An example to show you how to filter a Map with Java 8 stream API.
Before Java 8 :
String result = "";
for (Map.Entry<Integer, String> entry : A_MAP_EXAMPLE.entrySet()) {
if("something".equals(entry.getValue())){
result = entry.getValue();
}
}
With Java 8, you can convert a Map.entrySet() to a stream, follow by a filter() and collect() it.
//Map -> Stream -> Filter -> String
String result = A_MAP_EXAMPLE.entrySet().stream()
.filter(map -> "something".equals(map.getValue()))
.map(map->map.getValue())
.collect(Collectors.joining());
//Map -> Stream -> Filter -> MAP
Map<Integer, String> collect = A_MAP_EXAMPLE.entrySet().stream()
.filter(map -> map.getKey() == 2)
.collect(Collectors.toMap(p -> p.getKey(), p -> p.getValue()));
1. Java 8 – Filter a Map
A full example to filter a Map and return a String.
TestMapFilter.java
package com.mkyong.java8
import java.util.HashMap;
import java.util.Map;
import java.util.stream.Collectors;
public class TestMapFilter {
public static void main(String[] args) {
Map<Integer, String> HOSTING = new HashMap<>();
HOSTING.put(1, "linode.com");
HOSTING.put(2, "heroku.com");
HOSTING.put(3, "digitalocean.com");
HOSTING.put(4, "aws.amazon.com");
String result = "";
for (Map.Entry<Integer, String> entry : HOSTING.entrySet()) {
if ("aws.amazon.com".equals(entry.getValue())) {
result = entry.getValue();
}
}
System.out.println("Before Java 8 : " + result);
//Map -> Stream -> Filter -> String
result = HOSTING.entrySet().stream()
.filter(map -> "aws.amazon.com".equals(map.getValue()))
.map(map -> map.getValue())
.collect(Collectors.joining());
System.out.println("With Java 8 : " + result);
}
}
Output
Before Java 8 : aws.amazon.com
With Java 8 : aws.amazon.com
2. Java 8 – Filter a Map #2
Yet another example to filter a Map, but this will return a Map.
TestMapFilter.java
package com.mkyong.java8
import java.util.HashMap;
import java.util.Map;
import java.util.stream.Collectors;
public class TestMap {
public static void main(String[] args) {
Map<Integer, String> HOSTING = new HashMap<>();
HOSTING.put(1, "linode.com");
HOSTING.put(2, "heroku.com");
HOSTING.put(3, "digitalocean.com");
HOSTING.put(4, "aws.amazon.com");
//Map -> Stream -> Filter -> Map
Map<Integer, String> collect = HOSTING.entrySet().stream()
.filter(map -> map.getKey() == 2)
.collect(Collectors.toMap(p -> p.getKey(), p -> p.getValue()));
System.out.println(collect); //output : {2=heroku.com}
}
}
References
- Processing Data with Java SE 8 Streams
- Java Collectors JavaDoc
- Java 8 Streams filter examples