Usage and examples of Java HashMap computeIfAbsent()

Java HashMap Methods

The computeIfAbsent() method of Java HashMap calculates a new value and associates it with the specified key if the key is not associated with any value in the hash map.

The syntax of the computeIfAbsent() method is:

hashmap.computeIfAbsent(K key, Function remappingFunction)

computeIfAbsent() parameters

The computeIfAbsent() method has two parameters:

• key - Associated with the calculated value and the key

• remappingFunction - For the specifiedKeyFunction to calculate a new value

Note：The remapping function cannot accept two parameters.

computeIfAbsent() returns value

• Returns the new value or old value associated with the specified key

• Function to calculate a new value for the specified key

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

Example1：Java HashMap's computeIfAbsent()

import java.util.HashMap;
class Main {
　　public static void main(String[] args) {
　　　　//　Creating HashMap
　　　　HashMap<String, Integer> prices = new HashMap<>();
　　　　//　Inserting an entry into the HashMap
　　　　prices.put("Shoes",　200);
　　　　prices.put("Bag",　300);
　　　　prices.put("Pant",　150);
　　　　System.out.println("HashMap: ")　+　prices);
　　　　//　Calculate the value of shirt
　　　　int shirtPrice = prices.computeIfAbsent("Shirt", (key　->　280);
　　　　System.out.println("The price of shirt: ")　+　shirtPrice);
　　　　//　Print the updated HashMap
　　　　System.out.println("Updated HashMap:")　+　prices);
　　}
}

Output Result

HashMap: {Pant=150, Bag=300, Shoes=200}
The price of shirt:　280
Updated HashMap: {Pant=150, Shirt=280, Bag=300, Shoes=200}

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

prices.computeIfAbsent("Shirt", (key　->　280)

Here,

• key -> 280 - It is a lambda expression. It returns the value28For more information about lambda expressions, please visitJava Lambda Expressions.

• prices.computeIfAbsent() - 　Associate the new value returned by the lambda expression with the mapping of Shirt. This is possible because Shirt has not been mapped to any value in the hashmap yet.
  

Example2：If the key already exists, use computeIfAbsent()

import java.util.HashMap;
class Main {
　　public static void main(String[] args) {
　　　　//　Creating HashMap
　　　　HashMap<String, Integer> prices = new HashMap<>();
　　　　//　Inserting an entry into the HashMap
　　　　prices.put("Shoes",　180);
　　　　prices.put("Bag",　300);
　　　　prices.put("Pant",　150);
　　　　System.out.println("HashMap: ")　+　prices);
　　　　//The mapping of Shoes already exists
　　　　//Not to calculate the new value of Shoes
　　　　int shoePrice = prices.computeIfAbsent("Shoes", (key)　->　280);
　　　　System.out.println("Shoe price: ")　+　shoePrice);
　　　　//Print the updated HashMap
　　　　System.out.println("Updated HashMap:")　+　prices);
　　}
}

Output Result

HashMap: {Pant=150, Bag=300, Shoes=180}
Shoes Price:　180
Updated HashMap: {Pant=150, Bag=300, Shoes=180}

In the above example, the mapping of Shoes already exists in the hash map. Therefore, the computeIfAbsent() method will not compute a new value for Shoes.

Recommended Reading

• HashMap compute() - Compute the value of the specified key

• HashMap computeIfPresent() - If the specified key is already mapped to a value, compute that value

• Java HashMap merge() - Performs the same task as compute()

Java HashMap Methods