HashMap computeIfAbsent() method in java

Let’s learn HashMap computeIfAbsent() method in java.

HashMap computeIfAbsent(K key, Function<? super K,? extends V> mappingFunction) method in java

computeIfAbsent(K key, Function<? super K,? extends V> mappingFunction) method returns the current (existing or computed) value associated with the specified key, or null if the computed value is null.

If the specified key is not already associated with a value (or is mapped to null), attempts to compute its value using the given mapping function and enters it into this map unless null.

If the mapping function returns null, no mapping is recorded. If the mapping function itself throws an (unchecked) exception, the exception is rethrown, and no mapping is recorded.

This method will throw a ConcurrentModificationException if it is detected that the mapping function modifies this map during computation.

Syntax

public V computeIfAbsent(K key, Function<? super K,? extends V> mappingFunction)

Parameters

key key with which the specified value is to be associated
mappingFunction the mapping function to compute a value

Throws

ConcurrentModificationException – if it is detected that the mapping function modified this map

Example

import java.util.*;

class HashMapComputeIfAbsentDemo 
{
   public static void main(String[] args) 
   {
      HashMap<String, Integer> hm = new HashMap<>();
      hm.put("hello", 15000);
      hm.put("world", 24000);
      hm.put("core", 14300);
      hm.put("java", 61600);
      System.out.println("HashMap :  " + hm.toString());
      // value for new key which is absent using computeIfAbsent method
      hm.computeIfAbsent("abc", k -> 6000 + 12000);
      hm.computeIfAbsent("xyz", k -> 1000 * 14);
      // print new mapping
      System.out.println("New HashMap : " + hm);
   }
}

Output:

HashMap : {core=14300, world=24000, java=61600, hello=15000}
New HashMap : {core=14300, abc=18000, world=24000, java=61600, xyz=14000, hello=15000}

import java.util.*;

class HashMapComputeIfAbsentDemo 
{
   public static void main(String[] args) 
   {
      HashMap<Integer, String> hm = new HashMap<>();
      hm.put(15, "hello");
      hm.put(24, "world");
      hm.put(60, "java");
      System.out.println("HashMap : " + hm.toString());
      // value for new key which is absent using computeIfAbsent() method
      hm.computeIfAbsent(20, k -> "core");
      // no effect because key 15 is present
      hm.computeIfAbsent(15, k -> "abc");
      System.out.println("New HashMap : " + hm);
   }
}

Output:

HashMap : {24=world, 60=java, 15=hello}
New HashMap : {20=core, 24=world, 60=java, 15=hello}