The **`computeIfPresent()` method** of the `HashMap` class in Java maps a new value to a provided key, given that the key is present in the `HashMap`.

The process is illustrated below:




To use the `computeIfPresent()` method, you will need to import the `HashMap` class into the program, as shown below:

```Java
import java.util.HashMap;
```

The prototype of the `computeIfPresent()` method is shown below:

```Java
public computeIfPresent(K key, BiFunction remappingFunction)
```

<h2>Parameters</h2>

The `computeIfPresent()` method takes the following parameters:

* **`key`**: The key for which the value must be computed.
* **`remappingFunction`**: The function that computes the new value.

<h2>Return value</h2>

The `computeIfPresent()` method checks the value associated with the provided `key` parameter and returns one of the following:

* If `key` has an associated value, a new value is computed and returned.

* If `key` does not have an associated value, then `computeIfPresent()` returns `null`.

> Note: If the `remappingFunction` returns `null`, the *key-value* pair is removed from the `HashMap`.

<h2>Example</h2>

The code below shows how the `computeIfPresent()` method works in Java:

import java.util.HashMap;

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

    // populating HashMap
    quantities.put("Apples", 10);
    quantities.put("Bananas", 2);
    quantities.put("Oranges", 5);

    // original quantities
    System.out.println("The original quantities are: " + quantities);

    // key already present
    int banana_quantity = quantities.computeIfPresent("Bananas", (key, value) -> 30);
    
    // key not present
    if(quantities.computeIfPresent("Peach", (key, value) -> 15) == null){
      System.out.println("The function returned null.");
    }

    // new quantities
    System.out.println("The new quantities are: " + quantities);
  }
}

<h2>Explanation</h2>

First, a `HashMap` object `quantities` is initialized.

Next, *key-value* pairs are inserted into the `HashMap`. All entries have unique values.

The `computeIfPresent()` method in line $18$ proceeds to iterate over each mapping in the `HashMap` to find the value associated with the key "Bananas". Since "Bananas" exists in the `HashMap`, the `computeIfPresent()` method computes a new value for the specified key, i.e., $30$. The `computeIfPresent()` method also returns the newly computed value.

Similarly, the `computeIfPresent()` method in line $21$ proceeds to iterate over each mapping in the `HashMap` to find the value associated with the key "Peach". Since "Peach" does not exist in the `HashMap`, the `computeIfPresent()` method returns `null`. As a result, the `if-condition` in line $21$ is satisfied, and an error message is output.

What is the HashMap computeIfPresent() method in Java?

Maps a new value to key if present, computes new value, else returns null; removes key-value if function returns null.