Java - How to Get Keys and Values from Map

A map is an abstract data type that stores a collections of key => value pairs. Each key can appear only once and can be used to look up an associated value very quickly. In Java, we have the Map that’s implemented by several built-in classes that support the Map functionality. Its most popular implementation is the HashMap class.

Iterating over keys or values (or both) of a Map object is a pretty common use case and one that developers have to do every so often. Fortunately, the Map interface provides three collection views, which allow a map’s contents to be viewed:

as a set of keys
collection of values, or,
set of key-value mappings.

Let’s walkthrough some examples and see how to get keys, values or both from a map.

1. Using forEach (Java 8+) to get Keys and Values

Starting from Java 8, forEach is easiest and most convenient way to iterate over all keys and values in a map.

map.forEach((k,v) -> {
        System.out.println("key: " + k + ", value: " + v)
});

2. Using Map.entrySet() method

Map.entrySet() method returns a Set whose entries are key,value pairs of the mappings contained in the Map. This is equivalent to forEach method descried above.

// iterate over and get keys and values
for (Map.Entry<Integer, String> entry : map.entrySet()) {
    Integer k = entry.getKey();
    String v = entry.getValue();
    System.out.println("key: " + k + ", value: " + v);
}

3. Using map.keySet() method if you want to retrieve Keys only

If you only want to retrieve keys only, Map.keySet() method returns a Set of keys contained in the map. Use this if you want to obtain the keys only.

Set<Integer> keys = map.keySet();
for (Integer k : keys) {
    System.out.println("key: " + k);
}

4. Using map.values() method if you only want to retrieve Values only

If you only want to retrieve values, you can use the Map.values() method. It returns a Collection view of just the values contained in the map.

Collection<String> values = map.values();
for (String v : values) {
    System.out.println("value: " + v);
}

5. Check if Map contains a Key

If you just want to check whether or not a Map contains a specific key, you can use the containsKey() method which returns a boolean true of false indicating whether the key is present or not.

boolean hasJim = map.containsKey("jimHalpert");

Complete Example

package com.codeahoy.ex;

import java.util.Collection;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;

public class MapExample {
    public static void main(String[] args) {
        // new map
        Map<Integer, String> map = new HashMap<>();

        // add data
        map.put(1, "V1");
        map.put(5, "V2");
        map.put(10, "V3");

        System.out.println("#1. Using forEach");
        map.forEach( (k,v) -> {
            System.out.println("key: " + k + ", value: " + v);
        });

        System.out.println("#2. Using Map.entrySet() method");
        for (Map.Entry<Integer, String> entry : map.entrySet()) {
            Integer k = entry.getKey();
            String v = entry.getValue();
            System.out.println("key: " + k + ", value: " + v);
        }

        System.out.println("#3. Using map.keySet() method - Keys only");
        Set<Integer> keys = map.keySet();
        for (Integer k : keys) {
            System.out.println("key: " + k);
        }

        System.out.println("#4. Using map.values() method - Values only");
        Collection<String> values = map.values();
        for (String v : values) {
            System.out.println("value: " + v);
        }
    }
}

Output:

#1. Using forEach
key: 1, value: V1
key: 5, value: V2
key: 10, value: V3

#2. Using Map.entrySet() method
key: 1, value: V1
key: 5, value: V2
key: 10, value: V3

#3. Using map.keySet() method - Keys only
key: 1
key: 5
key: 10

#4. Using map.values() method - Values only
value: V1
value: V2
value: V3

That’s all. Hope you enjoyed this post.