Java 8 – Filter a Map examples

Posted 2021-01-22 Blogger

Java 8 – Filter a Map examples
Few Java examples to show you how to filter a Map with Java 8 stream API.

Before Java 8 :

Map<Integer, String> map = new HashMap<>();
map.put(1, "linode.com");
map.put(2, "heroku.com");

String result = "";
for (Map.Entry<Integer, String> entry : map.entrySet()) {
if("something".equals(entry.getValue())){
result = entry.getValue();
}
}

With Java 8, you can convert a Map.entrySet() into a stream, follow by a filter() and collect() it.

Map<Integer, String> map = new HashMap<>();
map.put(1, "linode.com");
map.put(2, "heroku.com");

//Map -> Stream -> Filter -> String
String result = map.entrySet().stream()
.filter(x -> "something".equals(x.getValue()))
.map(x->x.getValue())
.collect(Collectors.joining());

//Map -> Stream -> Filter -> MAP
Map<Integer, String> collect = map.entrySet().stream()
.filter(x -> x.getKey() == 2)
.collect(Collectors.toMap(x -> x.getKey(), x -> x.getValue()));

// or like this
Map<Integer, String> collect = map.entrySet().stream()
.filter(x -> x.getKey() == 3)
.collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

1. Java 8 – Filter a Map
A full example to filter a Map by values and return a String.

TestMapFilter.java
package com.mkyong;

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");

// Before Java 8
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);

// filter more values
result = HOSTING.entrySet().stream()
.filter(x -> {
if (!x.getValue().contains("amazon") && !x.getValue().contains("digital")) {
return true;
}
return false;
})
.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
With Java 8 : linode.com,heroku.com

2. Java 8 – Filter a Map #2
Yet another example to filter a Map by key, but this time will return a Map

TestMapFilter2.java
package com.mkyong;

import java.util.HashMap;
import java.util.Map;
import java.util.stream.Collectors;

public class TestMapFilter2 {

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}

Map<Integer, String> collect2 = HOSTING.entrySet().stream()
.filter(map -> map.getKey() <= 3)
.collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

System.out.println(collect2); //output : {1=linode.com, 2=heroku.com, 3=digitalocean.com}

}

}

Output

{2=heroku.com}
{1=linode.com, 2=heroku.com, 3=digitalocean.com}

3. Java 8 - Filter a Map #3 - Predicate
This time, try the new Java 8 Predicate

TestMapFilter3.java
package com.mkyong;

import java.util.HashMap;
import java.util.Map;
import java.util.function.Predicate;
import java.util.stream.Collectors;

public class TestMapFilter3 {

// Generic Map filterbyvalue, with predicate
public static <K, V> Map<K, V> filterByValue(Map<K, V> map, Predicate<V> predicate) {
return map.entrySet()
.stream()
.filter(x -> predicate.test(x.getValue()))
.collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
}

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");
HOSTING.put(5, "aws2.amazon.com");

// {1=linode.com}
Map<Integer, String> filteredMap = filterByValue(HOSTING, x -> x.contains("linode"));
System.out.println(filteredMap);

// {1=linode.com, 4=aws.amazon.com, 5=aws2.amazon.com}
Map<Integer, String> filteredMap2 = filterByValue(HOSTING, x -> (x.contains("aws") || x.contains("linode")));
System.out.println(filteredMap2);

// {4=aws.amazon.com}
Map<Integer, String> filteredMap3 = filterByValue(HOSTING, x -> (x.contains("aws") && !x.contains("aws2")));
System.out.println(filteredMap3);

// {1=linode.com, 2=heroku.com}
Map<Integer, String> filteredMap4 = filterByValue(HOSTING, x -> (x.length() <= 10));
System.out.println(filteredMap4);

}

}

Output

{1=linode.com}
{1=linode.com, 4=aws.amazon.com, 5=aws2.amazon.com}
{4=aws.amazon.com}
{1=linode.com, 2=heroku.com}
http://www.mkyong.com/java8/java-8-filter-a-map-examples/