Open In App

ConcurrentHashMap compute() method in Java with Examples

Last Updated : 29 Jun, 2022
Suggest changes
Share
Like Article
Like
Report

The compute(Key, BiFunction) method of ConcurrentHashMap class is used to compute a mapping for the specified key and its current mapped value (or null if there is no current mapping is found).

  • This method is used to atomically update a value for given key in ConcurrentHashMap.
  • If the remapping function throws an exception, the exception is re-thrown, and the current mapping is left unchanged.
  • During computation, update process on this map from other threads has been blocked so the computation must not attempt to update any other mappings of this Map
  • For example, This mapping append string value of mapping:
ConcurrentHashMap.compute(key, (key, value) -> (value == null) ? msg : value.concat(msg))

Syntax:

public V compute(K key, BiFunction<? super K, ? super V, ? 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 new value associated with the specified key, or null if none. Exception: This method throws following exceptions:

  • 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 compute(Key, BiFunction) method: Program 1: 

Java 
// Java program to demonstrate // compute(Key, BiFunction) 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("Book1", 10);  map.put("Book2", 500);  map.put("Book3", 400);  // print map details  System.out.println("ConcurrentHashMap: "  + map.toString());  // remap the values of ConcurrentHashMap  // using compute method  map.compute("Book2", (key, val)  -> val + 100);  map.compute("Book1", (key, val)  -> val + 512);  // print new mapping  System.out.println("New ConcurrentHashMap: "  + map);  } }
Output:
ConcurrentHashMap: {Book3=400, Book1=10, Book2=500} New ConcurrentHashMap: {Book3=400, Book1=522, Book2=600}

Program 2: 

Java 
// Java program to demonstrate // compute(Key, BiFunction) 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, "Kolkata");  map.put(2, "Nadia");  map.put(3, "Howrah");  // print map details  System.out.println("ConcurrentHashMap: "  + map.toString());  // remap the values of ConcurrentHashMap  // using compute method  map.compute(2, (key, val)  -> val.concat(" (West-Bengal)"));  map.compute(3, (key, val)  -> val.concat(" (West-Bengal)"));  // print new mapping  System.out.println("New ConcurrentHashMap: "  + map);  } }
Output:
ConcurrentHashMap: {1=Kolkata, 2=Nadia, 3=Howrah} New ConcurrentHashMap: {1=Kolkata, 2=Nadia (West-Bengal), 3=Howrah (West-Bengal)}

Program 3:To show NullPointerException 

Java 
// Java program to demonstrate NullPointerException // for compute(Key, BiFunction) 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, "Kolkata");  map.put(2, "Nadia");  map.put(3, "Howrah");  try {  // remap the values of ConcurrentHashMap  // using compute method  map.compute(null, (key, val)  -> val.concat(" (West-Bengal)"));  }  catch (NullPointerException e) {  // print Exception  System.out.println("Exception: " + e);  }  } }
Output:
Exception: java.lang.NullPointerException

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


A
AmanSingh2210
Article Tags :

Explore

Lightbox