Stream是Java8中处理集合的关键抽象概念,它可以指定你希望对集合进行的操作,可以执行非常复杂的查找、过滤和映射数据等操作,下面这篇文章主要给大家介绍了关于Java中List使用stream流转成map的几种方式,需要的朋友可以参考下
Java技术迷

实体例子

public class Person {
    private String name;
    private String address;
    public Person(String name, String address) {
        this.name = name;
        this.address = address;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getAddress() {
        return address;
    }
    public void setAddress(String address) {
        this.address = address;
    }
}

List 转成Map

	List list = new ArrayList<>();
	Person person1 = new Person("熊大","森林第一个小屋");
	Person person2 = new Person("熊二","森林第二个小屋");
	Person person3 = new Person("光头强","森林第三个小屋");
	Person person4 = new Person("熊大","森林第四个小屋");
	list.add(person1);
	list.add(person2);
	list.add(person3);
	list.add(person4);
	Map map =  list.stream().collect(Collectors.toMap(Person::getName,each->each,(value1, value2) -> value1));
	System.out.println(JSON.toJSONString(map));

控制台打印日志:

{“光头强”:{“address”:“森林第三个小屋”,“name”:“光头强”},“熊大”:{“address”:“森林第一个小屋”,“name”:“熊大”},“熊二”:{“address”:“森林第二个小屋”,“name”:“熊二”}}

List 转成Map

	List list = new ArrayList<>();
	Person person1 = new Person("熊大","森林第一个小屋");
	Person person2 = new Person("熊二","森林第二个小屋");
	Person person3 = new Person("光头强","森林第三个小屋");
	Person person4 = new Person("熊大","森林第四个小屋");
	list.add(person1);
	list.add(person2);
	list.add(person3);
	list.add(person4);
	Map map =  list.stream().collect(Collectors.toMap(Person::getName,Person::getAddress,(value1, value2) -> value1));
	System.out.println(JSON.toJSONString(map));

控制台打印日志:

{“光头强”:“森林第三个小屋”,“熊大”:“森林第一个小屋”,“熊二”:“森林第二个小屋”}

List 转成Map

方法一:

	List list = new ArrayList<>();
	Person person1 = new Person("熊大","森林第一个小屋");
	Person person2 = new Person("熊二","森林第二个小屋");
	Person person3 = new Person("光头强","森林第三个小屋");
	Person person4 = new Person("熊大","森林第四个小屋");
	list.add(person1);
	list.add(person2);
	list.add(person3);
	list.add(person4);
	Map> map = list.stream().collect(Collectors.groupingBy(Person::getName));
	System.out.println(JSON.toJSONString(map));

控制台打印日志:

{“光头强”:[{“address”:“森林第三个小屋”,“name”:“光头强”}],“熊大”:[{“address”:“森林第一个小屋”,“name”:“熊大”},{“address”:“森林第四个小屋”,“name”:“熊大”}],“熊二”:[{“address”:“森林第二个小屋”,“name”:“熊二”}]}

方法二:

	List list = new ArrayList<>();
	Person person1 = new Person("熊大","森林第一个小屋");
	Person person2 = new Person("熊二","森林第二个小屋");
	Person person3 = new Person("光头强","森林第三个小屋");
	Person person4 = new Person("熊大","森林第四个小屋");
	list.add(person1);
	list.add(person2);
	list.add(person3);
	list.add(person4);
	Map> map =  list.stream().collect(Collectors.toMap(Person::getName,each->Collections.singletonList(each),(value1, value2) -> {
            List union = new ArrayList<>(value1);
            union.addAll(value2);
            return union;
        }));
	System.out.println(JSON.toJSONString(map));

控制台打印日志:

{“光头强”:[{“address”:“森林第三个小屋”,“name”:“光头强”}],“熊大”:[{“address”:“森林第一个小屋”,“name”:“熊大”},{“address”:“森林第四个小屋”,“name”:“熊大”}],“熊二”:[{“address”:“森林第二个小屋”,“name”:“熊二”}]}

List 转成Map

	List list = new ArrayList<>();
	Person person1 = new Person("熊大","森林第一个小屋");
	Person person2 = new Person("熊二","森林第二个小屋");
	Person person3 = new Person("光头强","森林第三个小屋");
	Person person4 = new Person("熊大","森林第四个小屋");
	list.add(person1);
	list.add(person2);
	list.add(person3);
	list.add(person4);
	Map> map = list.stream().collect(Collectors.toMap(Person::getName,each->Collections.singletonList(each.getAddress()),(value1, value2) -> {
            List union = new ArrayList<>(value1);
            union.addAll(value2);
            return union;
        }));
	System.out.println(JSON.toJSONString(map));

