Arrays.asList()的坑

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异常代码

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List<Integer> list = Arrays.asList(1,2,3);
list.add(4);
list.remove(1);
list.clear();

运行这段代码，执行的三个方法都会抛异常：java.lang.UnsupportedOperationException

原因分析

跟踪源码，Arrays中asList实现：

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/**
 * Returns a fixed-size list backed by the specified array.  (Changes to
 * the returned list "write through" to the array.)  This method acts
 * as bridge between array-based and collection-based APIs, in
 * combination with {@link Collection#toArray}.  The returned list is
 * serializable and implements {@link RandomAccess}.
 *
 * <p>This method also provides a convenient way to create a fixed-size
 * list initialized to contain several elements:
 * <pre>
 *     List&lt;String&gt; stooges = Arrays.asList("Larry", "Moe", "Curly");
 * </pre>
 *
 * @param <T> the class of the objects in the array
 * @param a the array by which the list will be backed
 * @return a list view of the specified array
 */
@SafeVarargs
@SuppressWarnings("varargs")
public static <T> List<T> asList(T... a) {
    return new ArrayList<>(a);
}

乍一看确实返回了一个ArrayList，这么会不支持添加、删除、清空操作呢？再跟下ArrayList代码发现，这个ArrayList居然是在Arrays中定义的静态内部类:

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private static class ArrayList<E> extends AbstractList<E>
    implements RandomAccess, java.io.Serializable

而我们平时用的java.util.ArrayList是这个：

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public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable

两者区别在于Arrays中的ArrayList并未实现add、remove方法，调用这两个方法的时候实际是调用了其父类AbstractList的add、remove方法，AbstractList中直接抛了异常：

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public void add(int index, E element) {
    throw new UnsupportedOperationException();
}
    
public E remove(int index) {
    throw new UnsupportedOperationException();
}
}

而clear方法内部最终还是调用了remove来实现，所以同样会抛异常

解决方法

JDK当初这样设计估计只是简单提供一个可读权限集合，所以使用Arrays数组转集合时，如果会对集合进行添加或者删除时，可以将引用转移到其他地方：

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List list = Arrays.asList(T ... );
List newList = new ArrayList(list);