expectedModCount) { 1260             throw new ConcurrentModificationException(); 1261  } 1262  } 1263 
    /**
 * Creates a <em><a href="Spliterator.html#binding">late-binding</a></em> 1266  * and <em>fail-fast</em> {@link Spliterator} over the elements in this 1267  * list. 1268  * 1269  * <p>The {@code Spliterator} reports {@link Spliterator#SIZED}, 1270  * {@link Spliterator#SUBSIZED}, and {@link Spliterator#ORDERED}. 1271  * Overriding implementations should document the reporting of additional 1272  * characteristic values. 1273  * 1274  * @return a {@code Spliterator} over the elements in this list 1275  * @since 1.8 1276      */
 @Override 1278     public Spliterator<E> spliterator() { 1279         return new ArrayListSpliterator<>(this, 0, -1, 0); 1280  } 1281 
    /** Index-based split-by-two, lazily initialized Spliterator */
    static final class ArrayListSpliterator<E> implements Spliterator<E> { 1284 
        /*
 * If ArrayLists were immutable, or structurally immutable (no 1287  * adds, removes, etc), we could implement their spliterators 1288  * with Arrays.spliterator. Instead we detect as much 1289  * interference during traversal as practical without 1290  * sacrificing much performance. We rely primarily on 1291  * modCounts. These are not guaranteed to detect concurrency 1292  * violations, and are sometimes overly conservative about 1293  * within-thread interference, but detect enough problems to 1294  * be worthwhile in practice. To carry this out, we (1) lazily 1295  * initialize fence and expectedModCount until the latest 1296  * point that we need to commit to the state we are checking 1297  * against; thus improving precision. (This doesn't apply to 1298  * SubLists, that create spliterators with current non-lazy 1299  * values). (2) We perform only a single 1300  * ConcurrentModificationException check at the end of forEach 1301  * (the most performance-sensitive method). When using forEach 1302  * (as opposed to iterators), we can normally only detect 1303  * interference after actions, not before. Further 1304  * CME-triggering checks apply to all other possible 1305  * violations of assumptions for example null or too-small 1306  * elementData array given its size(), that could only have 1307  * occurred due to interference. This allows the inner loop 1308  * of forEach to run without any further checks, and 1309  * simplifies lambda-resolution. While this does entail a 1310  * number of checks, note that in the common case of 1311  * list.stream().forEach(a), no checks or other computation 1312  * occur anywhere other than inside forEach itself. The other 1313  * less-often-used methods cannot take advantage of most of 1314  * these streamlinings. 1315          */

        private final ArrayList<E> list; 1318         private int index; // current index, modified on advance/split
        private int fence; // -1 until used; then one past last index
        private int expectedModCount; // initialized when fence set