控制台打印日志:

{“光头强”:[“森林第三个小屋”],“熊大”:[“森林第一个小屋”,“森林第四个小屋”],“熊二”:[“森林第二个小屋”]}

List> 转成Map>

		List> list = new ArrayList<>();
        Person person1 = new Person("熊大","森林第一个小屋");
        Person person2 = new Person("熊二","森林第二个小屋");
        Person person3 = new Person("光头强","森林第三个小屋");
        Map map1 = new HashMap<>();
        map1.put("id","1");
        map1.put("person",person1);
        Map map2 = new HashMap<>();
        map2.put("id","2");
        map2.put("person",person2);
        Map map3 = new HashMap<>();
        map3.put("id","3");
        map3.put("person",person3);
        list.add(map1);
        list.add(map2);
        list.add(map3);
        Map> map = list.stream().collect(Collectors.toMap(each->Objects.toString(each.get("id"),""),each->each,(key1,key2)->key1));
        System.out.println(JSON.toJSONString(map));

控制台打印日志:

{“1”:{“person”:{“address”:“森林第一个小屋”,“name”:“熊大”},“id”:“1”},“2”:{“person”:{“address”:“森林第二个小屋”,“name”:“熊二”},“id”:“2”},“3”:{“person”:{“address”:“森林第三个小屋”,“name”:“光头强”},“id”:“3”}}

List> 转成Map

		List> list = new ArrayList<>();
        Person person1 = new Person("熊大","森林第一个小屋");
        Person person2 = new Person("熊二","森林第二个小屋");
        Person person3 = new Person("光头强","森林第三个小屋");
        Map map1 = new HashMap<>();
        map1.put("id","1");
        map1.put("person",person1);
        Map map2 = new HashMap<>();
        map2.put("id","2");
        map2.put("person",person2);
        Map map3 = new HashMap<>();
        map3.put("id","3");
        map3.put("person",person3);
        list.add(map1);
        list.add(map2);
        list.add(map3);
        Map map = list.stream().collect(Collectors.toMap(each->Objects.toString(each.get("id"),""),each->each.get("person"),(key1,key2)->key1));
        System.out.println(JSON.toJSONString(map));

控制台打印日志:

{“1”:{“address”:“森林第一个小屋”,“name”:“熊大”},“2”:{“address”:“森林第二个小屋”,“name”:“熊二”},“3”:{“address”:“森林第三个小屋”,“name”:“光头强”}}

List> 转成Map>

		List> list = new ArrayList<>();
        Map map1 = new HashMap<>();
        map1.put("id","1");
        map1.put("name","熊大");
        map1.put("address","森林第一个小屋");
        Map map2 = new HashMap<>();
        map2.put("id","2");
        map2.put("name","熊二");
        map2.put("address","森林第二个小屋");
        Map map3 = new HashMap<>();
        map3.put("id","3");
        map3.put("name","光头强");
        map3.put("address","森林第三个小屋");
        list.add(map1);
        list.add(map2);
        list.add(map3);
        Map> map = list.stream().collect(Collectors.toMap(each->each.get("id"),each->each,(key1,key2)->key1));
        System.out.println(JSON.toJSONString(map));

控制台打印日志:

{“1”:{“address”:“森林第一个小屋”,“name”:“熊大”,“id”:“1”},“2”:{“address”:“森林第二个小屋”,“name”:“熊二”,“id”:“2”},“3”:{“address”:“森林第三个小屋”,“name”:“光头强”,“id”:“3”}}

List> 转成Map

		List> list = new ArrayList<>();
        Map map1 = new HashMap<>();
        map1.put("id","1");
        map1.put("name","熊大");
        map1.put("address","森林第一个小屋");
        Map map2 = new HashMap<>();
        map2.put("id","2");
        map2.put("name","熊二");
        map2.put("address","森林第二个小屋");
        Map map3 = new HashMap<>();
        map3.put("id","3");
        map3.put("name","光头强");
        map3.put("address","森林第三个小屋");
        list.add(map1);
        list.add(map2);
        list.add(map3);
        Map map = list.stream().collect(Collectors.toMap(each->each.get("id"),each->each.get("name"),(key1,key2)->key1));
        System.out.println(JSON.toJSONString(map));

控制台打印日志:

{“1”:“熊大”,“2”:“熊二”,“3”:“光头强”}