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Java Map computeIfPresent() Example

By Arvind Rai, February 20, 2020
The computeIfPresent is the default method of java.util.Map and has been introduced in Java 8. The computeIfPresent method computes a specified mapping function for the given key and its associated value, and then updates the value for that key if the value for the specified key is present and non-null. Find the method declaration from Java doc.
default V computeIfPresent(K key, 
            BiFunction<? super K,? super V,? extends V> remappingFunction) 
The key is the specified key with which value to be associated.
The remappingFunction is the specified mapping function of Java BiFunction type to compute a new value.
The computeIfPresent method returns new value associated with the specified key, or null if none.

The computeIfPresent method works as following.
1. If specified key is associated with a not null value and new value computed by specified mapping function is also not null, in this case the computeIfPresent method will put the new value for the specified key.
2. If specified key is associated with a not null value and new value computed by specified mapping function is null, in this case the computeIfPresent method will remove the entry for the specified key.
3. If specified key is associated with a null value and new value computed by specified mapping function is not null, in this case the computeIfPresent method will do nothing.
4. If specified key is associated with a not null value and mapping function throws exception, in this case there will be no change in the Map.

Example-1

Specified key is associated with a not null value
and new value computed by specified mapping function is also not null.
In this case the computeIfPresent method will put the new value for the specified key.
ComputeIfPresent1.java
import java.util.HashMap;
import java.util.Map;

public class ComputeIfPresent1 {
  public static void main(String[] args) {
     Map<Integer, String> cityMap = new HashMap<>();
     cityMap.put(101, "Varanasi");
     cityMap.put(102, "Prayag");
     
     String newValue = cityMap.computeIfPresent(102, (k, v) -> v != null ? v.concat("raj") : null);
     
     System.out.println(newValue);
     System.out.println(cityMap);
  }
} 
Output
Prayagraj
{101=Varanasi, 102=Prayagraj} 
The same can be achieved without using computeIfPresent method as following.
ComputeIfPresent11.java
import java.util.HashMap;
import java.util.Map;
import java.util.function.BiFunction;

public class ComputeIfPresent11 {
  public static void main(String[] args) {
     Map<Integer, String> cityMap = new HashMap<>();
     cityMap.put(101, "Varanasi");
     cityMap.put(102, "Prayag");
    
     BiFunction<Integer, String, String> remappingFunction = (k, v) -> v != null ? v.concat("raj") : null;
     int key = 102;
     String oldValue = cityMap.get(key);
     String newValue = remappingFunction.apply(key, oldValue);  
     
     if (cityMap.get(key) != null) {
       if (newValue != null)
    	 cityMap.put(key, newValue);
       else
    	 cityMap.remove(key);
     }
     
     System.out.println(newValue);
     System.out.println(cityMap);
  }
} 
Output
Prayagraj
{101=Varanasi, 102=Prayagraj} 

Example-2

Specified key is associated with a not null value
and new value computed by specified mapping function is null.
In this case the computeIfPresent method will remove the entry for the specified key.
ComputeIfPresent2.java
import java.util.HashMap;
import java.util.Map;

public class ComputeIfPresent2 {
  public static void main(String[] args) {
     Map<Integer, String> cityMap = new HashMap<>();
     cityMap.put(101, "Varanasi");
     cityMap.put(102, "Prayag");
     
     cityMap.computeIfPresent(102, (k, v) -> null);
     
     System.out.println(cityMap);
  }
} 
Output
{101=Varanasi} 

Example-3

Specified key is associated with a null value
and new value computed by specified mapping function is not null.
In this case the computeIfPresent method will do nothing. There will be no change in the Map.
ComputeIfPresent3.java
import java.util.HashMap;
import java.util.Map;

public class ComputeIfPresent3 {
  public static void main(String[] args) {
     Map<Integer, String> cityMap = new HashMap<>();
     cityMap.put(101, "Varanasi");
     cityMap.put(102, null);
     
     cityMap.computeIfPresent(102, (k, v) -> "Noida");
     
     System.out.println(cityMap);
  }
} 
Output
{101=Varanasi, 102=null} 

Example-4

Specified key is associated with a not null value
and mapping function throws exception.
In this case there will be no change in the Map.
ComputeIfPresent4.java
import java.util.HashMap;
import java.util.Map;

public class ComputeIfPresent4 {
  public static void main(String[] args) {
	Map<Integer, String> cityMap = new HashMap<>();
	cityMap.put(101, "Varanasi");
	cityMap.put(102, "Noida");

	String newVal = null;
	try {
	  cityMap.computeIfPresent(102, (k, v) -> newVal.concat("Prayag")); //throws exception
	} catch (Throwable e) {
	  System.out.println(e);
	}

	System.out.println(cityMap);
  }
} 
Output
java.lang.NullPointerException
{101=Varanasi, 102=Noida} 

Reference

Java doc: Map
POSTED BY
ARVIND RAI
ARVIND RAI
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