删除 List 元素的三种正确方法

// 每次都会计算list.size()比较耗费性能
        public static void remove1 (List < String > list, String target){
            for (int i = list.size() - 1; i >= 0; i--) {
                String item = list.get(i);
                if (target.equals(item)) {
                    list.remove(item);
                }
            }
            print(list);
        }

　

//通过 CopyOnWriteArrayList 解决了 List的并发问题。每次remove的都是复制出的list
        public static void remove2 (ArrayList < String > list, String target){
            final CopyOnWriteArrayList<String> cowList = new CopyOnWriteArrayList<String>(list);
            for (String item : cowList) {
                if (item.equals(target)) {
                    cowList.remove(item);
                }
            }
            print(cowList);
        }

Iterator的定义代码

        public static void remove3 (List < String > list, String target){
            Iterator<String> iter = list.iterator();
            while (iter.hasNext()) {
                String item = iter.next();
                if (item.equals(target)) {
                    iter.remove();
                }
            }
            print(list);
        }

　　

 private class Itr implements Iterator<E> {
            int cursor; // index of next element to return
            int lastRet = -1; // index of last element returned; -1 if no such
            int expectedModCount = modCount;

            public boolean hasNext() {
                return cursor != size;
            }

            @SuppressWarnings("unchecked")
            public E next() {
                checkForComodification();
                int i = cursor;
                if (i >= size) throw new NoSuchElementException();
                Object[] elementData = ArrayList.this.elementData;
                if (i >= elementData.length) throw new ConcurrentModificationException();
                cursor = i + 1;
                return (E) elementData[lastRet = i];
            }

            public void remove() {
                if (lastRet < 0) throw new IllegalStateException();
                checkForComodification();
                try {
                    ArrayList.this.remove(lastRet);
                    cursor = lastRet;
                    lastRet = -1;
                    expectedModCount = modCount;
                } catch (IndexOutOfBoundsException ex) {
                    throw new ConcurrentModificationException();
                }
            }

            final void checkForComodification() {
                if (modCount != expectedModCount) throw new ConcurrentModificationException();
            }
        }

通过代码我们发现 Itr 是 ArrayList 中定义的一个私有内部类，
在 next、remove方法中都会调用 checkForComodification 方法，该方法的作用是判断 modCount != expectedModCount是否相等，如果不相等则抛出ConcurrentModificationException异常。
每次正常执行 remove 方法后，都会对执行expectedModCount = modCount赋值，保证两个值相等，那么问题基本上已经清晰了，在 foreach 循环中执行 list.remove(item);，对 list 对象的 modCount 值进行了修改，而 list 对象的迭代器的 expectedModCount 值未进行修改，因此抛出了ConcurrentModificationException异常。