Java 8 Filter a Map examples

摘要: Few Java examples to show you how to filter a Map with Java 8 stream API.

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}

References

  1. Processing Data with Java SE 8 Streams
  2. Java Collectors JavaDoc
  3. Java 8 Streams filter examples

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