java8使用stream的collect进行list转map注意事项

1.创建Person类

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.创建测试类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) {
        //创建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相同对象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);

        //转换为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);

        //转换为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.测试结果

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.注意事项

（1）list转map要注意重复对象，map转换方法要选择带mergeFunction参数的方法，如果key值重复，做合并处理，不然会抛异常！可以做到去重效果。

比如上面故意添加和id=8相同对象score为88，值不同。在map转换方法mergeFunction 为(oldValue, newValue)->newValue 使用新对象替换已有老对象，可以看到转换后id8的对象score变为88。

（2）list转map默认转换为HashMap，可以选择带mapSupplier参数的方法，选择要转换为的map类型。

比如上面TreeMap::new，选择转换为TreeMap。

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