List使用stream流转成map的几种方式

Posted 2023-04-02 小小小米fly

List使用stream流转成map的几种方式

• 实体例子
• List<Object> 转成Map<String,Object>
• List<Object> 转成Map<String,String>
• List<Object> 转成Map<String,List<Object>>
• • 方法一:
  • 方法二:
• List<Object> 转成Map<String,List<String>>
• List<Map<String,Object>> 转成Map<String,Map<String,Object>>
• List<Map<String,Object>> 转成Map<String,Object>
• List<Map<String,String>> 转成Map<String,Map<String,String>>
• List<Map<String,String>> 转成Map<String,String>

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<String,Object>

	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<String,String>

	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<String,List>

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<String,List>

	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<Map<String,Object>> 转成Map<String,Map<String,Object>>

		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<Map<String,Object>> 转成Map<String,Object>

		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<Map<String,String>> 转成Map<String,Map<String,String>>

		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<Map<String,String>> 转成Map<String,String>

		List<Map<String,String>> list = new ArrayList<>();
        Map<String,String> map1 = new HashMap<>(
Java之List和Map的几种遍历方式


/**
 * list和map遍历
 */

import java.util.ArrayList;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;

public class list_map遍历{
 public static void main(String[] args) {
     List<String> list=new ArrayList<String>();
     list.add("Hello");
     list.add("World");
     list.add("HAHAHAHA");
     
     //第一种遍历方法使用foreach遍历List
     for (String str : list) {            //也可以改写for(int i=0;i<list.size();i++)这种形式
        System.out.println(str);
     }
 
     //第二种遍历，把链表变为数组相关的内容进行遍历
     String[] strArray=new String[list.size()];
     list.toArray(strArray);
     for(int i=0;i<strArray.length;i++) //这里也可以改写为  foreach(String str:strArray)这种形式
     {
        System.out.println(strArray[i]);
     }
     
    //第三种遍历 使用迭代器进行相关遍历
     Iterator<String> ite=list.iterator();
     while(ite.hasNext())//判断下一个元素之后有值
     {
         System.out.println(ite.next());
     }
     
     
     
     Map<String, String> map = new HashMap<String, String>();
     map.put("1", "value1");
     map.put("2", "value2");
     map.put("3", "value3");
     
     //第一种：普遍使用，二次取值
     System.out.println("通过Map.keySet遍历key和value：");
     for (String key : map.keySet()) {
      System.out.println("key= "+ key + " and value= " + map.get(key));
     }
     
     //第二种
     System.out.println("通过Map.entrySet使用iterator遍历key和value：");
     Iterator<Map.Entry<String, String>> it = map.entrySet().iterator();
     while (it.hasNext()) {
      Map.Entry<String, String> entry = it.next();
      System.out.println("key= " + entry.getKey() + " and value= " + entry.getValue());
     }
     
     //第三种：推荐，尤其是容量大时
     System.out.println("通过Map.entrySet遍历key和value");
     for (Map.Entry<String, String> entry : map.entrySet()) {
      System.out.println("key= " + entry.getKey() + " and value= " + entry.getValue());
     }
   
     //第四种
     System.out.println("通过Map.values()遍历所有的value，但不能遍历key");
     for (String v : map.values()) {
      System.out.println("value= " + v);
     }
    }

}


　　
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