Java数组转集合与集合转数组的坑

在Java中将数组转为集合，会用到Arrays.asList()的方法，然而，这个方法却与我们的预期期望存在一些出入，当用到asList方法将数组转化成List列表时，对得到的List列表进行add()和remove()操作, JVM会抛出异常：java.lang.UnsupportedOperationException异常

Arrays.asList返回的是同样的ArrayList，为什么就不能使用add和remove方法呢？

接下来我们来看一下Arrays.asList 源码

  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源码：

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();
    }

由源码可见，UnsupportedOperationException 是AbstractList抛出的，因为Arrays中的ArrayList并没有实现remove()和add()方法，所以抛出了异常。 
所以说 Arrays.asList 返回的 List 是一个不可变长度的列表，此列表不再具备原 List 的很多特性，因此慎用 Arrays.asList 方法。

---

ArrayList中构造方法：

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

看一段这样的代码： 

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 的内部类 ArrayList 构造方法接收的是一个泛型数组，即数组类型不能为基本类型，应该为其对应的包装类型(传即入引用类型)，否则传入的基本类型的整个数组，将被编译器视为一个引用参数，

所以原始类型不能作为 Arrays.asList 方法的参数，否则会被当做一个参数

---

再来看看在Java中将集合转为数组，ArrayList提供了一个将List转为数组的一个非常方便的方法toArray。toArray有两个重载的方法：

  @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;
        }

对于第一个重载方法，是将list直接转为Object[] 数组；

第二种方法是将list转化为你所需要类型的数组，当然我们用的时候会转化为与list内容相同的类型。

 若采用第一种toArray方法，是这样写:

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

         String[] array= (String[]) list.toArray();

结果一运行，报错：

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

第一个方法不能将Object[] 转化为String[]，我们需要修改为：

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

建议用第二种方法：普通数组基本类型必须改为包装类

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好处是去除重复元素