        /** Create new spliterator covering the given range */
        ArrayListSpliterator(ArrayList<E> list, int origin, int fence, 1324                              int expectedModCount) { 1325             this.list = list; // OK if null unless traversed
            this.index = origin; 1327             this.fence = fence; 1328             this.expectedModCount = expectedModCount; 1329  } 1330 
        private int getFence() { // initialize fence to size on first use
            int hi; // (a specialized variant appears in method forEach)
            ArrayList<E> lst; 1334             if ((hi = fence) < 0) { 1335                 if ((lst = list) == null) 1336                     hi = fence = 0; 1337                 else { 1338                     expectedModCount = lst.modCount; 1339                     hi = fence = lst.size; 1340  } 1341  } 1342             return hi; 1343  } 1344 
        public ArrayListSpliterator<E> trySplit() { 1346             int hi = getFence(), lo = index, mid = (lo + hi) >>> 1; 1347             return (lo >= mid) ? null : // divide range in half unless too small
                new ArrayListSpliterator<E>(list, lo, index = mid, 1349  expectedModCount); 1350  } 1351 
        public boolean tryAdvance(Consumer<? super E> action) { 1353             if (action == null) 1354                 throw new NullPointerException(); 1355             int hi = getFence(), i = index; 1356             if (i < hi) { 1357                 index = i + 1; 1358                 @SuppressWarnings("unchecked") E e = (E)list.elementData[i]; 1359  action.accept(e); 1360                 if (list.modCount != expectedModCount) 1361                     throw new ConcurrentModificationException(); 1362                 return true; 1363  } 1364             return false; 1365  } 1366 
        public void forEachRemaining(Consumer<? super E> action) { 1368             int i, hi, mc; // hoist accesses and checks from loop
            ArrayList<E> lst; Object[] a; 1370             if (action == null) 1371                 throw new NullPointerException(); 1372             if ((lst = list) != null && (a = lst.elementData) != null) { 1373                 if ((hi = fence) < 0) { 1374                     mc = lst.modCount; 1375                     hi = lst.size; 1376  } 1377                 else
                    mc = expectedModCount; 1379                 if ((i = index) >= 0 && (index = hi) <= a.length) { 1380                     for (; i < hi; ++i) { 1381                         @SuppressWarnings("unchecked") E e = (E) a[i]; 1382  action.accept(e); 1383  } 1384                     if (lst.modCount == mc) 1385                         return; 1386  } 1387  } 1388             throw new ConcurrentModificationException(); 1389  } 1390 
        public long estimateSize() { 1392             return (long) (getFence() - index); 1393  } 1394 
        public int characteristics() { 1396             return Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED; 1397  } 1398  } 1399 
 @Override 1401     public boolean removeIf(Predicate<? super E> filter) { 1402  Objects.requireNonNull(filter); 1403         // figure out which elements are to be removed 1404         // any exception thrown from the filter predicate at this stage 1405         // will leave the collection unmodified
        int removeCount = 0; 1407         final BitSet removeSet = new BitSet(size); 1408         final int expectedModCount = modCount; 1409         final int size = this.size; 1410         for (int i=0; modCount == expectedModCount && i < size; i++) { 1411             @SuppressWarnings("unchecked") 1412             final E element = (E) elementData[i]; 1413             if (filter.test(element)) { 1414  removeSet.set(i); 1415                 removeCount++; 1416  } 1417  } 1418         if (modCount != expectedModCount) { 1419             throw new ConcurrentModificationException(); 1420  } 1421 
        // shift surviving elements left over the spaces left by removed elements
        final boolean anyToRemove = removeCount > 0; 1424         if (anyToRemove) { 1425             final int newSize = size - removeCount; 1426             for (int i=0, j=0; (i < size) && (j < newSize); i++, j++) { 1427                 i = removeSet.nextClearBit(i); 1428                 elementData[j] = elementData[i]; 1429  } 1430             for (int k=newSize; k < size; k++) { 1431                 elementData[k] = null;  // Let gc do its work
 } 1433             this.size = newSize; 1434             if (modCount != expectedModCount) { 1435                 throw new ConcurrentModificationException(); 1436  } 1437             modCount++; 1438  } 1439 
        return anyToRemove; 1441  } 1442 
 @Override 1444     @SuppressWarnings("unchecked") 1445     public void replaceAll(UnaryOperator<E> operator) { 1446  Objects.requireNonNull(operator); 1447         final int expectedModCount = modCount; 1448         final int size = this.size; 1449         for (int i=0; modCount == expectedModCount && i < size; i++) { 1450             elementData[i] = operator.apply((E) elementData[i]); 1451  } 1452         if (modCount != expectedModCount) { 1453             throw new ConcurrentModificationException(); 1454  } 1455         modCount++; 1456  } 1457 
 @Override 1459     @SuppressWarnings("unchecked") 1460     public void sort(Comparator<? super E> c) { 1461         final int expectedModCount = modCount; 1462         Arrays.sort((E[]) elementData, 0, size, c); 1463         if (modCount != expectedModCount) { 1464             throw new ConcurrentModificationException(); 1465  } 1466         modCount++; 1467  } 1468 }

