Java 集合系列（二）—— ArrayList

本文轉載自查看原文 2019-03-29 00:20 572 Java

ArrayList

　　ArrayList 是通過一個數組來實現的，因此它是在連續的存儲位置存放對象的引用，只不過它比 Array 更智能，能夠根據集合長度進行自動擴容。

　　假設讓我們來實現一個簡單的能夠自動擴容的數組，我們最容易想到的點就是：

add()的時候需要判斷當前數組size+1是否等於此時定義的數組大小；
若小於直接添加即可；否則，需要先擴容再進行添加。

實際上，ArrayList的內部實現原理也是這樣子，我們可以來研究分析一下ArrayList的源碼

`add(E e)` 源碼分析

 1   /**
 2      * Appends the specified element to the end of this list.
 3      *
 4      * @param e element to be appended to this list
 5      * @return <tt>true</tt> (as specified by {@link Collection#add})
 6      */
 7     public boolean add(E e) {
 8         ensureCapacityInternal(size + 1);   // 進行擴容校驗
 9         elementData[size++] = e;            // 將值添加到數組后面，並將 size+1
10         return true;
11     }
12 
13 
14 
15     /**
16      * The array buffer into which the elements of the ArrayList are stored.
17      * The capacity of the ArrayList is the length of this array buffer. Any
18      * empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
19      * will be expanded to DEFAULT_CAPACITY when the first element is added.
20      */
21     transient Object[] elementData; // non-private to simplify nested class access
22     
23     private void ensureCapacityInternal(int minCapacity) {
24         ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));    // elementData 數組
25     }
26 
27 
28 
29     /**
30      * Default initial capacity.
31      */
32     private static final int DEFAULT_CAPACITY = 10;
33     
34     /**
35      * Shared empty array instance used for default sized empty instances. We
36      * distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when
37      * first element is added.
38      */
39     private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
40 
41     // 返回最大的 index
42     private static int calculateCapacity(Object[] elementData, int minCapacity) {
43         if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {   //  與空數組實例對比
44             return Math.max(DEFAULT_CAPACITY, minCapacity);
45         }
46         return minCapacity;
47     }
48 
49 
50 
51     private void ensureExplicitCapacity(int minCapacity) {
52         modCount++;
53 
54         // overflow-conscious code
55         if (minCapacity - elementData.length > 0)
56             grow(minCapacity);
57     }

擴容調用方法，實際也就是數組復制的過程

 1     /**
 2      * Increases the capacity to ensure that it can hold at least the
 3      * number of elements specified by the minimum capacity argument.
 4      *
 5      * @param minCapacity the desired minimum capacity
 6      */
 7     private void grow(int minCapacity) {
 8         // overflow-conscious code
 9         int oldCapacity = elementData.length;
10         int newCapacity = oldCapacity + (oldCapacity >> 1);
11         if (newCapacity - minCapacity < 0)
12             newCapacity = minCapacity;
13         if (newCapacity - MAX_ARRAY_SIZE > 0)
14             newCapacity = hugeCapacity(minCapacity);
15         // minCapacity is usually close to size, so this is a win:
16         elementData = Arrays.copyOf(elementData, newCapacity);
17     }

`add(int index, E element)` 源碼分析

 1   /**
 2      * Inserts the specified element at the specified position in this
 3      * list. Shifts the element currently at that position (if any) and
 4      * any subsequent elements to the right (adds one to their indices).
 5      *
 6      * @param index index at which the specified element is to be inserted
 7      * @param element element to be inserted
 8      * @throws IndexOutOfBoundsException {@inheritDoc}
 9      */
10     public void add(int index, E element) {
11         rangeCheckForAdd(index);    // 校驗index是否超過當前定義的數組大小范圍，超過則拋出 IndexOutOfBoundsException
12 
13         ensureCapacityInternal(size + 1);  // Increments modCount!!
14         System.arraycopy(elementData, index, elementData, index + 1,
15                          size - index);     // 復制，向后移動
16         elementData[index] = element;
17         size++;
18     }
19     
20 
21     /**
22      * A version of rangeCheck used by add and addAll.
23      */
24     private void rangeCheckForAdd(int index) {
25         if (index > size || index < 0)
26             throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
27     }