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ConcurrentHashMap computeIfAbsent() method in Java with Examples

Last Updated : 25 Jun, 2021
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The computeIfAbsent(Key, Function) method of ConcurrentHashMap class which attempts to compute its value using the given mapping function for specified key if key is not already associated with a value (or is mapped to null) and enter that computed value in map else null.
 

  • If mapping function of this method returns null, then no value is recorded for new key in map.
  • This method is used to atomically update a value for given key in ConcurrentHashMap.
  • If the 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 because other threads can also uses this map.


Syntax: 
 

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


Parameters: This method accepts two parameters: 
 

  • key: key with which the value is to be associated.
  • remappingFunction: function to do the operation on value.


Returns: This method returns current (existing or computed) value associated with the specified key, or null if mapping returns null.
Exception: This method throws:


  • NullPointerException: if the specified key or remappingFunction is null.

  • llegalStateException: if the computation try to do a recursive update to this map that would otherwise never complete.

  • RuntimeException: if the remappingFunction does so, in which case the mapping is unchanged.


Below programs illustrate the computeIfAbsent(Key, Function) method:
Program 1: 
 

Java 
// Java program to demonstrate // computeIfAbsent(Key, Function) method. import java.util.concurrent.*; public class GFG {  // Main method  public static void main(String[] args)  {  // create a ConcurrentHashMap and  // add some values  ConcurrentHashMap<String, Integer> map  = new ConcurrentHashMap<>();  map.put("Pencil", 1000);  map.put("Laptop", 55000);  map.put("Clothes", 4400);  map.put("Mobile", 5300);  // print map details  System.out.println("ConcurrentHashMap :\n "  + map.toString());  // provide value for new key which is absent  // using computeIfAbsent method  map.computeIfAbsent("PC", k -> 60000);  map.computeIfAbsent("Charger", k -> 800);  // print new mapping  System.out.println("new ConcurrentHashMap :\n "  + map);  } }

Output: 
ConcurrentHashMap : {Laptop=55000, Clothes=4400, Pencil=1000, Mobile=5300} new ConcurrentHashMap : {Laptop=55000, PC=60000, Clothes=4400, Pencil=1000, Charger=800, Mobile=5300}

 

Program 2: 
 

Java 
// Java program to demonstrate // computeIfAbsent(Key, Function) method. import java.util.concurrent.*; public class GFG {  // Main method  public static void main(String[] args)  {  // create a ConcurrentHashMap and  // add some values  ConcurrentHashMap<Integer, String> map  = new ConcurrentHashMap<>();  map.put(1, "1000RS");  map.put(2, "5009RS");  map.put(3, "1300RS");  // print map details  System.out.println("ConcurrentHashMap : \n"  + map.toString());  // provide value for new key which is absent  // using computeIfAbsent method  map.computeIfAbsent(4, k -> "6000RS");  // this will not effect anything  // because key 1 is present  map.computeIfAbsent(1, k -> "8000RS");  // print new mapping  System.out.println("new ConcurrentHashMap :\n"  + map);  } }

Output: 
ConcurrentHashMap : {1=1000RS, 2=5009RS, 3=1300RS} new ConcurrentHashMap : {1=1000RS, 2=5009RS, 3=1300RS, 4=6000RS}

 

Program 3: To show NullPointerException
 

Java 
// Java program to demonstrate NullPointerException of // computeIfAbsent(Key, Function) method. import java.util.concurrent.*; public class GFG {  // Main method  public static void main(String[] args)  {  // create a ConcurrentHashMap and  // add some values  ConcurrentHashMap<String, Integer> map  = new ConcurrentHashMap<>();  map.put("Pencil", 1000);  map.put("Laptop", 55000);  map.put("Clothes", 4400);  map.put("Mobile", 5300);  try {  // provide value for new key which is absent  // using computeIfAbsent method  map.computeIfAbsent(null, k -> 60000);  }  catch (Exception e) {  // print new mapping  System.out.println("Exception: "  + e);  }  } }

Output: 
Exception: java.lang.NullPointerException

 

References: https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ConcurrentHashMap.html#computeIfAbsent-K-java.util.function.Function-
 


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AmanSingh2210
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