java-合并两个Map

Java8中如何合并两个map

如何处理Map含有重复元素的情况

1. 初始化

我们定义两个map实例

private static Map<String, People> map1 = new HashMap<>();
private static Map<String, People> map2 = new HashMap<>();

People类

class People {private Long id;private String name;public People(Long id, String name) {this.id = id;this.name = name;}......}然后往map中存入一些数据static {People people1 = new People(1L, "Henry");map1.put(people1.getName(), people1);People people2 = new People(2L, "Annie");map1.put(people2.getName(), people2);People people3 = new People(3L, "John");map1.put(people3.getName(), people3);People people4 = new People(4L, "George");map2.put(people4.getName(), people4);People people5 = new People(5L, "Henry");map2.put(people5.getName(), people5);}

特别需要注意的是_people1_和_people5__在map中有完全相同的key（name）。_

2.Map.merge()

Java8为**java.util.Map接口新增了merge()函数。**

merge() 函数的作用是: 如果给定的key之前没设置value 或者value为null, 则将给定的value关联到这个key上.

否则，通过给定的remaping函数计算的结果来替换其value。如果remapping函数的计算结果为null，将解除此结果。

// 通过拷贝map1中的元素来构造一个新的HashMap
Map<String, People> map3 = new HashMap<>(map1);
// 引入merge函数和合并规则 map3.merge(key, value, (v1, v2) -> new People(v1.getId(), v2.getName())
// map2进行迭代将其元素合并到map3中
map2.forEach((key, value) -> map3.merge(key, value, (v1, v2) -> new People(v1.getId(), v2.getName())));
printMap("map3", map3);
/*** map3:* John:People{id=3, name='John'}* Annie:People{id=2, name='Annie'}* George:People{id=4, name='George'}* Henry:People{id=1, name='Henry'}*/

最终，通过结果可以看出，实现了两个map的合并，对重复的key也合并为同一个元素。

注意最后一个_People_的id来自map1而name来自map2.

原因是我们的merge函数的定义:

(v1, v2) -> new People(v1.getId(), v2.getName())

3.Stream.concat()

_Java8的Stream_API 也为解决该问题提供了较好的解决方案。

首先需要将两个map合为一个**Stream。**

Stream combined = Stream.concat(map1.entrySet().stream(), map2.entrySet().stream());

我们需要将entry sets作为参数，然后利用_Collectors.toMap()_:将结果放到新的map中。

 try {Map<String, People> map4 = Stream.concat(map1.entrySet().stream(), map2.entrySet().stream()).collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));printMap("map4", map4);} catch (IllegalStateException e) {System.out.println("Error: " + e);}

该方法可以实现map的合并，但是有重复key会报_IllegalStateException异常。_

为了解决这个问题，我们需要加入lambda表达式merger作为第三个参数

(value1, value2) -> new Peopel(value2.getId(), value1.getName())

当检测到有重复Key时就会用到该lambda表达式。

Map<String, People> map5 = Stream.concat(map1.entrySet().stream(), map2.entrySet().stream()).collect(Collectors.toMap(Map.Entry::getKey,Map.Entry::getValue,(value1, value2) -> new People(value2.getId(), value1.getName())));
printMap("map5", map5);
/*** map5:* George:People{id=4, name='George'}* John:People{id=3, name='John'}* Annie:People{id=2, name='Annie'}* Henry:People{id=5, name='Henry'}*/

从结果可以看出重复的key_“Henry”将合并为一个新的键值对，id取自map2，name取自map1。_

4. Stream.of()

通过Stream.of()方法不需要借助其他stream就可以实现map的合并。

Map<String, People> map6 = Stream.of(map1, map2).flatMap(map -> map.entrySet().stream()).collect(Collectors.toMap(Map.Entry::getKey,Map.Entry::getValue,(v1, v2) -> new People(v1.getId(), v2.getName())));
printMap("map6", map6);
/*** map6:* George:People{id=4, name='George'}* John:People{id=3, name='John'}* Annie:People{id=2, name='Annie'}* Henry:People{id=1, name='Henry'}*/

