Java踩坑之List的removeAll方法

最近在公司写东西，发现List的removeAll方法报错 Demo代码如下：

1. List<Long> ids1 = Arrays.asList(1L, 3L, 2L);
2. List<Long> ids2 = Collections.singletonList(2L);
3. List<Long> ids3 = new ArrayList<>();
4. ids3.add(1L);
5. ids3.add(2L);
6. List<Long> ids = new ArrayList<>();
7. ids.add(2L);
8. System.out.println("==== 001");
9. ids1.removeAll(ids); // 这一步会报错
10. System.out.println("==== 002");
11. ids2.removeAll(ids); // 这一步也会报错
12. System.out.println("==== 003");
13. ids3.removeAll(ids);

001报错的原因是：Arrays.asList 返回的List是自己内部实现的ArrayList 而不是util下的ArrayList对象

1. /**
2. * Returns a fixed-size list backed by the specified array. (Changes to //明确指出 返回的是固定大小的list
3. * the returned list "write through" to the array.) This method acts
4. * as bridge between array-based and collection-based APIs, in
5. * combination with {@link Collection#toArray}. The returned list is
6. * serializable and implements {@link RandomAccess}.
7. *
8. * <p>This method also provides a convenient way to create a fixed-size
9. * list initialized to contain several elements:
10. * <pre>
11. * List<String> stooges = Arrays.asList("Larry", "Moe", "Curly");
12. * </pre>
13. *
14. * @param <T> the class of the objects in the array
15. * @param a the array by which the list will be backed
16. * @return a list view of the specified array
17. */
18. @SafeVarargs
19. @SuppressWarnings("varargs")
20. public static <T> List<T> asList(T... a) {
21. return new ArrayList<>(a);
22. }

1. private static class ArrayList<E> extends AbstractList<E>
2. implements RandomAccess, java.io.Serializable
3. {
4. private static final long serialVersionUID = -2764017481108945198L;
5. private final E[] a;
6.  
7. ArrayList(E[] array) {
8. a = Objects.requireNonNull(array);
9. }
10.  
11. @Override
12. public int size() {
13. return a.length;
14. }
15.  
16. @Override
17. public Object[] toArray() {
18. return a.clone();
19. }
20.  
21. @Override
22. @SuppressWarnings("unchecked")
23. public <T> T[] toArray(T[] a) {
24. int size = size();
25. if (a.length < size)
26. return Arrays.copyOf(this.a, size,
27. (Class<? extends T[]>) a.getClass());
28. System.arraycopy(this.a, 0, a, 0, size);
29. if (a.length > size)
30. a[size] = null;
31. return a;
32. }
33.  
34. @Override
35. public E get(int index) {
36. return a[index];
37. }
38.  
39. @Override
40. public E set(int index, E element) {
41. E oldValue = a[index];
42. a[index] = element;
43. return oldValue;
44. }
45.  
46. @Override
47. public int indexOf(Object o) {
48. E[] a = this.a;
49. if (o == null) {
50. for (int i = 0; i < a.length; i++)
51. if (a[i] == null)
52. return i;
53. } else {
54. for (int i = 0; i < a.length; i++)
55. if (o.equals(a[i]))
56. return i;
57. }
58. return -1;
59. }
60. ...... //看的出来，这个list是可以修改的 但是要通过set方法
61. }

所以调用removeAll方法的时候 会调用AbstractList的父类AbstractCollection的removeAll方法：

1. public boolean removeAll(Collection<?> c) {
2. Objects.requireNonNull(c);
3. boolean modified = false;
4. Iterator<?> it = iterator();
5. while (it.hasNext()) {
6. if (c.contains(it.next())) {
7. it.remove();
8. modified = true;
9. }
10. }
11. return modified;
12. }

ArrayList是数组 在循环的时候删除元素 一定会出现问题

002报错的原因是与001类似：Collections.singletonList的返回值SingletonSet也是自己的内部类

1. /**
2. * Returns an immutable list containing only the specified object. // 返回不可变的list
3. * The returned list is serializable.
4. *
5. * @param <T> the class of the objects in the list
6. * @param o the sole object to be stored in the returned list.
7. * @return an immutable list containing only the specified object.
8. * @since 1.3
9. */
10. public static <T> List<T> singletonList(T o) {
11. return new SingletonList<>(o);
12. }

1. private static class SingletonList<E>
2. extends AbstractList<E>
3. implements RandomAccess, Serializable {
4.  
5. private static final long serialVersionUID = 3093736618740652951L;
6.  
7. private final E element;
8.  
9. SingletonList(E obj) {element = obj;}
10.  
11. public Iterator<E> iterator() {
12. return singletonIterator(element);
13. }
14.  
15. public int size() {return 1;}
16.  
17. public boolean contains(Object obj) {return eq(obj, element);}
18.  
19. public E get(int index) {
20. if (index != 0)
21. throw new IndexOutOfBoundsException("Index: "+index+", Size: 1");
22. return element;
23. }
24.  
25. // Override default methods for Collection
26. @Override
27. public void forEach(Consumer<? super E> action) {
28. action.accept(element);
29. }
30. @Override
31. public boolean removeIf(Predicate<? super E> filter) {
32. throw new UnsupportedOperationException();
33. }
34. @Override
35. public void replaceAll(UnaryOperator<E> operator) {
36. throw new UnsupportedOperationException();
37. }
38. @Override
39. public void sort(Comparator<? super E> c) {
40. }
41. @Override
42. public Spliterator<E> spliterator() {
43. return singletonSpliterator(element);
44. }
45. }
46. // 可以看出 这个list是一个只读的list 并未对外提供编辑方法

同样会调用AbstractCollection的removeAll方法

003是我new的ArrayList对象，会调用自己内部重写的removeAll方法，针对数组重写了删除方法，不会出问题，解决001、002的问题 最简单的办法可以用new ArrayList()包一层就ok了！

究其原因还是自己对源码的研究不足！

更多技术资源请访问：https://www.zhaochao.top/articles