public class Student {
    private String id;
    private String name;
    private String level;
}

List<Student> list = new ArrayList<>();
list.add(new Student("1","张三" + 1,"A" + 1));
list.add(new Student("2","张三" + 2,"A" + 2));
list.add(new Student("1","张三" + 3,"A" + 3));
list.add(new Student("4","张三" + 4,"A" + 4));
list.add(new Student("5","张三" + 5,"A" + 5));

/**
     * map有3种重载
     */
    //1 仅返回key，value的映射关系
    Map<String, String> map1 = list.stream()
      .collect(
          Collectors
            .toMap(
                Student::getId, 
                Student::getName));
    
    //2 用于针对有重复键的映射关系
    Map<String, String> map2 = list.stream()
      .collect(
          Collectors
            .toMap(
                //用id做map的key，name做map的val，
                //同一个key，可能会有多个val，用新val还是老val
                //可以这样设置
                item -> item.getId(), 
                item -> item.getName(), 
                (oldVal, currVal) -> oldVal));
    
    //3 用于返回某些特定map
    Map<String, String> map3 = list.stream()
        .collect(
            Collectors
              .toMap(
                  //用id做map的key，name做map的val，
                  //同一个key，可能会有多个val，用新val还是老val
                  //可以这样设置
                  item -> item.getId(), 
                  item -> item.getName(), 
                  (oldVal, currVal) -> oldVal,
                  HashMap::new));