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<Person> 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<String,Person> 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<Person> 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<String,String> 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<Person> 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<String, List<Person>> 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<Person> 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<String,List<Person>> map =  list.stream().collect(Collectors.toMap(Person::getName,each->Collections.singletonList(each),(value1, value2) -> {            List<Person> 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<Person> 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<String,List<String>> map = list.stream().collect(Collectors.toMap(Person::getName,each->Collections.singletonList(each.getAddress()),(value1, value2) -> {            List<String> 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<Map<String,Object>> 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<String,Object> map1 = new HashMap<>();        map1.put("id","1");        map1.put("person",person1);        Map<String,Object> map2 = new HashMap<>();        map2.put("id","2");        map2.put("person",person2);        Map<String,Object> map3 = new HashMap<>();        map3.put("id","3");        map3.put("person",person3);        list.add(map1);        list.add(map2);        list.add(map3);        Map<String,Map<String,Object>> 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<Map<String,Object>> 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<String,Object> map1 = new HashMap<>();        map1.put("id","1");        map1.put("person",person1);        Map<String,Object> map2 = new HashMap<>();        map2.put("id","2");        map2.put("person",person2);        Map<String,Object> map3 = new HashMap<>();        map3.put("id","3");        map3.put("person",person3);        list.add(map1);        list.add(map2);        list.add(map3);        Map<String,Object> 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<Map<String,String>> list = new ArrayList<>();        Map<String,String> map1 = new HashMap<>();        map1.put("id","1");        map1.put("name","熊大");        map1.put("address","森林第一个小屋");        Map<String,String> map2 = new HashMap<>();        map2.put("id","2");        map2.put("name","熊二");        map2.put("address","森林第二个小屋");        Map<String,String> 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<String,Map<String,String>> 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<Map<String,String>> list = new ArrayList<>();        Map<String,String> map1 = new HashMap<>();        map1.put("id","1");        map1.put("name","熊大");        map1.put("address","森林第一个小屋");        Map<String,String> map2 = new HashMap<>();        map2.put("id","2");        map2.put("name","熊二");        map2.put("address","森林第二个小屋");        Map<String,String> 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<String,String> 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”}

来源地址：https://blog.csdn.net/mianxiao15908/article/details/128673800

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