The DoubleFunction interface in Java is a functional interface in the java.util.function package. It represents a function that takes in a double as an argument and produces a result of some type. Since it is a functional interface, you can use it as a lambda expression or method reference.

Method in DoubleFunction

The DoubleFunction interface has one abstract method:

R apply(double value);

• value: The double value passed as input to the function.
• The method returns an object of type R (the return type).

How to Use the DoubleFunction Interface

We typically use DoubleFunction in situations where we need to transform or process a double and produce a result of a specific type.

Example 1: Using DoubleFunction to Convert double to a String

package org.kodejava.util.function;

import java.util.function.DoubleFunction;

public class DoubleFunctionExample {
    public static void main(String[] args) {
        // Create a DoubleFunction that converts a double to a String representation
        DoubleFunction<String> doubleToString = (value) -> "Value is: " + value;

        // Use the DoubleFunction
        String result = doubleToString.apply(42.5);

        // Print the result
        // Output: Value is: 42.5
        System.out.println(result);
    }
}

Example 2: Using DoubleFunction to Compute Complex Results

You can use DoubleFunction to compute and return various types of results, such as objects of a custom type.

package org.kodejava.util.function;

import java.util.function.DoubleFunction;

public class DoubleFunctionExample2 {
    public static void main(String[] args) {
        // Create a DoubleFunction that creates an Area object from a radius
        DoubleFunction<Area> calculateArea =
                (radius) -> new Area(Math.PI * radius * radius);

        // Compute the area using the DoubleFunction
        Area area = calculateArea.apply(5.0);

        // Print the result
        // Output: Area is: 78.53981633974483
        System.out.println("Area is: " + area.getValue());
    }
}

// A simple class to store area
class Area {
    private double value;

    public Area(double value) {
        this.value = value;
    }

    public double getValue() {
        return value;
    }
}

Example 3: Method References with DoubleFunction

You can simplify usage by using method references when possible.

package org.kodejava.util.function;

import java.util.function.DoubleFunction;

public class DoubleFunctionExample3 {
    public static void main(String[] args) {
        // Using a method reference for Math.sqrt
        DoubleFunction<Double> sqrtFunction = Math::sqrt;

        // Apply the DoubleFunction
        double result = sqrtFunction.apply(25.0);

        // Print the result
        // Output: Square root is: 5.0
        System.out.println("Square root is: " + result);
    }
}

Use Cases

• Converting a double to a custom object (e.g., Area, Volume).
• Performing mathematical computations with a double input and returning relevant results.
• Simplifying transformations in streams or other functional programming contexts.

Key Points to Note

1. DoubleFunction<R> is particularly useful to avoid unnecessary boxing of primitive double values when dealing with functions that process them since it directly deals with primitive types.
2. It produces a result of any type R, which makes it very flexible.

By using DoubleFunction, we can write concise, reusable, and readable code for operations that require processing a double to produce a result.