Java 8 Map filter examples

This article shows a few examples of filtering a Map by its keys or values using the Java 8 stream APIs.

Table of contents:

• 1. Java Map filter (Classic)
• 2. Java Map filter (Java 8)
• 3. Java Map filter (Java 8 Predicate)
• 4. References

1. Java Map filter (Classic)

Before Java 8, we looped the Map, filtered it, and collected it manually.

package com.mkyong.examples;

import java.util.HashMap;
import java.util.Map;

public class MapFilterExample {

    public static void main(String[] args) {

        Map<Integer, String> map = new HashMap<>();
        map.put(1, "linode");
        map.put(2, "heroku");
        map.put(3, "aws");

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

        // linode
        System.out.println(result);

    }

}

output

linode

2. Java Map filter (Java 8)

With Java 8, we can convert a Map.entrySet() into a stream, followed by a filter() and collect().

package com.mkyong.examples;

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

public class MapFilterExample2 {

    public static void main(String[] args) {

        Map<Integer, String> map = new HashMap<>();
        map.put(1, "linode");
        map.put(2, "heroku");
        map.put(3, "aws");

        //Map -> Stream -> Filter -> String
        String result1 = map.values().stream()
                .filter("linode"::equals)
                .collect(Collectors.joining());

        // linode
        System.out.println(result1);

        //Map -> Stream -> Filter -> MAP
        Map<Integer, String> result2 = map.entrySet().stream()
                .filter(x -> x.getKey() == 1)
                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

        // k:1 v:linode
        result2.forEach((k, v) -> System.out.println("k:" + k + " v:" + v));

        // or like this
        Map<Integer, String> result3 = map.entrySet().stream()
                .filter(x -> x.getKey() == 3)
                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

        //k:3 v:aws
        result3.forEach((k, v) -> System.out.println("k:" + k + " v:" + v));

    }

}

output

linode
k:1 v:linode
k:3 v:aws

3. Java Map filter (Java 8 Predicate)

Java 8 map filtering using Predicate.

package com.mkyong.examples;

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

public class MapFilterExample3 {

    // 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> map = new HashMap<>();
        map.put(1, "linode");
        map.put(2, "heroku");
        map.put(3, "aws");

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

        // {1=linode, 3=aws}
        Map<Integer, String> filteredMap2 = filterByValue(map, x -> (x.contains("aws") || x.contains("linode")));
        System.out.println(filteredMap2);

        // {3=aws}
        Map<Integer, String> filteredMap3 = filterByValue(map, x -> (x.length() <= 5));
        System.out.println(filteredMap3);

    }

}

output

{1=linode}
{1=linode, 3=aws}
{3=aws}

4. References

• java.util.Map JavaDoc
• Java 8 stream filter examples
• Java 8 forEach examples
• Java 8 Streams map() examples
• Java – Get keys from value HashMap