合并两个Map
map自己的方法
实现方式是通过 putAll() 方法将多个 map 对象中的数据放到另外一个全新的 map 对象中，代码如下所示，展示了两个 map 对象的合并，如果是多个 map 合并也是用这种方式。
public static void main(String[] args) {Map<String, String> map1 = new HashMap<String, String>();map1.put("one", "一");map1.put("two", "二");map1.put("three", "三");Map<String, String> map2 = new HashMap<String, String>();map1.put("ten", "十");map1.put("nine", "九");map1.put("eight", "八");// 合并Map<String, String> combineResultMap = new HashMap<String, String>();combineResultMap.putAll(map1);combineResultMap.putAll(map2);// 合并后打印出所有内容for (Map.Entry<String, String> entry : combineResultMap.entrySet()) {System.out.println(entry.getKey() + ":" + entry.getValue());}}java8合并两个Map
需求：如果两个Map有冲突则Value取第二个Map中的Value
例子准备
实体类
import lombok.*;
@Getter
@Setter
@ToString
@AllArgsConstructor
@NoArgsConstructor
public class Employee {private Long id;private String name;
}
新建两个Map<String,Employee>
private static Map<String, Employee> map1 = new HashMap<>();private static Map<String, Employee> map2 = new HashMap<>();static {Employee employee1 = new Employee(1L, "Henry");map1.put(employee1.getName(), employee1);Employee employee2 = new Employee(22L, "Annie");map1.put(employee2.getName(), employee2);Employee employee3 = new Employee(8L, "John");map1.put(employee3.getName(), employee3);Employee employee4 = new Employee(2L, "George");map2.put(employee4.getName(), employee4);Employee employee5 = new Employee(3L, "Henry");map2.put(employee5.getName(), employee5);}
在java8之前我们合并
@Testpublic void fun0() {Map<String, Employee> map3 = new HashMap<>(map1);for (Map.Entry<String, Employee> entry : map2.entrySet()) {if (map3.containsKey(entry.getKey())) {map3.put(entry.getKey(), new Employee(map3.get(entry.getKey()).getId(), entry.getValue().getName()));}else{map3.put(entry.getKey(), entry.getValue());}}}
java8之后
1.用map提供的merge()方法来合并
@Testpublic void fun1() {Map<String, Employee> map3 = new HashMap<>(map1);map2.forEach((key, value) -> map3.merge(key, value, (v1, v2) -> new Employee(v1.getId(),v2.getName())));map3.forEach((k,v)-> System.out.println(k+"=:"+v));}
merge方法有三个参数，第一个是要合并的Key，第二个是合并的Value，第三个是一个接收两个参数的函数，它的作用就如我们上面的对于重复的key处理的逻辑一样，你可以自定义，v1是map2中的值，v2是map3中重复的值。
2.用Stream.concat来合并
Stream.contcat接收两个值，顺序的将两个集合中的数据加入流中，然后再用收集器变成Map
@Testpublic void fun2() {Stream<Map.Entry<String, Employee>> concat = Stream.concat(map1.entrySet().stream(), map2.entrySet().stream());Map<String, Employee> collect = concat.collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (value1, value2) -> new Employee(value1.getId(), value1.getName())));collect.forEach((k,v)-> System.out.println(k+"=:"+v));}
3.用Stream.of()创建流后合并
和contact不同的是stream.of可以初始化多个元素,然后用扁平化的处理成需要的流，然后用收集器来转为Map
@Testpublic void fun3() {Map<String, Employee> collect = Stream.of(map1, map2).flatMap(x -> x.entrySet().stream()).collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (v1, v2) -> new Employee(v1.getId(), v2.getName())));collect.forEach((k,v)-> System.out.println(k+"=:"+v));}
4.直接用Collector来收集
@Testpublic void fun4() {Map<String, Employee> map3 = map2.entrySet().stream().collect(Collectors.toMap(Map.Entry::getKey,Map.Entry::getValue,(v1, v2) -> new Employee(v1.getId(), v2.getName()),() -> new HashMap<>(map1)));map3.forEach((k,v)-> System.out.println(k+"=:"+v));}
方法实例)
