Arrays.asList返回结果执行clear方法抛java.lang.UnsupportedOperationException

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代码

public class ErrorTest {    public static void main(String[] args) {        String[] strArr=new String[]{"1","2","3"};        List<String> list=Arrays.asList(strArr);        list.clear();        System.out.println(list);    }}

Exception in thread “main” java.lang.UnsupportedOperationException
at java.util.AbstractList.remove(AbstractList.java:161)
at java.util.AbstractList$Itr.remove(AbstractList.java:374)
at java.util.AbstractList.removeRange(AbstractList.java:571)
at java.util.AbstractList.clear(AbstractList.java:234)
at cn.htd.goodscenter.test.ErrorTest.main(ErrorTest.java:10)

原因：

Arrays.asList(strArr);返回的List是Arrays类定义的内部类ArrayList，这个ArrayList继承了AbstractList。

 @SafeVarargs    public static <T> List<T> asList(T... a) {        return new ArrayList<>(a);    }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) {            if (array==null)                throw new NullPointerException();            a = array;        }        public int size() {            return a.length;        }        public Object[] toArray() {            return a.clone();        }        public <T> T[] toArray(T[] a) {        }       ...      }

AbstractList的clear方法代码如下

 public void clear() {        removeRange(0, size());    } protected void removeRange(int fromIndex, int toIndex) {        ListIterator<E> it = listIterator(fromIndex);        for (int i=0, n=toIndex-fromIndex; i<n; i++) {            it.next();            it.remove();        }    }   public ListIterator<E> listIterator(final int index) {        rangeCheckForAdd(index);        return new ListItr(index);    }

ListIterator实际类是ListItr，ListItr类是AbstractList类的一个内部类，代码如下：

 private class ListItr extends Itr implements ListIterator<E> {        ListItr(int index) {            cursor = index;        }        public boolean hasPrevious() {            return cursor != 0;        }        public E previous() {            checkForComodification();            try {                int i = cursor - 1;                E previous = get(i);                lastRet = cursor = i;                return previous;            } catch (IndexOutOfBoundsException e) {                checkForComodification();                throw new NoSuchElementException();            }        }

ListItr继承了ListItr，我们可以看下ListItr的remove方法

   public void remove() {            if (lastRet < 0)                throw new IllegalStateException();            checkForComodification();            try {                AbstractList.this.remove(lastRet);                if (lastRet < cursor)                    cursor--;                lastRet = -1;                expectedModCount = modCount;            } catch (IndexOutOfBoundsException e) {                throw new ConcurrentModificationException();            }        }

重点看 AbstractList.this.remove(lastRet);这一行，调用了AbstractList的remove方法

public E remove(int index) {        throw new UnsupportedOperationException();    }

该方法在AbstractList类中并没有实现。

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