2. 擴容

添加元素時使用 ensureCapacityInternal() 方法來保證容量足夠，如果不夠時，需要使用 grow() 方法進行擴容，新容量的大小為 oldCapacity + (oldCapacity >> 1)，即 oldCapacity+oldCapacity/2。其中 oldCapacity >> 1 需要取整，所以新容量大約是舊容量的 1.5 倍左右。（oldCapacity 為偶數就是 1.5 倍，為奇數就是 1.5 倍-0.5）

擴容操作需要調用 Arrays.copyOf() 把原數組整個復制到新數組中，這個操作代價很高，因此最好在創建 ArrayList 對象時就指定大概的容量大小，減少擴容操作的次數。

public boolean add(E e) {  2     ensureCapacityInternal(size + 1);  // Increments modCount!!
    elementData[size++] = e;  4     return true;  5 }  6 
private void ensureCapacityInternal(int minCapacity) {  8     if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {  9         minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity); 10  } 11  ensureExplicitCapacity(minCapacity); 12 } 13 
private void ensureExplicitCapacity(int minCapacity) { 15     modCount++; 16     // overflow-conscious code
    if (minCapacity - elementData.length > 0) 18  grow(minCapacity); 19 } 20 
private void grow(int minCapacity) { 22     // overflow-conscious code
    int oldCapacity = elementData.length; 24     int newCapacity = oldCapacity + (oldCapacity >> 1); 25     if (newCapacity - minCapacity < 0) 26         newCapacity = minCapacity; 27     if (newCapacity - MAX_ARRAY_SIZE > 0) 28         newCapacity = hugeCapacity(minCapacity); 29     // minCapacity is usually close to size, so this is a win:
    elementData = Arrays.copyOf(elementData, newCapacity); 31 }

3. 刪除元素

需要調用 System.arraycopy() 將 index+1 后面的元素都復制到 index 位置上，該操作的時間復雜度為 O(N)，可以看到 ArrayList 刪除元素的代價是非常高的。

public E remove(int index) {  2  rangeCheck(index);  3     modCount++;  4     E oldValue = elementData(index);  5     int numMoved = size - index - 1;  6     if (numMoved > 0)  7         System.arraycopy(elementData, index+1, elementData, index, numMoved);  8     elementData[--size] = null; // clear to let GC do its work
    return oldValue; 10 }

4. 序列化

ArrayList 基於數組實現，並且具有動態擴容特性，因此保存元素的數組不一定都會被使用，那么就沒必要全部進行序列化。

保存元素的數組 elementData 使用 transient 修飾，該關鍵字聲明數組默認不會被序列化。

transient Object[] elementData; // non-private to simplify nested class access

ArrayList 實現了 writeObject() 和 readObject() 來控制只序列化數組中有元素填充那部分內容。

private void readObject(java.io.ObjectInputStream s)  2     throws java.io.IOException, ClassNotFoundException {  3     elementData = EMPTY_ELEMENTDATA;  4 
    // Read in size, and any hidden stuff
 s.defaultReadObject();  7 
    // Read in capacity
    s.readInt(); // ignored

    if (size > 0) { 12         // be like clone(), allocate array based upon size not capacity
 ensureCapacityInternal(size); 14 
        Object[] a = elementData; 16         // Read in all elements in the proper order.
        for (int i=0; i<size; i++) { 18             a[i] = s.readObject(); 19  } 20  } 21 }

private void writeObject(java.io.ObjectOutputStream s)  2     throws java.io.IOException{  3     // Write out element count, and any hidden stuff
    int expectedModCount = modCount;  5  s.defaultWriteObject();  6 
    // Write out size as capacity for behavioural compatibility with clone()
 s.writeInt(size);  9 
    // Write out all elements in the proper order.
    for (int i=0; i<size; i++) { 12  s.writeObject(elementData[i]); 13  } 14 
    if (modCount != expectedModCount) { 16         throw new ConcurrentModificationException(); 17  } 18 }

序列化時需要使用 ObjectOutputStream 的 writeObject() 將對象轉換為字節流並輸出。而 writeObject() 方法在傳入的對象存在 writeObject() 的時候會去反射調用該對象的 writeObject() 來實現序列化。反序列化使用的是 ObjectInputStream 的 readObject() 方法，原理類似。

