java8使用stream的collect進行list轉map注意事項

1.創建Person類

package com.xkzhangsan.normal.collectors;

public class Person {
    
    private Integer id;
    private String name;
    private Integer score;
    public Integer getId() {
        return id;
    }
    public void setId(Integer id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public Integer getScore() {
        return score;
    }
    public void setScore(Integer score) {
        this.score = score;
    }
    
    @Override
    public String toString() {
        return "Person [id=" + id + ", name=" + name + ", score=" + score + "]";
    }
    

}

2.創建測試類ListToMap

package com.xkzhangsan.normal.collectors;

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.TreeMap;
import java.util.stream.Collectors;

public class ListToMap {
    
    public static void main(String[] args) {
        //創建list
        List<Person> personList = new ArrayList<>();
        for (int i = 0; i < 10; i++) {
            Person p = new Person();
            p.setId(i);
            p.setName("p"+i);
            p.setScore(i*10);
            personList.add(p);
        }
        
        //添加和id=8相同對象score值不同
        Person p = new Person();
        p.setId(8);
        p.setName("p"+8);
        p.setScore(88);
        personList.add(p);
        
        System.out.println("list：========================");
        personList.stream().forEach(System.out::println);
        
        //轉換為HashMap
        Map<Integer, Person> map = personList.stream().collect(Collectors.toMap(Person::getId, d->d, (oldValue, newValue)->newValue));
        System.out.println("hashMap：========================");
        map.entrySet().stream().forEach(System.out::println);
        
        //轉換為TreeMap
        Map<Integer, Person> treeMap = personList.stream().collect(Collectors.toMap(Person::getId, d->d, (oldValue, newValue)->newValue, TreeMap::new));
        System.out.println("treeMap：========================");
        treeMap.entrySet().stream().forEach(System.out::println);
    }

}

3.測試結果

list：========================
Person [id=0, name=p0, score=0]
Person [id=1, name=p1, score=10]
Person [id=2, name=p2, score=20]
Person [id=3, name=p3, score=30]
Person [id=4, name=p4, score=40]
Person [id=5, name=p5, score=50]
Person [id=6, name=p6, score=60]
Person [id=7, name=p7, score=70]
Person [id=8, name=p8, score=80]
Person [id=9, name=p9, score=90]
Person [id=8, name=p8, score=88]
hashMap：========================
0=Person [id=0, name=p0, score=0]
1=Person [id=1, name=p1, score=10]
2=Person [id=2, name=p2, score=20]
3=Person [id=3, name=p3, score=30]
4=Person [id=4, name=p4, score=40]
5=Person [id=5, name=p5, score=50]
6=Person [id=6, name=p6, score=60]
7=Person [id=7, name=p7, score=70]
8=Person [id=8, name=p8, score=88]
9=Person [id=9, name=p9, score=90]
treeMap：========================
0=Person [id=0, name=p0, score=0]
1=Person [id=1, name=p1, score=10]
2=Person [id=2, name=p2, score=20]
3=Person [id=3, name=p3, score=30]
4=Person [id=4, name=p4, score=40]
5=Person [id=5, name=p5, score=50]
6=Person [id=6, name=p6, score=60]
7=Person [id=7, name=p7, score=70]
8=Person [id=8, name=p8, score=88]
9=Person [id=9, name=p9, score=90]

 

4.注意事項

（1）list轉map要注意重復對象，map轉換方法要選擇帶mergeFunction參數的方法，如果key值重復，做合並處理，不然會拋異常！可以做到去重效果。

比如上面故意添加和id=8相同對象score為88，值不同。在map轉換方法mergeFunction 為(oldValue, newValue)->newValue 使用新對象替換已有老對象，可以看到轉換后id8的對象score變為88。

 

（2）list轉map默認轉換為HashMap，可以選擇帶mapSupplier參數的方法，選擇要轉換為的map類型。

比如上面TreeMap::new，選擇轉換為TreeMap。

 github地址：https://github.com/xkzhangsan/java8-practice