The computeIfAbsent(Key, Function) method of HashMap class is used to compute the value for a given key using the given mapping function. Store the computed value for the key in Hashmap if the key is not already associated with a value (or is mapped to null) else null.
Syntax:
public V
computeIfAbsent(K key,
Function<? super K, ? extends V> remappingFunction)
Parameters: This method accepts two parameters:
- key : key for which we want to compute value using mapping.
- remappingFunction : function to do the operation on value.
Returns: This method returns the current (existing or computed) value associated with the specified key, or null if mapping returns null.Â
- If remapping function returns null for a key, then no mapping is recorded for that key.
- At time of computation if remapping function throws an exception, the exception is rethrown, and the no mapping is recorded.
- During computation, modification this map using this method is not allowed.
- This method will throw a ConcurrentModificationException if the remapping function modified this map during computation.
Examples of Java HashMap computeIfAbsent() method
Below programs illustrate the computeIfAbsent(Key, Function) method:Â
Example 1:Â
Java
// Java program to demonstrate// computeIfAbsent(Key, Function) method.import java.util.*;Â
// Driver Classpublic class GFG {    // Main method    public static void main(String[] args)    {        // create a HashMap and add some values        HashMap<String, Integer> map = new HashMap<>();        map.put("key1", 10000);        map.put("key2", 55000);        map.put("key3", 44300);        map.put("key4", 53200);Â
        // print map details        System.out.println("HashMap:\n " + map.toString());Â
        // provide value for new key which is absent        // using computeIfAbsent method        map.computeIfAbsent("key5", k -> 2000 + 33000);        map.computeIfAbsent("key6", k -> 2000 * 34);Â
        // print new mapping        System.out.println("New HashMap:\n " + map);    }} |
Output
HashMap:
{key1=10000, key2=55000, key3=44300, key4=53200}
New HashMap:
{key1=10000, key2=55000, key5=35000, key6=68000, key3=44300, key4=53200}
Example 2:Â
Java
// Java program to demonstrate// computeIfAbsent(Key, Function) method.import java.util.*;Â
// Driver Classpublic class GFG {    // Main method    public static void main(String[] args)    {        // create a HashMap and add some values        HashMap<Integer, String> map = new HashMap<>();        map.put(10, "Aman");        map.put(20, "Suraj");        map.put(30, "Harsh");Â
        // print map details        System.out.println("HashMap:\n" + map.toString());Â
        // provide value for new key which is absent        // using computeIfAbsent method        map.computeIfAbsent(40, k -> "Sanjeet");Â
        // this will not effect anything        // because key 10 is present        map.computeIfAbsent(10, k -> "Amarjit");Â
        // print new mapping        System.out.println("New HashMap:\n" + map);    }} |
Output
HashMap:
{20=Suraj, 10=Aman, 30=Harsh}
New HashMap:
{20=Suraj, 40=Sanjeet, 10=Aman, 30=Harsh}
