1. Introduction

While programming you may come across a requirement when you want to remove a key value pair while iterating over a HashMap. The requirement is to search a key in HashMap and if found, remove the key value pair.

The way you may think of is to iterate over the HashMap and compare each value, if found, remove the key value pair using the remove(Object key, Object value) method. The approach is right but the method to remove the key value pair is wrong. While iterating the HashMap, if you use remove() method of java.util.Map, it will throw ConcurrentModficationException.

We’ll discuss following ways to remove key value pair from a HashMap while iterating:

• Using remove() method of Iterator
• Using Java 8 lambda expression – removeIf() method

2. Using remove() method of iterator

• Get the HashMap and the key to be removed.
• Create an iterator over the HashMap using iterate() method.
• Iterate over the HashMap using the Iterator.hasNext() method.
• While iterating, compare the key at that iteration with the key you are searching.
• If a match is found, remove the entry using remove() method.

import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;

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

		// put values in HashMap
		map.put(1, "one");
		map.put(2, "two");
		map.put(3, "three");

		// Get the key to be removed
		int keyToBeRemoved = 2;

		// Print the initial HashMap
		System.out.println("Initial HashMap: " + map);

		// Get the iterator over the HashMap
		Iterator<Map.Entry<Integer, String>> iterator = map.entrySet().iterator();

		// Iterate over the HashMap
		while (iterator.hasNext()) {

			// Get the entry
			Map.Entry<Integer, String> entry = iterator.next();

			// compare the key with the specified key
			if (keyToBeRemoved == entry.getKey()) {

				// Remove this entry from HashMap
				iterator.remove();
			}
		}

		// Print the modified HashMap
		System.out.println("Modified HashMap: " + map);
	}
}

Output

Initial HashMap: {1=one, 2=two, 3=three}
Modified HashMap: {1=one, 3=three}

3. Using Java 8 lambda expressions – removeIf() method

• Get the HashMap and the key to be removed.
• Get the entry set of this map using HashMap.entrySet() method.
• Using lambda expression, remove the entry from the map if the key is equal to the specified key.

import java.util.HashMap;

public class RemoveIfExample {

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

		// put values in HashMap
		map.put(1, "one");
		map.put(2, "two");
		map.put(3, "three");

		// Print the initial HashMap
		System.out.println("Initial HashMap: " + map);

		// Get the key to be removed
		int keyToBeRemoved = 2;

		// Remove the specified entry from the Map
		map.entrySet().removeIf(entry -> (keyToBeRemoved == entry.getKey()));

		// Print the modified HashMap
		System.out.println("Modified HashMap: " + map);

	}

}

Output

Initial HashMap: {1=one, 2=two, 3=three}
Modified HashMap: {1=one, 3=three}