Java中toMap构建字典的正确方法

toMap(Function<T, K> keyMapper, Function<T, U> valueMapper)

class User {
    int id;
    String name;

    User(int id, String name) {
        this.id = id;
        this.name = name;
    }
}

List<User> users = Arrays.asList(
    new User(1, "Alice"),
    new User(2, "Bob"),
    new User(3, "Charlie")
);

Map<Integer, String> userMap = users.stream()
    .collect(Collectors.toMap(u -> u.id, u -> u.name));

// 结果：{1=Alice, 2=Bob, 3=Charlie}

toMap(Function<T, K> keyMapper,
      Function<T, U> valueMapper,
      BinaryOperator<U> mergeFunction)

Map<Integer, String> userMap = users.stream()
    .collect(Collectors.toMap(
        u -> u.id,
        u -> u.name,
        (existing, replacement) -> existing  // 冲突时保留旧值
    ));

TreeMap<Integer, String> sortedMap = users.stream()
    .collect(Collectors.toMap(
        u -> u.id,
        u -> u.name,
        (a, b) -> a,
        TreeMap::new
    ));