ArrayList list = new ArrayList(); 2 ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream(file)); 3 oos.writeObject(list);

5. modCount

modCount 用來記錄 ArrayList 結構發生變化的次數。結構發生變化是指添加或者刪除至少一個元素的所有操作，或者是調整內部數組的大小，僅僅只是設置元素的值不算結構發生變化。

在進行序列化或者迭代等操作時，需要比較操作前后 modCount 是否改變，如果改變了需要拋出 ConcurrentModificationException。代碼參以上序列化中的 writeObject() 方法。

Vector

1. 同步

它的實現與 ArrayList 類似，但是使用了 synchronized 進行同步。

public synchronized boolean add(E e) {  2     modCount++;  3     ensureCapacityHelper(elementCount + 1);  4     elementData[elementCount++] = e;  5     return true;  6 }  7 
public synchronized E get(int index) {  9     if (index >= elementCount) 10         throw new ArrayIndexOutOfBoundsException(index); 11 
    return elementData(index); 13 }

2. 擴容

Vector 的構造函數可以傳入 capacityIncrement 參數，它的作用是在擴容時使容量 capacity 增長 capacityIncrement。如果這個參數的值小於等於 0，擴容時每次都令 capacity 為原來的兩倍。

public Vector(int initialCapacity, int capacityIncrement) { 2     super(); 3     if (initialCapacity < 0) 4         throw new IllegalArgumentException("Illegal Capacity: "+
 initialCapacity); 6     this.elementData = new Object[initialCapacity]; 7     this.capacityIncrement = capacityIncrement; 8 }

private void grow(int minCapacity) {  2     // overflow-conscious code
    int oldCapacity = elementData.length;  4     int newCapacity = oldCapacity + ((capacityIncrement > 0) ?
 capacityIncrement : oldCapacity);  6     if (newCapacity - minCapacity < 0)  7         newCapacity = minCapacity;  8     if (newCapacity - MAX_ARRAY_SIZE > 0)  9         newCapacity = hugeCapacity(minCapacity); 10     elementData = Arrays.copyOf(elementData, newCapacity); 11 }

調用沒有 capacityIncrement 的構造函數時，capacityIncrement 值被設置為 0，也就是說默認情況下 Vector 每次擴容時容量都會翻倍。

public Vector(int initialCapacity) { 2     this(initialCapacity, 0); 3 } 4 
public Vector() { 6     this(10); 7 }

3. 與 ArrayList 的比較

• Vector 是同步的，因此開銷就比 ArrayList 要大，訪問速度更慢。最好使用 ArrayList 而不是 Vector，因為同步操作完全可以由程序員自己來控制；
• Vector 每次擴容請求其大小的 2 倍（也可以通過構造函數設置增長的容量），而 ArrayList 是 1.5 倍。

4. 替代方案

可以使用 Collections.synchronizedList(); 得到一個線程安全的 ArrayList。

List<String> list = new ArrayList<>(); 2 List<String> synList = Collections.synchronizedList(list);

也可以使用 concurrent 並發包下的 CopyOnWriteArrayList 類。

List<String> list = new CopyOnWriteArrayList<>();

CopyOnWriteArrayList

1. 讀寫分離

寫操作在一個復制的數組上進行，讀操作還是在原始數組中進行，讀寫分離，互不影響。

寫操作需要加鎖，防止並發寫入時導致寫入數據丟失。

寫操作結束之后需要把原始數組指向新的復制數組。

public boolean add(E e) {  2     final ReentrantLock lock = this.lock;  3  lock.lock();  4     try {  5         Object[] elements = getArray();  6         int len = elements.length;  7         Object[] newElements = Arrays.copyOf(elements, len + 1);  8         newElements[len] = e;  9  setArray(newElements); 10         return true; 11     } finally { 12  lock.unlock(); 13  } 14 } 15 
final void setArray(Object[] a) { 17     array = a; 18 }

