# 
## Overview

**`mapToDouble()`** of the `Stream` interface returns a `DoubleStream`. The `DoubleStream` contains the results of the the [ToDoubleFunction](https://www.educative.io/answers/what-is-doublefunction-functional-interface-in-java) function's provided implementation on the stream's items.


## Syntax
---

``` java
DoubleStream mapToInt(ToDoubleFunction<? super T> mapper)
```

---


## Parameters 

- **`ToDoubleFunction<? super T> mapper`**: The mapper function to apply.


## Return value

This method returns a new stream:

import java.util.Arrays;
import java.util.List;
import java.util.function.ToDoubleFunction;

public class Main{

    public static void main(String[] args) {
        // define the list of doubles
        List<Double> doubleList = Arrays.asList(10.4, 24.5, 54.1, 8.0,23.2);

        // Define the implementation of ToDoubleFunction interface
        ToDoubleFunction<Double> multiplyByTwo = e -> e * 2;

        // apply the multiplyByTwo using mapToDouble function and print each value
        doubleList.stream().mapToDouble(multiplyByTwo).forEach(System.out::println);
    }
}


## Explanation

- **Lines 1-3**: We import the relevant packages.
- **Line 9**: We define a list of double values.
- **Line 12**: We define the implementation of the `ToDoubleFunction` interface called `multiplyByTwo`. Here, we multiply the input double value by `2`.
- **Line 15**: We apply the `multiplyByTwo` using the `mapToDouble` function and print the result.

What is the Stream.mapToDouble method in Java?

Converts each stream item using a provided function to a DoubleStream. Example multiplies values by 2.

What is the Stream.mapToDouble method in Java?

Overview

Syntax

Parameters

Return value

Code

Explanation