赞
踩
今天无意中测试了一下数组转集合的jdk封装好的工具类Arrays,意外发生了。。。
代码如下:
- public static void main(String[] args) {
- Integer[] i = {128,12,1};
- List<Integer> list = Arrays.asList(i);
- list.add(123);
- System.out.println(list);
- }
运行结果:
- Exception in thread "main" java.lang.UnsupportedOperationException
- at java.util.AbstractList.add(AbstractList.java:148)
- at java.util.AbstractList.add(AbstractList.java:108)
- at test.demo.Test.main(Test.java:10)
让我们一起走进源码查看情况
首先查看Arrays的asList方法实现
- public static <T> List<T> asList(T... a) {
- return new ArrayList<>(a);
- }
这明明是创建的ArrayList对象啊,没什么不一样啊。鼠标放上去你会发现显示java.util Arrays.ArrayList ArrayList,原来是静态内部类啊。
那我们继续往下看具体实现如下:
- 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);
- }
- }

只实现了这些方法,根本就没有add和remove方法
但是ArrayList继承了AbstractList类,让我们看看他的add和remove方法是怎么写的吧。
- public void add(int index, E element) {
- throw new UnsupportedOperationException();
- }
-
- public E remove(int index) {
- throw new UnsupportedOperationException();
- }
知道了吧,这就是Arrays.asList为什么不能进行add和remove操作了。
但是就是想使用add和remove操作,我们可以先把这个集合放到java.util.ArrayList中在操作哦OvO
List list = new ArrayList(Arrays.asList(arr));
?
Copyright © 2003-2013 www.wpsshop.cn 版权所有,并保留所有权利。