@SuppressWarnings("unchecked") 2 private E get(Object[] a, int index) { 3     return (E) a[index]; 4 }

2. 適用場景

CopyOnWriteArrayList 在寫操作的同時允許讀操作，大大提高了讀操作的性能，因此很適合讀多寫少的應用場景。

但是 CopyOnWriteArrayList 有其缺陷：

• 內存占用：在寫操作時需要復制一個新的數組，使得內存占用為原來的兩倍左右；
• 數據不一致：讀操作不能讀取實時性的數據，因為部分寫操作的數據還未同步到讀數組中。

所以 CopyOnWriteArrayList 不適合內存敏感以及對實時性要求很高的場景。

LinkedList

1. 概覽

基於雙向鏈表實現，使用 Node 存儲鏈表節點信息。

private static class Node<E> { 2  E item; 3     Node<E> next; 4     Node<E> prev; 5 }

每個鏈表存儲了 first 和 last 指針：

transient Node<E> first; 2 transient Node<E> last;

 

2. 與 ArrayList 的比較

ArrayList 基於動態數組實現，LinkedList 基於雙向鏈表實現。ArrayList 和 LinkedList 的區別可以歸結為數組和鏈表的區別：

• 數組支持隨機訪問，但插入刪除的代價很高，需要移動大量元素；
• 鏈表不支持隨機訪問，但插入刪除只需要改變指針。

HashMap

1、HashMap 底層原理備戰鏈接

https://www.cnblogs.com/taojietaoge/p/11359542.html

2、與 Hashtable 的比較

• Hashtable 使用 synchronized 來進行同步。
• HashMap 可以插入鍵為 null 的 Entry。
• HashMap 的迭代器是 fail-fast 迭代器。
• HashMap 不能保證隨着時間的推移 Map 中的元素次序是不變的。

ConcurrentHashMap

參考鏈接：https://www.cnblogs.com/taojietaoge/p/10301711.html

1. 存儲結構

static final class HashEntry<K,V> { 2     final int hash; 3     final K key; 4     volatile V value; 5     volatile HashEntry<K,V> next; 6 }

ConcurrentHashMap 和 HashMap 實現上類似，最主要的差別是 ConcurrentHashMap 采用了分段鎖（Segment），每個分段鎖維護着幾個桶（HashEntry），多個線程可以同時訪問不同分段鎖上的桶，從而使其並發度更高（並發度就是 Segment 的個數）。

Segment 繼承自 ReentrantLock。

static final class Segment<K,V> extends ReentrantLock implements Serializable {  2 
    private static final long serialVersionUID = 2249069246763182397L;  4 
    static final int MAX_SCAN_RETRIES =
        Runtime.getRuntime().availableProcessors() > 1 ? 64 : 1;  7 
    transient volatile HashEntry<K,V>[] table;  9 
    transient int count; 11 
    transient int modCount; 13 
    transient int threshold; 15 
    final float loadFactor; 17 }

final Segment<K,V>[] segments;

默認的並發級別為 16，也就是說默認創建 16 個 Segment。

static final int DEFAULT_CONCURRENCY_LEVEL = 16;

2. size 操作

每個 Segment 維護了一個 count 變量來統計該 Segment 中的鍵值對個數。

/**
 * The number of elements. Accessed only either within locks 3  * or among other volatile reads that maintain visibility. 4  */
transient int count;

