Java HashMap.entrySet() – Examples

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

entrySet()

HashMap.entrySet() returns a Set view of the mappings contained in this map.

The HashMap object should not be null, else, the function throws java.lang.NullPointerException.

Syntax

The syntax of entrySet() function is

entrySet()

Returns

The function returns Set<Map.Entry<K,V>> value.

Example 1 – entrySet()

In this example, we will initialize a HashMap hashMap with mappings from String to Integer. To get Set view of this hashMap, we will call entrySet() method on this HashMap.

Java Program

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

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

Output

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

Example 2 – entrySet() – Null HashMap

In this example, we will initialize a HashMap hashMap with null. Calling HashMap.entrySet() method on null object should throw java.lang.NullPointerException.

Java Program

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

public class Example{ 
	public static void main(String[] args) {
		HashMap<String,Integer> hashMap = null;
		
		Set<Entry<String,Integer>> set = hashMap.entrySet();
		System.out.println("Set view :" + set);
	}
}

Output

Exception in thread "main" java.lang.NullPointerException
	at Example.main(Example.java:9)

Conclusion

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