Java – Get Set view of the keys in HashMap using keySet()

Java HashMap.keySet()

In this tutorial, we will learn about the Java HashMap.keySet() function, and learn how to use this function to get the keys of this HashMap as a Set, with the help of examples.

keySet()

HashMap.keySet() Returns a Set view of the keys contained in this map.

The syntax of keySet() function is

keySet()

Returns

The function returns Set<K>.

Examples

1. keySet() basic example

In this example, we will initialize a HashMap hashMap with four mappings in it. To get the set of keys, call keySet() method on this HashMap.

Java Program

import java.util.HashMap;
import java.util.Set;

public class Example{ 
	public static void main(String[] args) {
		HashMap<Integer, String> hashMap = new HashMap<>();
		hashMap.put(1, "A");
		hashMap.put(2, "B");
		hashMap.put(3, "C");
		hashMap.put(4, "D");
		System.out.println("HashMap     : " + hashMap);
		
		Set<Integer> keys = hashMap.keySet();
		System.out.println("Set of Keys : " + keys);
	}
}

Output

HashMap     : {1=A, 2=B, 3=C, 4=D}
Set of Keys : [1, 2, 3, 4]

2. keySet() – When HashMap is null

In this example, we will initialize a HashMap hashMap with null value. keySet() method throws java.lang.NullPointerException when called on a null HashMap.

Java Program

import java.util.HashMap;
import java.util.Set;

public class Example{ 
	public static void main(String[] args) {
		HashMap<Integer, String> hashMap = null;
		
		Set<Integer> keys = hashMap.keySet();
		System.out.println("Set of Keys : " + keys);
	}
}

Output

Exception in thread "main" java.lang.NullPointerException
	at Example.main(Example.java:8)

Conclusion

In this Java Tutorial, we have learnt the syntax of Java HashMap.keySet() function, and also learnt how to use this function with the help of examples.