如何使用Stream流将List转换为Map

以下程序用到的基础代码：

final static List<Student> studentList = new ArrayList<Student>();

/**
 * 初始化集合数据
 */
static {
   
   Student stu1 = new Student("0001", "张三", 12, "江苏南京");
   Student stu2 = new Student("0002", "李四", 14, "江苏无锡");
   Student stu3 = new Student("0003", "王二", 11, "浙江台州");
   Student stu4 = new Student("0004", "李五", 12, "浙江温州");


   studentList.add(stu1);
   studentList.add(stu2);
   studentList.add(stu3);
   studentList.add(stu4);
}

        

          
        
        
        

          

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List<Object> 转化为Map

Map<String, Student> map = studentList.stream().collect(Collectors.toMap(Student::getId, each -> each, (value1, value2) -> value1));

        

          
        
        
        

          

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List<Object>转化为Map

Map<String, String> map = studentList.stream().collect(Collectors.toMap(Student::getName, Student::getAddress, (value1, value2) -> value1));

        

          
        
        
        

          

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List<Object>转化为Map>`

Map<Integer, List<Student>> map = studentList.stream().collect(Collectors.groupingBy(Student::getAge));

        

          
        
        
        

          

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List<Object>转化为Map>`

Map<String, List<String>> map3 = studentList.stream().collect(Collectors.toMap(Student::getId, each -> Collections.singletonList(each.getName()), (value1, value2) -> {
   
            List<String> union = new ArrayList<>(value1);
            union.addAll(value2);
            return union;
}));

        

          
        
        
        

          

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List> 转化为Map

final static List<Map<String, Object>> mapStudentList = new ArrayList<>();
public static void main(String[] args) {
   
        Map<String, Object> map4 = mapStudentList.stream().collect(Collectors.toMap(each -> Objects.toString(each.get("id"), ""), each -> each.get("student"), (key1, key2) -> key1));
    }


    /**
     * 初始化集合数据
     */
    static {
   
        Student stu1 = new Student("0001", "张三", 12, "江苏南京");
        Student stu2 = new Student("0002", "李四", 14, "江苏无锡");
        Student stu3 = new Student("0003", "王二", 11, "浙江台州");
        Student stu4 = new Student("0004", "李五", 12, "浙江温州");


        Map<String, Object> map1 = new HashMap<>();
        map1.put("id", "0001");
        map1.put("student", stu1);

        Map<String, Object> map2 = new HashMap<>();
        map2.put("id", "0002");
        map2.put("student", stu2);

        Map<String, Object> map3 = new HashMap<>();
        map3.put("id", "0003");
        map3.put("student", stu3);

        Map<String, Object> map4 = new HashMap<>();
        map4.put("id", "0004");
        map4.put("student", stu4);

        mapStudentList.add(map1);
        mapStudentList.add(map2);
        mapStudentList.add(map3);
        mapStudentList.add(map4);
    }

        

          
        
        
        

          

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List> 转化为Map>

    final static List<Map<String, String>> listMapList = new ArrayList<>();


    public static void main(String[] args) {
   
        Map<String, Map<String, String>> map5 = listMapList.stream().collect(Collectors.toMap(each -> each.get("id"), each -> each, (key1, key2) -> key1));
        System.out.println("map5 = " + map5);

    }

    /**
     * 初始化集合数据
     */
    static {
   
        Map<String, String> map1 = new HashMap<>();
        map1.put("id", "0001");
        map1.put("name", "张三");
        map1.put("age", "12");
        map1.put("address", "江苏南京");

        Map<String, String> map2 = new HashMap<>();
        map2.put("id", "0002");
        map2.put("name", "李四");
        map2.put("age", "14");
        map2.put("address", "江苏无锡");


        Map<String, String> map3 = new HashMap<>();
        map3.put("id", "0003");
        map3.put("name", "王二");
        map3.put("age", "11");
        map3.put("address", "浙江台州");

        Map<String, String> map4 = new HashMap<>();
        map4.put("id", "0004");
        map4.put("name", "李五");
        map4.put("age", "12");
        map4.put("address", "浙江温州");


        listMapList.add(map1);
        listMapList.add(map2);
        listMapList.add(map3);
        listMapList.add(map4);
    }

        

          
        
        
        

          

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List> 转化为Map

     final static List<Map<String, String>> listmapstringlist = new ArrayList<>();

     public static void main(String[] args) {
   
     Map<String, String> map6 = listmapstringlist.stream().collect(Collectors.toMap(each -> each.get("id"), each -> each.get("name"), (key1, key2) -> key1));

    }

    /**
     * 初始化集合数据
     */
    static {
   
        Map<String, String> map1 = new HashMap<>();
        map1.put("id", "0001");
        map1.put("name", "张三");
        map1.put("age", "12");
        map1.put("address", "江苏南京");

        Map<String, String> map2 = new HashMap<>();
        map2.put("id", "0002");
        map2.put("name", "李四");
        map2.put("age", "14");
        map2.put("address", "江苏无锡");


        Map<String, String> map3 = new HashMap<>();
        map3.put("id", "0003");
        map3.put("name", "王二");
        map3.put("age", "11");
        map3.put("address", "浙江台州");

        Map<String, String> map4 = new HashMap<>();
        map4.put("id", "0004");
        map4.put("name", "李五");
        map4.put("age", "12");
        map4.put("address", "浙江温州");
        listmapstringlist.add(map1);
        listmapstringlist.add(map2);
        listmapstringlist.add(map3);
        listmapstringlist.add(map4);
    }

        

          
        
        
        

          

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