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HashMap 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!
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。