刪除 List 元素的三種正確方法

// 每次都會計算list.size()比較耗費性能
        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);
        }

//通過 CopyOnWriteArrayList 解決了 List的併發問題。每次remove的都是複製出的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);
        }

        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);
        }

Iterator的定義代碼

 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();
            }
        }

通過代碼我們發現 Itr 是 ArrayList 中定義的一個私有內部類，
在 next、remove方法中都會調用 checkForComodification 方法，該方法的作用是判斷 modCount != expectedModCount是否相等，如果不相等則拋出ConcurrentModificationException異常。
每次正常執行 remove 方法後，都會對執行expectedModCount = modCount賦值，保證兩個值相等，那麼問題基本上已經清晰了，在 foreach 循環中執行 list.remove(item);，對 list 對象的 modCount 值進行了修改，而 list 對象的迭代器的 expectedModCount 值未進行修改，因此拋出了ConcurrentModificationException異常。