Java Map集合逻辑方法

1. Java Map集合逻辑方法

分组、List转Map

import com.alibaba.fastjson.JSON;
import com.example.demo.pojo.StudentPlus;
import lombok.Data;
 
import java.math.BigDecimal;
import java.util.*;
import java.util.stream.Collectors;
 
public class Test100 {
    public static void main(String[] args) {
 
        // 1.使用Lambda给List中相同的一个字段进行分组, 结果赋值Map集合中
        // 案例：根据key判断优惠卷是否可用返回客户端 ----》可使用、map.get("!可使用").key;
        StudentPlus studentPlus01 = new StudentPlus(1, "张三", new Date());
        StudentPlus studentPlus00 = new StudentPlus(1, "靳露", new Date());
        StudentPlus studentPlus02 = new StudentPlus(2, "李四", new Date());
        List<StudentPlus> arrayList = new ArrayList<>();
        Collections.addAll(arrayList, studentPlus01, studentPlus00, studentPlus02);
        // 逻辑开始
        Map<Integer, List<StudentPlus>> collect = arrayList.stream().collect(Collectors.groupingBy(StudentPlus::getId)); //根据Id进行分组
        collect.forEach((key, value) -> System.out.println("key: " + key + "; value: " + JSON.toJSON(value)));
        /*TODO: key: 1; value: [{"date":1627614007999,"name":"张三","id":1},{"date":1627614007999,"name":"靳露","id":1}]
                key: 2; value: [{"date":1627614007999,"name":"李四","id":2}]*/
 
        // 2.把StudentPlus实体类的值转换为Map集合, 然后通过map.key获取员工姓名
        Map<Integer, String> map = arrayList.stream().collect(Collectors.toMap(StudentPlus::getId, StudentPlus::getName));
        Arrays.stream("1,2".split(",")).forEach(item -> System.out.println("员工名称: " + map.get(Integer.valueOf(item))));
        /*TODO: {1=靳露, 2=李四} 员工名称: 靳露 员工名称: 李四*/
    }
}
 
@Data
class Producer {
    private String name;
    private BigDecimal mony;
 
    public Producer(String name, BigDecimal mony) {
        this.name = name;
        this.mony = mony;
    }
}