Java HashMap remove entry examples

There are various ways of removing an item from a Map type in Java. We will use HashMap to show various ways of removing an item from a Map and same can be used for other Map implementations.

1) Remove an entry from HashMap by using Collection removeIf() method

You can remove an entry from HashMap by using Collection removeIf() method which was added in Java 8. It has following signature default boolean removeIf(Predicate filter) and removes all entries from the Collection which match the predicate. removeIf() method will be called on Map.Entry Set. It uses Iterator remove() internally and hence doesn't throw ConcurrentModificationException.

In the example below we first pass a predicte (e -> e.getKey().equals("Mark")) that checks for the matching key and another predicate (e -> e.getValue().equals(6)) that checks for the matching value for entry that we want to remove from the HashMap.

package java8;

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

public class RemoveHashMapEntry {

    public static void main(String[] args) {
        Map<String, Integer> map = new HashMap<>();
        map.put("John", 2);
        map.put("Mark", 1);
        map.put("Thomas", 4);
        map.put("James", 3);
        map.put("Edwin", 5);
        map.put("Vinod", 6);
        System.out.println(map);
        map.entrySet().removeIf(e -> e.getKey().equals("Mark"));
        map.entrySet().removeIf(e -> e.getValue().equals(6));
        System.out.println(map);
    }

}

Output :

{Thomas=4, James=3, Edwin=5, John=2, Mark=1, Vinod=6}
{Thomas=4, James=3, Edwin=5, John=2}

2) Remove an entry from HashMap by using HashMap remove() method

You can remove an entry from HashMap by using HashMap remove() method. It has following signature public remove(Object key) and removes the entry from HashMap for the key specified if the key is present. It returns the previous value associated with key passed.

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

// Removes entry from HashMap
public class MyClass {
    public static void main(String[] args) {
        Map<String, String> map = new HashMap<String, String>();
        map.put("key1", "value1");
        map.put("key2", "value2");
        map.put("key3", "value3");
        map.remove("key2"); // Remove entry from HashMap

        System.out.println("map " + map);
    }
}

Output :

map {key1=value1, key3=value3}

3) Remove an entry from HashMap by using Iterator remove() method

You can remove an entry from HashMap by using Iterator remove() method. It has following signature default void remove(). It removes the last element returned by the iterator from the specified collection and can be used only once for one Iterator.next() call.

You can get Map.Entry Iterator by calling iterator() method on Map entrySet() method. Also this is a preferred approach to remove entry from HashMap as with this approach you will not get ConcurrentModificationException.

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

// Removes entry from HashMap
public class MyClass {
    public static void main(String[] args) {
        Map<String, String> map = new HashMap<String, String>();
        map.put("key1", "value1");
        map.put("key2", "value2");
        map.put("key3", "value3");
        Iterator<Map.Entry<String, String>> iterator = map.entrySet().iterator();

        while (iterator.hasNext()){
            Map.Entry<String, String> entry = iterator.next();
            if (entry.getKey().equalsIgnoreCase("key2")) {
                iterator.remove(); // Remove entry from HashMap
            }
        }
        System.out.println("map " + map);
    } 
}

Output :

map {key1=value1, key3=value3}

If you remove an entry from HashMap while iterating using HashMap remove() method, you will get ConcurrentModfiicationException.

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

// Removes entry from HashMap
public class MyClass {
    public static void main(String[] args) {
        Map<String, String> map = new HashMap<String, String>();
        map.put("key1", "value1");
        map.put("key2", "value2");
        map.put("key3", "value3");
        Iterator<Map.Entry<String, String>> iterator = map.entrySet().iterator();

        while(iterator.hasNext()){
            Map.Entry<String, String> entry = iterator.next();
            if ("key2".equals(entry.getKey())) {            
                map.remove("key2"); // Remove entry from HashMap
            }
        }
        System.out.println("map " + map);
    } 
}

Output :

Exception in thread "main" java.util.ConcurrentModificationException
at java.util.HashMap$HashIterator.nextNode(Unknown Source)
at java.util.HashMap$EntryIterator.next(Unknown Source)
at java.util.HashMap$EntryIterator.next(Unknown Source)
at MyClass.main(MyClass.java:16)

4) Remove all the entries from HashMap by using HashMap clear()

You can remove all the entries from HashMap by using HashMap clear() method. It has following signature public void clear(). It removes all of the entries from the map.

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

// Removes all the entries from HashMap
public class MyClass {
    public static void main(String[] args) {
        Map<String, String> map = new HashMap<String, String>();
        map.put("key1", "value1");
        map.put("key2", "value2");
        map.put("key3", "value3");
        map.clear();

        System.out.println("map " + map);
    }
}

Output :

map {}