Map API Improvements: Fetch Operations

This lesson explains the new methods added in Map API for fetch operations.

We'll cover the following

If you have used Map then you must have faced a challenge where you needed to update the value of a key in the Map. Now, before updating, you must first check if the value is present in the Map, get the current value, update it, and again put the value in the Map. This is quite a cumbersome process, and it involves using lots of if/else statements. This kind of code is difficult to understand and fix if any issues occur.

Thankfully, Java 8 has introduced some new methods in the Map interface to make our lives easier. In this lesson, we will discuss some of those new methods.

1. getOrDefault()

This method is a lifesaver if you need to update a certain key in the Map. The getOrDefault() method either returns the value of the key, or it returns the default value if the key is not present.

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import java.util.HashMap;

import java.util.Map;



public class MapUpgrades {

  

  public static void main(String args[]){

    Map<String , Integer> fruits = new HashMap<>();

    fruits.put("apple", 20);



    // We need to add 20 bananas in map.

    // Below line will throw NullPointerException if banana 

    //is already not present in the map.

    

    //fruits.put("banana", fruits.get("banana") + 20); 



    //This is the correct way to update map value fefore Java 8

    if (fruits.containsKey("banana")) {

      fruits.put("banana", fruits.get("banana") + 20);

    } else {

      fruits.put("banana", 20);

    }

  }

}

Below is the same example using the getOrDefault() method. Now we don’t need the if/else checks to update a value in the Map.

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import java.util.HashMap;

import java.util.Map;



public class MapUpgrades {

  

  public static void main(String args[]){

    Map<String , Integer> fruits = new HashMap<>();

    fruits.put("apple", 20);



    fruits.put("banana", fruits.getOrDefault("banana", 0) + 20);

  }

}

2. putIfAbsent()

You might be aware that the put() method in Map either inserts a key-value pair in the Map or updates the value if the key is already present. Now, what if you don’t want to update the value in the Map. You want to insert a key-value pair only if it is not present in the Map.

This can be achieved by theputIfAbsent() method. This method inserts a key and value in the Map only if it is not present. Let’s look at hoow it works in the example below.

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import java.util.HashMap;

import java.util.Map;



public class MapUpgrades {

  

  public static void main(String args[]){

    Map<String , Integer> fruits = new HashMap<>();

    

    fruits.put("apple", 20);



    System.out.println(fruits.get("apple"));



    fruits.putIfAbsent("apple", 30);



    System.out.println(fruits.get("apple"));

  }

}

3. compute(), computeIfAbsent() and computeIfPresent()

The compute() method computes a new mapping given the key and its existing value. This method returns the computed value. If the key is not present in the map, then an exception is thrown.

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import java.util.HashMap;

import java.util.Map;



public class ComputeExample {



  public static void main(String args[]) {

    Map<String, Integer> fruits = new HashMap<>();

    fruits.put("apple", 20);



    int val = fruits.compute("apple", (k, v) -> v + 10);



    System.out.println(val);

    // Below line will throw Null Pointer Exception.

    //val = fruits.compute("banana", (k, v) -> v + 10);



  }

}

The computeIfAbsent() method returns:

  • The original value if the key is already present in the map.

  • The computed value if the key is not present in the map.

This method takes a key and a Function as a parameters.

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import java.util.HashMap;

import java.util.Map;



public class ComputeIfAbsentExample {



  public static void main(String args[]) {

    Map<String, Integer> fruits = new HashMap<>();

    fruits.put("apple", 20);



    int val = fruits.computeIfAbsent("apple", v ->  10);



    System.out.println(val);

    

    val = fruits.computeIfAbsent("banana", v ->  10);



    System.out.println(val);

  }

}

The computeIfPresent() method returns:

  • A null value if the key is not present in the map.

  • The computed value if the key is present in the map.

This method takes a key and a BiFunction as parameters.

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import java.util.HashMap;

import java.util.Map;



public class ComputeIfPresentExample {



  public static void main(String args[]) {

    Map<String, Integer> fruits = new HashMap<>();

    fruits.put("apple", 20);



    Integer val = fruits.computeIfPresent("apple", (k, v) -> v + 10);



    System.out.println(val);

    

    val = fruits.computeIfPresent("banana", (k, v) -> v + 10);



    System.out.println(val);

  }

}

In the next lesson, we will look at some more improvements done to the Map Interface.

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