Arrays.asList()返回ArrayList呼叫remove()或add()出現 java.lang.UnsupportedOperationException異常
阿新 • • 發佈:2018-11-09
- Iterator和ListIterator可以對集合遍歷同時可以做remove操作, 但在呼叫Arrays.asList()轉化為ArrayList的過程中並不能呼叫remove()
public class Test { public static void main(String[] args) { List<String> list = test(); Iterator<String> it = list.iterator(); while (it.hasNext()) { String str = it.next(); System.out.println(str); if (str.equals("a")) { it.remove(); // TODO: 在這出錯了!!! java.lang.UnsupportedOperationException } } } private static List<String> test() { // TODO: 返回的是java.util.Arrays$ArrayList List<String> list = Arrays.asList("a", "b", "c"); list.sort( (e1, e2 ) -> e1.compareTo(e2) ); return list; } }
異常:
Exception in thread "main" java.lang.UnsupportedOperationException
at java.util.AbstractList.remove(AbstractList.java:161)
at java.util.AbstractList$Itr.remove(AbstractList.java:374)
at lambda.main(lambda.java:18)
解決方式:
private static List<String> test() { // TODO: 轉化成java.util.ArrayList List<String> list = new ArrayList(Arrays.asList("a", "b", "c")); list.sort( (e1, e2 ) -> e1.compareTo(e2) ); return list; }
什麼原因呢? Arrays.asList()返回的是ArrayList沒錯, 但是返回的ArrayList是Array的一個內部類(java.util.Arrays$ArrayList), 原始碼貼到下面了, 觀察一下, 根本沒有java.util.ArrayList類中的add()、remove()方法, 呼叫remove()方法當然出錯啦!
... 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) { a = Objects.requireNonNull(array); } @Override public int size() { return a.length; } @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; } @Override public E get(int index) { return a[index]; } @Override public E set(int index, E element) { E oldValue = a[index]; a[index] = element; return oldValue; } @Override public int indexOf(Object o) { E[] a = this.a; if (o == null) { for (int i = 0; i < a.length; i++) if (a[i] == null) return i; } else { for (int i = 0; i < a.length; i++) if (o.equals(a[i])) return i; } return -1; } @Override public boolean contains(Object o) { return indexOf(o) != -1; } @Override public Spliterator<E> spliterator() { return Spliterators.spliterator(a, Spliterator.ORDERED); } @Override public void forEach(Consumer<? super E> action) { Objects.requireNonNull(action); for (E e : a) { action.accept(e); } } @Override public void replaceAll(UnaryOperator<E> operator) { Objects.requireNonNull(operator); E[] a = this.a; for (int i = 0; i < a.length; i++) { a[i] = operator.apply(a[i]); } } @Override public void sort(Comparator<? super E> c) { Arrays.sort(a, c); } } ...