首先创建两个实体类文章来源地址https://www.toymoban.com/news/detail-514444.html
@Data
public class Student {
//学号
private String no;
//姓名
private String name;
}
@Data
public class Teacher{
//教师号
private String no;
//姓名
private String name;
}
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 【key为1个属性,value为1个属性】
Map<String, String> map =
list.stream().collect(Collectors.toMap(
Student::getNo,
Student::getName,
(key1 , key2) -> key1 //(map的键重复不会报错,下面已经处理)
));
//将list转map 【key为多个属性,value为1个属性】
Map<String, String> map =
list.stream().collect(Collectors.toMap(
obj -> obj.getNo() + "_" + obj.getName(),
Student::getName,
(key1 , key2) -> key1 //(map的键重复不会报错,下面已经处理)
));
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 【key为1个属性,value为对象本身】 (map的键去重)
Map<String, Student> map =
list.stream().collect(Collectors.toMap(
Student::getNo,
obj -> obj,
(key1 , key2) -> key1
));
或者
//将list转map 【key为1个属性,value为对象本身】 (map的键去重)
Map<String, Student> map =
list.stream().collect(Collectors.toMap(
Student::getNo,
Function.identity(),
(key1 , key2) -> key1
));
或者
//将list转map 【key为多个属性,value为对象本身】 (map的键去重)
Map<String, Student> map =
list.stream().collect(Collectors.toMap(
obj -> obj.getNo() + "_" + obj.getName(),
obj -> obj,
(key1 , key2) -> key1
));
三、List<Object1> 转 Map<String, Object2> (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 【key为1个属性,value为另一个对象】 (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) -> key1
));
四、List<Object> 转 Map<String, List<Object>> (分组)【以1个字段分/以多个字段分】
//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 【key为1个属性,value为相同key的集合】 (以某1个属性来分组,将分组后相同的对象放在一起)
Map<String, List<Student>> map =
list.stream().collect( Collectors.groupingBy(Student::getNo) );
//将list转map 【key为多个属性,value为相同key的集合】 (以多个属性来分组,将分组后相同的对象放在一起)
Map<String, List<Student>> map =
list.stream().collect( Collectors.groupingBy(obj -> obj.getNo + "_" + obj.getName) );
文章来源:https://www.toymoban.com/news/detail-514444.html
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