Java HashMap compute() Usage and Example

Java HashMap Methods

The Java HashMap compute() method calculates a new value and associates it with the specified key in the hash map.

The syntax of the compute() method is:

hashmap.compute(K key, BiFunction remappingFunction)

compute() parameters

The compute() method has two parameters:

• key - Key associated with the calculated value

• remappingFunction - For the specifiedKeyFunction to calculate the new value

NoteThe remappingFunction can accept two parameters.

compute() return value

• Return the new value associated with the key

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

NoteIf the remappingFunction result is null, the specifiedKeymapping.

Example: HashMap compute() to insert a new value

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);
　　　　//With10Percentage discount to recalculate the price of the shoes
　　　　int newPrice = prices.compute("Shoes", (key, value)　-> value　-　value　*　10/100);
　　　　System.out.println("Discounted shoe price: ")　+　newPrice);
　　　　//Print the updated HashMap
　　　　System.out.println("Updated HashMap: ")　+　prices);
　　}
}

Output result

HashMap: {Pant=150, Bag=300, Shoes=200}
Discounted shoe price:　180
Updated HashMap: {Pant=150, Bag=300, Shoes=180

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

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

Here,

• (key, value) -> value - value * 10/100 - 　This is a lambda expression. It will reduce the original price of the shoes10% and return. For more information on lambda expressions, please visitJava Lambda Expressions.

• prices.compute() - Associate the new value returned by the lambda expression with the Shoes mapping.

NoteAccording to the official Java documentation,HashMap merge()The method is simpler than the compute() method.

Recommended Reading

• HashMap computeIfAbsent() - Compute the value if the specified key is not mapped to any value

• HashMap computeIfPresent() - Compute the value if the specified key is already mapped to a value

Java HashMap Methods