HashMap详解

①特性

• 非线程安全
• hashMap的映射不是有序的
• key、value都可以为null

②常用方法

• get(Object key)
• put(Object key,Object value)
• remove(Object key)
• remove(Object key,Object value)
• isEmpty()
• size()
• clone()
• clear()
• entrySet()
• keySet()
• values()
• containsKey(Object key)
• containsValue(Object value)
• getOrDefault(Object key,Object defaultValue)
• replace(Object key,Object oldValue,Object newValue)
• replace(Object key,Object value)
• replaceAll(BigFunction function)
• putAll(Map m)
• putIfAbsent(Object key,Object value)
• compute(Objetc key,BigFunction bf)
• computeIfAbsent(Object key,Function f)
• computeIfPresent(Object key,BigFunction bf)
• merge(Object key,Object value,BigFunction bf)

get/put/remove/isEmpty/size

 @Test
    public void test() {
        HashMap<String, Integer> map = new HashMap();
        map.put("key1", 1);
        map.put("key2", 2);
        System.out.println(map);
        System.out.println("key1 的值" + map.get("key1"));
        map.remove("key1");
        map.remove("key2", 3);
        System.out.println(map);
        System.out.println(map.isEmpty());
        System.out.println("map size" + map.size());
        map.remove("key2", 2);
        System.out.println(map.isEmpty());
    }

clone/clear

 public void test2() {
        HashMap<String, Integer> map = new HashMap<>();
        map.put("key1", 1);
        map.put("key2", 2);
        HashMap<String, Integer> cloneMap = (HashMap) map.clone();
        System.out.println(map);
        System.out.println(cloneMap);
        cloneMap.remove("key1");
        System.out.println(map);
        System.out.println(cloneMap);
        map.clear();
        cloneMap.clear();
        System.out.println(map);
        System.out.println(cloneMap);
    }

entrySet/keySet/values

 @Test
    public void test3() {
        HashMap<String, Integer> map = new HashMap<>();
        map.put("key1", 1);
        map.put("key2", 2);
        for (Map.Entry<String, Integer> entry : map.entrySet()) {
            System.out.println(entry.getKey());
            System.out.println(entry.getValue());
        }
        for (String key : map.keySet()) {
            System.out.println(key);
        }
        for (Integer value : map.values()) {
            System.out.println(value);
        }
    }

containsKey/containsValue

  @Test
    public void test4() {
        HashMap<String, Student> map = new HashMap<>();
        Student ryan = new Student(18);
        map.put("Ryan", ryan);
        System.out.println(map.containsKey("Ryan"));
        System.out.println(map.containsKey("ryan"));
        System.out.println(map.containsValue(ryan));
    }

getOrDefault

@Test
    public void test5() {
        HashMap<String, Integer> map = new HashMap<>();
        map.put("key1", 1);
        System.out.println(map.get("key1"));
        Integer n = map.getOrDefault("key1", 2);//map 中存在key = "key1" 返回对应的值，否则返回 2
        Integer  m = map.getOrDefault("key2",2);
        System.out.println(n);
        System.out.println(m);
    }

replace

@Test
    public void test6() {
        HashMap<String, Integer> map = new HashMap<>();
        map.put("key1", 1);
        map.put("key2", 2);
        System.out.println(map);
        map.replace("key1", 1, 4);
        System.out.println(map);
        map.replace("key2", 5);
        System.out.println(map);
        //replaceAll 对集合中的键值对进行计算
        map.replaceAll((k, v) -> {
            System.out.println(k);
            System.out.println(v);
            return null;
        });
    }

putAll/putIfAbsent

@Test
    public void test7() {
        HashMap<String, Integer> map = new HashMap<>();
        map.put("key1", 1);
        HashMap<String, Integer> map2 = new HashMap<>();
        map.put("key2", 2);
        map.putAll(map2);
        System.out.println(map);
        map.putIfAbsent("key1", 2);
        map.putIfAbsent("key3", 3);
        System.out.println(map);
    }

merge/computeIfAbsent

https://blog.youkuaiyun.com/mengxb12138/article/details/80893937