集合类中的modcount属性

灰太狼 2023-01-21 05:23 127阅读 0赞

经常看jdk源码的同学应该发现在很多的集合类里面会有一个modcount的成员变量。这个变量的含义在AbstractList类中有详细的描述。

/**
 * The number of times this list has been structurally modified.
 * Structural modifications are those that change the size of the
 * list, or otherwise perturb it in such a fashion that iterations in
 * progress may yield incorrect results.
 *
 * This field is used by the iterator and list iterator implementation
 * returned by the {@code iterator} and {@code listIterator} methods.
 * If the value of this field changes unexpectedly, the iterator (or list
 * iterator) will throw a {@code ConcurrentModificationException} in
 * response to the {@code next}, {@code remove}, {@code previous},
 * {@code set} or {@code add} operations. This provides
 * fail-fast behavior, rather than non-deterministic behavior in
 * the face of concurrent modification during iteration.
 *
 * Use of this field by subclasses is optional. If a subclass
 * wishes to provide fail-fast iterators (and list iterators), then it
 * merely has to increment this field in its {@code add(int, E)} and
 * {@code remove(int)} methods (and any other methods that it overrides
 * that result in structural modifications to the list). A single call to
 * {@code add(int, E)} or {@code remove(int)} must add no more than
 * one to this field, or the iterators (and list iterators) will throw
 * bogus {@code ConcurrentModificationExceptions}. If an implementation
 * does not wish to provide fail-fast iterators, this field may be
 * ignored.
 */
 protected transient int modCount = 0;

简单来说，就是为了提供了一种快速失败的机制，当iterator在遍历时，为了防止集合中的元素的个数发生变化，导致了一些不明确的情况出现，所以才需要维护这个modcount的变量。

当我们调用add,remove等方法时，会改变这个值，然后集合会在某些操作时对这个值进行检查。