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
     
       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
      
        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
       
         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：