在執行 size 操作時，需要遍歷所有 Segment 然后把 count 累計起來。

ConcurrentHashMap 在執行 size 操作時先嘗試不加鎖，如果連續兩次不加鎖操作得到的結果一致，那么可以認為這個結果是正確的。

嘗試次數使用 RETRIES_BEFORE_LOCK 定義，該值為 2，retries 初始值為 -1，因此嘗試次數為 3。

如果嘗試的次數超過 3 次，就需要對每個 Segment 加鎖。

/**
 * Number of unsynchronized retries in size and containsValue  3  * methods before resorting to locking. This is used to avoid  4  * unbounded retries if tables undergo continuous modification  5  * which would make it impossible to obtain an accurate result.  6  */
static final int RETRIES_BEFORE_LOCK = 2;  8 
public int size() { 10     // Try a few times to get accurate count. On failure due to 11     // continuous async changes in table, resort to locking.
    final Segment<K,V>[] segments = this.segments; 13     int size; 14     boolean overflow; // true if size overflows 32 bits
    long sum;         // sum of modCounts
    long last = 0L;   // previous sum
    int retries = -1; // first iteration isn't retry
    try { 19         for (;;) { 20             // 超過嘗試次數，則對每個 Segment 加鎖
            if (retries++ == RETRIES_BEFORE_LOCK) { 22                 for (int j = 0; j < segments.length; ++j) 23                     ensureSegment(j).lock(); // force creation
 } 25             sum = 0L; 26             size = 0; 27             overflow = false; 28             for (int j = 0; j < segments.length; ++j) { 29                 Segment<K,V> seg = segmentAt(segments, j); 30                 if (seg != null) { 31                     sum += seg.modCount; 32                     int c = seg.count; 33                     if (c < 0 || (size += c) < 0) 34                         overflow = true; 35  } 36  } 37             // 連續兩次得到的結果一致，則認為這個結果是正確的
            if (sum == last) 39                 break; 40             last = sum; 41  } 42     } finally { 43         if (retries > RETRIES_BEFORE_LOCK) { 44             for (int j = 0; j < segments.length; ++j) 45  segmentAt(segments, j).unlock(); 46  } 47  } 48     return overflow ? Integer.MAX_VALUE : size; 49 }

3. JDK 1.8 的改動

JDK 1.7 使用分段鎖機制來實現並發更新操作，核心類為 Segment，它繼承自重入鎖 ReentrantLock，並發度與 Segment 數量相等。

JDK 1.8 使用了 CAS 操作來支持更高的並發度，在 CAS 操作失敗時使用內置鎖 synchronized。

並且 JDK 1.8 的實現也在鏈表過長時會轉換為紅黑樹。

LinkedHashMap

1、存儲結構

繼承自 HashMap，因此具有和 HashMap 一樣的快速查找特性。

public class LinkedHashMap<K,V> extends HashMap<K,V> implements Map<K,V>

內部維護了一個雙向鏈表，用來維護插入順序或者 LRU 順序。

/**
 * The head (eldest) of the doubly linked list. 3  */
transient LinkedHashMap.Entry<K,V> head; 5 
/**
 * The tail (youngest) of the doubly linked list. 8  */
transient LinkedHashMap.Entry<K,V> tail;

accessOrder 決定了順序，默認為 false，此時維護的是插入順序。

final boolean accessOrder;

LinkedHashMap 最重要的是以下用於維護順序的函數，它們會在 put、get 等方法中調用。

void afterNodeAccess(Node<K,V> p) { } 2 void afterNodeInsertion(boolean evict) { }

2、afterNodeAccess()

當一個節點被訪問時，如果 accessOrder 為 true，則會將該節點移到鏈表尾部。也就是說指定為 LRU 順序之后，在每次訪問一個節點時，會將這個節點移到鏈表尾部，保證鏈表尾部是最近訪問的節點，那么鏈表首部就是最近最久未使用的節點。 

void afterNodeAccess(Node<K,V> e) { // move node to last
    LinkedHashMap.Entry<K,V> last;  3     if (accessOrder && (last = tail) != e) {  4         LinkedHashMap.Entry<K,V> p =
            (LinkedHashMap.Entry<K,V>)e, b = p.before, a = p.after;  6         p.after = null;  7         if (b == null)  8             head = a;  9         else
            b.after = a; 11         if (a != null) 12             a.before = b; 13         else
            last = b; 15         if (last == null) 16             head = p; 17         else { 18             p.before = last; 19             last.after = p; 20  } 21         tail = p; 22         ++modCount; 23  } 24 }

3、afterNodeInsertion()

