//removeAll()方法
private static void testList(){

        List<String> list = new ArrayList<String>();

        List<String> list1 = new ArrayList<String>();

        list.add("123");

        list.add("456");

        list1.add("123");

        list.removeAll(list1);

        Iterator<String> iterator = list1.iterator();

        while(iterator.hasNext()){

            System.out.println(iterator.next());

        }

    }

//输出结果

456

//clear()方法
private static void testList(){

        List<String> list = new ArrayList<String>();

        List<String> list1 = new ArrayList<String>();

        list.add("123");

        list1.add("123");

        list.add("456");

        list.clear();

        Iterator<String> iterator = list.iterator();

        while(iterator.hasNext()){

            System.out.println(iterator.next());

        }

        if(list == null){

            System.out.println("null");

        }else if(list.size()==0){

            System.out.println("list不为null"+"元素个数为"+list.size());

        }else{

            System.out.println("仍有元素");

        }

    }

//输出结果

list不为null元素个数为0