Java數組轉集合與集合轉數組的坑

在Java中將數組轉為集合，會用到Arrays.asList()的方法，然而，這個方法卻與我們的預期期望存在一些出入，當用到asList方法將數組轉化成List列表時，對得到的List列表進行add()和remove()操作, JVM會拋出異常：java.lang.UnsupportedOperationException異常 

Arrays.asList返回的是同樣的ArrayList，為什么就不能使用add和remove方法呢？

接下來我們來看一下Arrays.asList 源碼 

  public static <T> List<T> asList(T... a) {
        return new ArrayList<>(a);
    }

 /**
     * @serial include
     */
    private static class ArrayList<E> extends AbstractList<E>
        implements RandomAccess, java.io.Serializable
    {
        private static final long serialVersionUID = -2764017481108945198L;
        private final E[] a;

        ArrayList(E[] array) {
            a = Objects.requireNonNull(array);
        }

        @Override
        public int size() {
            return a.length;
        }

        @Override
        public Object[] toArray() {
            return a.clone();
        }

        @Override
        @SuppressWarnings("unchecked")

AbstractList源碼：

public abstract class AbstractList<E> extends AbstractCollection<E> implements List<E> {
    /**
     * Sole constructor.  (For invocation by subclass constructors, typically
     * implicit.)
     */
    protected AbstractList() {
    }

    /**
     * Appends the specified element to the end of this list (optional
     * operation).
     *
     * <p>Lists that support this operation may place limitations on what
     * elements may be added to this list.  In particular, some
     * lists will refuse to add null elements, and others will impose
     * restrictions on the type of elements that may be added.  List
     * classes should clearly specify in their documentation any restrictions
     * on what elements may be added.
     *
     * <p>This implementation calls {@code add(size(), e)}.
     *
     * <p>Note that this implementation throws an
     * {@code UnsupportedOperationException} unless
     * {@link #add(int, Object) add(int, E)} is overridden.
     *
     * @param e element to be appended to this list
     * @return {@code true} (as specified by {@link Collection#add})
     * @throws UnsupportedOperationException if the {@code add} operation
     *         is not supported by this list
     * @throws ClassCastException if the class of the specified element
     *         prevents it from being added to this list
     * @throws NullPointerException if the specified element is null and this
     *         list does not permit null elements
     * @throws IllegalArgumentException if some property of this element
     *         prevents it from being added to this list
     */
    public boolean add(E e) {
        add(size(), e);
        return true;
    }

    /**
     * {@inheritDoc}
     *
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    abstract public E get(int index);

    /**
     * {@inheritDoc}
     *
     * <p>This implementation always throws an
     * {@code UnsupportedOperationException}.
     *
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws ClassCastException            {@inheritDoc}
     * @throws NullPointerException          {@inheritDoc}
     * @throws IllegalArgumentException      {@inheritDoc}
     * @throws IndexOutOfBoundsException     {@inheritDoc}
     */
    public E set(int index, E element) {
        throw new UnsupportedOperationException();
    }

    /**
     * {@inheritDoc}
     *
     * <p>This implementation always throws an
     * {@code UnsupportedOperationException}.
     *
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws ClassCastException            {@inheritDoc}
     * @throws NullPointerException          {@inheritDoc}
     * @throws IllegalArgumentException      {@inheritDoc}
     * @throws IndexOutOfBoundsException     {@inheritDoc}
     */
    public void add(int index, E element) {
        throw new UnsupportedOperationException();
    }

    /**
     * {@inheritDoc}
     *
     * <p>This implementation always throws an
     * {@code UnsupportedOperationException}.
     *
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws IndexOutOfBoundsException     {@inheritDoc}
     */
    public E remove(int index) {
        throw new UnsupportedOperationException();
    }

 

由源碼可見，UnsupportedOperationException 是AbstractList拋出的，因為Arrays中的ArrayList並沒有實現remove()和add()方法，所以拋出了異常。 
所以說 Arrays.asList 返回的 List 是一個不可變長度的列表，此列表不再具備原 List 的很多特性，因此慎用 Arrays.asList 方法。

---

ArrayList中構造方法：

 

 ArrayList(E[] array) {
            a = Objects.requireNonNull(array);
        }

 

看一段這樣的代碼： 

 

import java.util.Arrays;
import java.util.List;

public class Test2{
    public static void main(String[] args) {
        int[] array = new int[]{1,2,3,4,5};
        List list = Arrays.asList(array);
        System.out.println(list.size());
    }
}

 

 

 

Arrays 的內部類 ArrayList 構造方法接收的是一個泛型數組，即數組類型不能為基本類型，應該為其對應的包裝類型(傳即入引用類型)，否則傳入的基本類型的整個數組，將被編譯器視為一個引用參數，

所以原始類型不能作為 Arrays.asList 方法的參數，否則會被當做一個參數

---

再來看看在Java中將集合轉為數組，ArrayList提供了一個將List轉為數組的一個非常方便的方法toArray。toArray有兩個重載的方法：

  @Override
        public Object[] toArray() {
            return a.clone();
        }

   @Override
        @SuppressWarnings("unchecked")
        public <T> T[] toArray(T[] a) {
            int size = size();
            if (a.length < size)
                return Arrays.copyOf(this.a, size,
                                     (Class<? extends T[]>) a.getClass());
            System.arraycopy(this.a, 0, a, 0, size);
            if (a.length > size)
                a[size] = null;
            return a;
        }

對於第一個重載方法，是將list直接轉為Object[] 數組；

第二種方法是將list轉化為你所需要類型的數組，當然我們用的時候會轉化為與list內容相同的類型。

 若采用第一種toArray方法，是這樣寫:

ArrayList<String> list=new ArrayList<String>();
        for (int i = 0; i < 10; i++) {
            list.add(""+i);
        }
         
        String[] array= (String[]) list.toArray();

結果一運行，報錯：

Exception in thread "main" java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.String;

第一個方法不能將Object[] 轉化為String[]，我們需要修改為：

Object[] arr = list.toArray();
        for (int i = 0; i < arr.length; i++) {
            String e = (String) arr[i];
            System.out.println(e);
        }

建議用第二種方法：普通數組基本類型必須改為包裝類

import java.util.ArrayList;
import java.util.List;

public class Test2{
    public static void main(String[] args) {
        List<Integer> list = new ArrayList();
        list.add(1);
        list.add(2);
        list.add(3);
        Integer[] array = new Integer[list.size()];
        list.toArray(array);
        for (int i:array){
            System.out.print(i+" ");
        }
    }
}

 或者

set.toArray(new Integer[set.size()]);

set好處是去除重復元素