在 put 等操作之后執行，當 removeEldestEntry() 方法返回 true 時會移除最晚的節點，也就是鏈表首部節點 first。

evict 只有在構建 Map 的時候才為 false，在這里為 true。

void afterNodeInsertion(boolean evict) { // possibly remove eldest
    LinkedHashMap.Entry<K,V> first; 3     if (evict && (first = head) != null && removeEldestEntry(first)) { 4         K key = first.key; 5         removeNode(hash(key), key, null, false, true); 6  } 7 }

removeEldestEntry() 默認為 false，如果需要讓它為 true，需要繼承 LinkedHashMap 並且覆蓋這個方法的實現，這在實現 LRU 的緩存中特別有用，通過移除最近最久未使用的節點，從而保證緩存空間足夠，並且緩存的數據都是熱點數據。

protected boolean removeEldestEntry(Map.Entry<K,V> eldest) { 2     return false; 3 }

4、LRU 緩存

以下是使用 LinkedHashMap 實現的一個 LRU 緩存：

• 設定最大緩存空間 MAX_ENTRIES 為 3；
• 使用 LinkedHashMap 的構造函數將 accessOrder 設置為 true，開啟 LRU 順序；
• 覆蓋 removeEldestEntry() 方法實現，在節點多於 MAX_ENTRIES 就會將最近最久未使用的數據移除。

class LRUCache<K, V> extends LinkedHashMap<K, V> {  2     private static final int MAX_ENTRIES = 3;  3 
    protected boolean removeEldestEntry(Map.Entry eldest) {  5         return size() > MAX_ENTRIES;  6  }  7 
 LRUCache() {  9         super(MAX_ENTRIES, 0.75f, true); 10  } 11 }

public static void main(String[] args) { 2     LRUCache<Integer, String> cache = new LRUCache<>(); 3     cache.put(1, "a"); 4     cache.put(2, "b"); 5     cache.put(3, "c"); 6     cache.get(1); 7     cache.put(4, "d"); 8     System.out.println(cache.keySet());  // [3, 1, 4]
}

WeakHashMap

1、存儲結構

WeakHashMap 的 Entry 繼承自 WeakReference，被 WeakReference 關聯的對象在下一次垃圾回收時會被回收。

WeakHashMap 主要用來實現緩存，通過使用 WeakHashMap 來引用緩存對象，由 JVM 對這部分緩存進行回收。 

private static class Entry<K,V> extends WeakReference<Object> implements Map.Entry<K,V>

2、ConcurrentCache

Tomcat 中的 ConcurrentCache 使用了 WeakHashMap 來實現緩存功能。

ConcurrentCache 采取的是分代緩存：

• 經常使用的對象放入 eden 中，eden 使用 ConcurrentHashMap 實現，不用擔心會被回收（伊甸園）；
• 不常用的對象放入 longterm，longterm 使用 WeakHashMap 實現，這些老對象會被垃圾收集器回收。
• 當調用 get() 方法時，會先從 eden 區獲取，如果沒有找到的話再到 longterm 獲取，當從 longterm 獲取到就把對象放入 eden 中，從而保證經常被訪問的節點不容易被回收。
• 當調用 put() 方法時，如果 eden 的大小超過了 size，那么就將 eden 中的所有對象都放入 longterm 中，利用虛擬機回收掉一部分不經常使用的對象。

public final class ConcurrentCache<K, V> {  2 
    private final int size;  4 
    private final Map<K, V> eden;  6 
    private final Map<K, V> longterm;  8 
    public ConcurrentCache(int size) { 10         this.size = size; 11         this.eden = new ConcurrentHashMap<>(size); 12         this.longterm = new WeakHashMap<>(size); 13  } 14 
    public V get(K k) { 16         V v = this.eden.get(k); 17         if (v == null) { 18             v = this.longterm.get(k); 19             if (v != null) 20                 this.eden.put(k, v); 21  } 22         return v; 23  } 24 
    public void put(K k, V v) { 26         if (this.eden.size() >= size) { 27             this.longterm.putAll(this.eden); 28             this.eden.clear(); 29  } 30         this.eden.put(k, v); 31  } 32 }

 

 

 

 

玉階生白露

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