Java HashMap computeIfPresent()

The syntax of the computeIfPresent() method is:

hashmap.computeIfPresent(K key, BiFunction remappingFunction)

Here, hashmap is an object of the HashMap class.

computeIfPresent() Parameters

The computeIfPresent() method takes 2 parameters:

• key - key with which the computed value is to be associated
• remappingFunction - function that computes the new value for the specified key

Note: The remappingFunction can take two arguments. Hence, considered as BiFunction.

computeIfPresent() Return Value

• returns the new value associated with the specified key
• returns null if no value associated with key

Note: If remappingFunction results null, then the mapping for the specified key is removed.

Example 1: Java HashMap computeIfPresent()

import java.util.HashMap;

class Main {
  public static void main(String[] args) {
    // create an HashMap
    HashMap<String, Integer> prices = new HashMap<>();

    // insert entries to the HashMap
    prices.put("Shoes", 200);
    prices.put("Bag", 300);
    prices.put("Pant", 150);
    System.out.println("HashMap: " + prices);

    // recompute the value of Shoes with 10% VAT
    int shoesPrice = prices.computeIfPresent("Shoes", (key, value) -> value + value * 10/100);
    System.out.println("Price of Shoes after VAT: " + shoesPrice);

    // print updated HashMap
    System.out.println("Updated HashMap: " + prices);
  }
}

Output

HashMap: {Pant=150, Bag=300, Shoes=200}
Price of Shoes after VAT: 220
Updated HashMap: {Pant=150, Bag=300, Shoes=220}}

In the above example, we have created a hashmap named prices. Notice the expression,

prices.computeIfPresent("Shoes", (key, value) -> value + value * 10/100)

Here,

• (key, value) -> value + value*10/100 is a lambda expression. It computes the new value of Shoes and returns it. To learn more about the lambda expression, visit Java Lambda Expressions.
• prices.computeIfPresent() associates the new value returned by lambda expression to the mapping for Shoes. It is only possible because Shoes is already mapped to a value in the hashmap.

Here, the lambda expression acts as remapping function. And, it takes two parameters.

Note: We cannot use the computeIfPresent() method if the key is not present in the hashmap.

Recommended Reading

• HashMap compute() - computes the value for the specified key
• HashMap computeIfAbsent() - computes the value if the specified key is not mapped to any value
• Java HashMap merge() - performs the same task as compute()