Map集合中定义的方法

Posted 2021-06-23 koito

添加、删除、修改操作

Object put(Object key,Object value):将指定key-value添加到(或修改)当前map对象中
void putAll(Map m):将m中的所有key-value对存放到当前map中
Object remove(Object key):移除指定key的key-value对，并返回value
void clear():清空当前map中的所有数据

    @Test
    public void test3(){
        Map map = new HashMap();
        map.put("AA",123);//添加
        map.put(45,123);
        map.put("BB",56);
        map.put("AA",87);//修改
        System.out.println(map);

        Map map1 = new HashMap();
        map1.put("CC",123);
        map1.put("DD",123);
        map.putAll(map1);
        System.out.println(map);

        //remove(Object key)
        Object value = map.remove("CC");
        System.out.println(value);
        System.out.println(map);

        //clear()
        map.clear();//与map = null操作不同
        System.out.println(map.size());
        System.out.println(map);
    }

元素查询操作

Object get(Object key):获取指定key对应的value
boolean containsKey(Object key):是否包含指定的key
boolean containsValue(Object value):是否包含指定的value
int size():返回map中key-value对的个数
boolean isEmpty():判断当前map是否为空
boolean equals(Object obj):判断当前map和参数对象obj是否相等

    @Test
    public void test4(){
        Map map = new HashMap();
        map.put("AA",123);
        map.put(45,123);
        map.put("BB",56);
        //Object get(Object key)
        System.out.println(map.get(45));
        //boolean containsKey(Object key)
        boolean isExist = map.containsKey("BB");
        System.out.println(isExist);

        isExist = map.containsValue(123);
        System.out.println(isExist);

        map.clear();
        System.out.println(map.isEmpty());
    }

元视图操作的方法：

Set keySet():返回所有key构成的Set集合
Collection values():返回所有value构成的Collection集合
Set entrySet():返回所有key-value对构成的Set集合

    @Test
    public void test5(){
        Map map = new HashMap();
        map.put("AA",123);
        map.put(45,123);
        map.put("BB",56);

        //遍历所有的key集：keySet()
        Set set = map.keySet();
        Iterator iterator = set.iterator();
        while (iterator.hasNext()) {
            System.out.println(iterator.next());
        }
        System.out.println("=======================");

        //遍历所有的value集：values()
        Collection values = map.values();
        for(Object obj : values){
            System.out.println(obj);
        }
        System.out.println("=======================");

        //遍历所有的key-value
        //方式一：entrySet():
        Set entrySet = map.entrySet();
        Iterator iterator1 = entrySet.iterator();
        while (iterator1.hasNext()){
            Object o = iterator1.next();
            //entrySet集合中的元素都是entry
            Map.Entry entry = (Map.Entry) o;
            System.out.println(entry.getKey() + "----->" + entry.getValue());
        }
        System.out.println("=======================");

        //方式二：
        Set keySet = map.keySet();
        Iterator iterator2 = keySet.iterator();
        while (iterator2.hasNext()) {
            Object key = iterator2.next();
            Object value = map.get(key);
            System.out.println(key + "====" + value);
        }
    }

总结

• 增：put(Object key,Object value)
• 删：remove(Object key)
• 改：put(Object key,Object value)
• 查：get(Object key)
• 长度：size()
• 遍历：keySet() / values() / entrySet()