Java：List转Map （用stream实现）

Java：List转Map （用stream实现）

//实体类
public class Student {
    private  String no;  //学号
    private  String name;	//姓名
    //构造方法忽略
    //set、get 方法忽略
}
public class Teacher{
    private  String no;  //教师号
    private  String name; //姓名
}
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一、List<Object> 转 Map<String, String>

//声明一个List集合
List<Student> list = new ArrayList();  
	 list.add(new Student("1001", "小A"));  
	 list.add(new Student("1001", "小B"));//学号重复（下面特殊处理）
	 list.add(new Student("1002", "小C"));
	 list.add(new Student("1003", "小D"));


//将list转map （map的键重复不会报错，下面已经处理）

Map<String, String> map =

list.stream().collect(Collectors.toMap(

Student::getNo,

Student::getName,

(key1 , key2) -> key2

));


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---

二、List<Object> 转 Map<String, Object> (返回对象本身)

//声明一个List集合
List<Student> list = new ArrayList();
	list.add(new Student("1001", "小A"));
	list.add(new Student("1001", "小B"));
	list.add(new Student("1002", "小C"));
	list.add(new Student("1003", "小D"));


//将list转map （map的键去重）

Map<String, Student> map =

list.stream().collect(Collectors.toMap(

Student::getNo,

stu -> stu,

(key1 , key2) -> key2

));




或者




//将list转map （map的键去重）

Map<String, Student> map =

list.stream().collect(Collectors.toMap(

Student::getNo,

Function.identity(),

(key1 , key2) -> key2

));


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---

三、List<Object2> 转 Map<String, Object1> (返回另一个对象)

//声明一个List集合
List<Student> list = new ArrayList();
	list.add(new Student("1001", "小A"));
	list.add(new Student("1001", "小B"));
	list.add(new Student("1002", "小C"));
	list.add(new Student("1003", "小D"));


//将list转map （map的键去重）

Map<String, Teacher> map =

list.stream().collect(Collectors.toMap(

Student::getNo,

stu -> {

Teacher teacher = new Teacher();

teacher.setNo(stu.getNo());

teacher.setName(stu.getName());

return teacher;

},

(key1 , key2) -> key2

));


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---

四、List<Object> 转 Map<String, List<Object>> (分组)

//声明一个List集合
List<Student> list = new ArrayList();
   list.add(new Student("1001", "小A"));
   list.add(new Student("1001", "小B"));
   list.add(new Student("1002", "小C"));
   list.add(new Student("1003", "小D"));


//将list转map （以某个属性来分组，将分组后相同的map放在一起）

Map<String, List<Student>> map =

list.stream().collect( Collectors.groupingBy(Student::getNo) );


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---

五、学习链接：

https://blog.csdn.net/qq_39629277/article/details/83012548
https://blog.csdn.net/lu930124

https://blog.csdn.net/SeniorShen/article/details/120061248