Java中Map转List及List转Map的方法(简单好用!)

    public void testMapToList(){
        // 创建一个Map
        Map<String, Integer> map = new HashMap<>();
        map.put("key1", 1);
        map.put("key2", 2);
        map.put("key3", 3);
        // 将Map的key转换为List
        List<String> keyList = new ArrayList<>(map.keySet());
        // 打印结果
        System.out.println(keyList);
    }

    public void testMapToList(){
        // 创建一个Map
        Map<String, Integer> map = new HashMap<>();
        map.put("key1", 1);
        map.put("key2", 2);
        map.put("key3", 3);
        // 将Map的value转换为List
        List<Integer> keyList = new ArrayList<>(map.values());
        // 打印结果
        System.out.println(keyList);
    }

    public void testListToMap(){
        // 创建一个List
        List<String> list = new ArrayList<>();
        list.add("key1");
        list.add("key2");
        list.add("key3");
        // 将List转换为Map
        Map<String, Integer> map = list.stream()
                .collect(Collectors.toMap(key -> key, value -> 0, (v1, v2) -> v1)); // value里默认放一个0
        // 打印结果
        System.out.println(map);
    }

List<String> list = new ArrayList<>();
    list.add("key3");
    list.add("key2");
    list.add("key1");

public void testListToMap(){
        // 创建一个List
        List<String> list = new ArrayList<>();
        list.add("key3");
        list.add("key2");
        list.add("key1");
        // 将List转换为Map
        Map<String, Integer> map = list.stream()
                .collect(Collectors.toMap(key -> key, value -> 0, (v1, v2) -> v1, LinkedHashMap::new)); // value里默认放一个0
        // 打印结果
        System.out.println(map);
    }

{<!-- -->key3&#61;0, key2&#61;0, key1&#61;0}

@Data
    @AllArgsConstructor
    @NoArgsConstructor
    @Builder
    public static class User{
        private Integer id;
        private String name;
        private Integer age;
    }

public void testListToMap2() {
        // 创建一个List
        List<User> list = new ArrayList<>();
        list.add(new User(1,"Tom",10));
        list.add(new User(2,"Tim",10));
        list.add(new User(3,"Jim",10));
        // 将List转换为Map
        Map<String, Integer> map = list.stream()
                .collect(Collectors.toMap(key -> key.getId()+"_"+key.getName(), value -> value.getAge(), (v1, v2) -> v1)); 
        System.out.println(JSON.toJSON(map));
    }

public void testListToMap2() {
        // 创建一个List
        List<User> list = new ArrayList<>();
        list.add(new User(1,"Tom",10));
        list.add(new User(2,"Tim",10));
        list.add(new User(3,"Jim",10));
        // 将List转换为Map
        Map<Integer, User> map = list.stream()
                .collect(Collectors.toMap(User::getId, value -> value, (v1, v2) -> v1));
        System.out.println(JSON.toJSON(map));
    }