Usage and examples of Java HashMap computeIfPresent()

Java HashMap Methods

If the key already exists in the hash key, the Java HashMap computeIfPresent() method will calculate a new value and associate it with the specified key.

The syntax of computeIfPresent() method is: }}

hashmap.computeIfPresent(K key, BiFunction remappingFunction)

computeIfPresent() parameters

computeIfPresent() method has two parameters:

• key - key associated with the calculated value

• remappingFunction - For the specifiedKeyFunction to calculate the new value

Note：The remappingFunction can accept two parameters.

computeIfPresent() return value

• Return the new value associated with the specified key

• If there is no value associated with the key, then return null
  

Note：If the remappingFunction result is null, the specifiedKeymapping.

Example1：Java HashMap's computeIfPresent()

import java.util.HashMap;
class Main {
　　public static void main(String[] args) {
　　　　//　Creating HashMap
　　　　HashMap<String, Integer> prices = new HashMap<>();
　　　　//　Inserting entries into the HashMap
　　　　prices.put("Shoes",　200);
　　　　prices.put("Bag",　300);
　　　　prices.put("Pant",　150);
　　　　System.out.println("HashMap: ")　+　prices);
　　　　//Using10Recompute the value of shoes with the VAT rate
　　　　int shoesPrice = prices.computeIfPresent("Shoes", (key, value)　-> value　+　value　*　10/100);
　　　　System.out.println("Price of shoes after adding VAT: ")　+　shoesPrice);
　　　　//　Print the updated HashMap
　　　　System.out.println("Updated HashMap: ")　+　prices);
　　}
}

Output result

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

In the above example, we created a hash map named prices. Note the expression

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

Here,

• (key, value) -> value + value*10/100　 - It is a lambda expression. It calculates the new value of Shoes and returns. For more information about lambda expressions, please visitJava Lambda Expressions.

• prices.computeIfPresent()　 - Associates the new value returned by the lambda expression with the Shoes mapping. This is the only possible because Shoes is already mapped to the value in the hash map.

The computeIfPresent() method cannot be used if the key does not exist in the hash table.

Recommended Reading

• HashMap compute()　 - Computes the value of 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()

Java HashMap Methods