HashMap接口

// 1. put
Map map = new HashMap<>();
        map.put("a","A");
        map.put("b","B");
        String v = map.put("b","v"); // 输出 B
        System.out.println(v);
        String v1 = map.put("c","v");
        System.out.println(v1); // 输出：NULL
// 2. putIfAbsent 相比于put就是多个absent的判断， 如果有key对应的键值对存进去了就不put了


// 3. computeIfAbsent
Map map = new HashMap<>();
        map.put("a","A");
        map.put("b","B");
        String v = map.computeIfAbsent("b",k->"v");  // 输出 B
        System.out.println(v);
        String v1 = map.computeIfAbsent("c",k->"v"); // 输出 v
        System.out.println(v1);