首先将map1和map2的元素合并为同一个流，然后再转成map。通过使用v1的id和v2的name来解决重复key的问题。

5. Simple Streaming

我们还可以借助stream的管道操作来实现map合并。

Map<String, People> map7 = map2.entrySet().stream().collect(Collectors.toMap(Map.Entry::getKey,Map.Entry::getValue,(v1, v2) -> new People(v1.getId(), v2.getName()),() -> new HashMap<>(map1)));
printMap("map7", map7);
/*** map7:* John:People{id=3, name='John'}* Annie:People{id=2, name='Annie'}* George:People{id=4, name='George'}* Henry:People{id=1, name='Henry'}*/

参考：

https://www.baeldung.com/java-merge-maps

所有代码：

package learn;import java.util.HashMap;
import java.util.Map;
import java.util.stream.Collectors;
import java.util.stream.Stream;class People {private Long id;private String name;public People(Long id, String name) {this.id = id;this.name = name;}public Long getId() {return id;}public void setId(Long id) {this.id = id;}public String getName() {return name;}public void setName(String name) {this.name = name;}@Overridepublic String toString() {return "People{" +"id=" + id +", name='" + name + ''' +'}';}
}public class TestMergeMap {private static Map<String, People> map1 = new HashMap<>();private static Map<String, People> map2 = new HashMap<>();static {People people1 = new People(1L, "Henry");map1.put(people1.getName(), people1);People people2 = new People(2L, "Annie");map1.put(people2.getName(), people2);People people3 = new People(3L, "John");map1.put(people3.getName(), people3);People people4 = new People(4L, "George");map2.put(people4.getName(), people4);People people5 = new People(5L, "Henry");map2.put(people5.getName(), people5);}public static void main(String[] args) {// 通过拷贝map1中的元素来构造一个新的HashMapMap<String, People> map3 = new HashMap<>(map1);// 引入merge函数和合并规则 map3.merge(key, value, (v1, v2) -> new People(v1.getId(), v2.getName())// map2进行迭代将其元素合并到map3中map2.forEach((key, value) -> map3.merge(key, value, (v1, v2) -> new People(v1.getId(), v2.getName())));printMap("map3", map3);/*** map3:* John:People{id=3, name='John'}* Annie:People{id=2, name='Annie'}* George:People{id=4, name='George'}* Henry:People{id=1, name='Henry'}*/try {Map<String, People> map4 = Stream.concat(map1.entrySet().stream(), map2.entrySet().stream()).collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));printMap("map4", map4);} catch (IllegalStateException e) {System.out.println("Error: " + e);}Map<String, People> map5 = Stream.concat(map1.entrySet().stream(), map2.entrySet().stream()).collect(Collectors.toMap(Map.Entry::getKey,Map.Entry::getValue,(value1, value2) -> new People(value2.getId(), value1.getName())));printMap("map5", map5);/*** map5:* George:People{id=4, name='George'}* John:People{id=3, name='John'}* Annie:People{id=2, name='Annie'}* Henry:People{id=5, name='Henry'}*/Map<String, People> map6 = Stream.of(map1, map2).flatMap(map -> map.entrySet().stream()).collect(Collectors.toMap(Map.Entry::getKey,Map.Entry::getValue,(v1, v2) -> new People(v1.getId(), v2.getName())));printMap("map6", map6);/*** map6:* George:People{id=4, name='George'}* John:People{id=3, name='John'}* Annie:People{id=2, name='Annie'}* Henry:People{id=1, name='Henry'}*/Map<String, People> map7 = map2.entrySet().stream().collect(Collectors.toMap(Map.Entry::getKey,Map.Entry::getValue,(v1, v2) -> new People(v1.getId(), v2.getName()),() -> new HashMap<>(map1)));printMap("map7", map7);/*** map7:* John:People{id=3, name='John'}* Annie:People{id=2, name='Annie'}* George:People{id=4, name='George'}* Henry:People{id=1, name='Henry'}*/}private static void printMap(String topic, Map<String, People> map) {System.out.println(topic + ":");map.forEach((key, value) -> {System.out.println(key + ":" + value);});}
}