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

1. List ids1 = Arrays.asList(1L, 3L, 2L);

2. List ids2 = Collections.singletonList(2L);

3. List ids3 = new ArrayList<>();

4. ids3.add(1L);

5. ids3.add(2L);

6. List 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. *

   This method also provides a convenient way to create a fixed-size

9. * list initialized to contain several elements:

10. *

11. * List stooges = Arrays.asList("Larry", "Moe", "Curly");

12. *

13. *

14. * @param 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 List asList(T… a) {

21. return new ArrayList<>(a);

22. }

23. private static class ArrayList extends AbstractList

24. implements RandomAccess, java.io.Serializable

25. {

26. private static final long serialVersionUID = -2764017481108945198L;

27. private final E[] a;

28. ArrayList(E[] array) {

29. a = Objects.requireNonNull(array);

30. }

31. @Override

32. public int size() {

33. return a.length;

34. }

35. @Override

36. public Object[] toArray() {

37. return a.clone();

38. }

39. @Override

40. @SuppressWarnings("unchecked")

41. public T[] toArray(T[] a) {

42. int size = size();

43. if (a.length < size)

44. return Arrays.copyOf(this.a, size,

45. (Class) a.getClass());

46. System.arraycopy(this.a, 0, a, 0, size);

47. if (a.length > size)

48. a[size] = null;

49. return a;

50. }

51. @Override

52. public E get(int index) {

53. return a[index];

54. }

55. @Override

56. public E set(int index, E element) {

57. E oldValue = a[index];

58. a[index] = element;

59. return oldValue;

60. }

61. @Override

62. public int indexOf(Object o) {

63. E[] a = this.a;

64. if (o == null) {

65. for (int i = 0; i < a.length; i++)

66. if (a[i] == null)

67. return i;

68. } else {

69. for (int i = 0; i < a.length; i++)

70. if (o.equals(a[i]))

71. return i;

72. }

73. return -1;

74. }

75. …… //看的出来，这个list是可以修改的 但是要通过set方法

76. }

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 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 List singletonList(T o) {

11. return new SingletonList<>(o);

12. }

13. private static class SingletonList

14. extends AbstractList

15. implements RandomAccess, Serializable {

16. private static final long serialVersionUID = 3093736618740652951L;

17. private final E element;

18. SingletonList(E obj) {element = obj;}

19. public Iterator iterator() {

20. return singletonIterator(element);

21. }

22. public int size() {return 1;}

23. public boolean contains(Object obj) {return eq(obj, element);}

24. public E get(int index) {

25. if (index != 0)

26. throw new IndexOutOfBoundsException("Index: "+index+", Size: 1");

27. return element;

28. }

29. // Override default methods for Collection

30. @Override

31. public void forEach(Consumer action) {

32. action.accept(element);

33. }

34. @Override

35. public boolean removeIf(Predicate filter) {

36. throw new UnsupportedOperationException();

37. }

38. @Override

39. public void replaceAll(UnaryOperator operator) {

40. throw new UnsupportedOperationException();

41. }

42. @Override

43. public void sort(Comparator c) {

44. }

45. @Override

46. public Spliterator spliterator() {

47. return singletonSpliterator(element);

48. }

49. }

50. // 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