Java - How to Get All Keys and Values from Map

Quick recap: In Java, a Map object is used for mapping keys to values. Duplicate keys are not allowed and each key can have at most one value in a map.

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+)

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 pair 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 - 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 - Values only

Map.keySet() method returns a Collection view of the values contained in the map.

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

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.