//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