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Java | Collectors averagingDouble() with Examples
• Last Updated : 06 Dec, 2018

Collectors averagingDouble(ToDoubleFunction<? super T> mapper) method is used to find the mean of the double values passed in the parameters. This method returns a Collector that produces the arithmetic mean of an double-valued function applied to the input elements. If no elements are passed as the input elements, then this method returns 0.

The formula used by this method to calculate arithmetic mean is: Syntax:

public static <T>
Collector<T, ?, Double>
averagingDouble(ToDoubleFunction<? super T> mapper)


where the terms are as follows:

• Interface Collector<T, A, R>: A mutable reduction operation that accumulates input elements into a mutable result container, optionally transforming the accumulated result into a final representation after all input elements have been processed. Reduction operations can be performed either sequentially or in parallel.
• T: The type of input elements to the reduction operation.
• A: The mutable accumulation type of the reduction operation.
• R: The result type of the reduction operation.
• Double: The Double class wraps a value of the primitive type double in an object. An object of type Double contains a single field whose type is double.
• ToDoubleFunction: Represents a function that produces a double-valued result.

Parameters: This method accepts a parameter mapper which is double-valued stream converted into Double using ToDoubleFunctions. ToDoubleFunction is a function which extracts a double type of value as it works on the objects of the stream.

Below are examples to illustrate averagingDouble() method:

Program 1:

 // Java code to show the implementation of// averagingDouble(ToDoubleFunction mapper) function  import java.util.stream.Collectors;import java.util.stream.Stream;  class GFG {      // Driver code    public static void main(String[] args)    {          // creating a string stream        Stream s = Stream.of("3", "4", "5");          // using Collectors averagingDouble(ToDoubleFunction mapper)        // method to find arithmetic mean of inputs given        double ans = s                         .collect(Collectors                                      .averagingDouble(                                          num -> Double.parseDouble(num)));          // displaying the result        System.out.println(ans);    }}
Output:
4.0


Program 2:

 // Java code to show the implementation of// averagingDouble(ToDoubleFunction mapper) function  import java.util.stream.Collectors;import java.util.stream.Stream;  class GFG {      // Driver code    public static void main(String[] args)    {          // creating a string stream        Stream s = Stream.of("7", "8", "9", "10");          // using Collectors averagingDouble(ToDoubleFunction mapper)        // method to find arithmetic mean of inputs given        double ans = s                         .collect(Collectors                                      .averagingDouble(                                          num -> Double.parseDouble(num)));          // displaying the result        System.out.println(ans);    }}
Output:
8.5


Program 3: When no value is passed as parameter.

 // Java code to show the implementation of// averagingDouble(ToDoubleFunction mapper) function  import java.util.stream.Collectors;import java.util.stream.Stream;  class GFG {      // Driver code    public static void main(String[] args)    {          // creating a string stream        Stream s = Stream.of();          // using Collectors averagingDouble(ToDoubleFunction mapper)        // method to find arithmetic mean of inputs given        double ans = s                         .collect(Collectors                                      .averagingDouble(                                          num -> Double.parseDouble(num)));          // displaying the result        System.out.println(ans);    }}
Output:
0.0


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