The ToDoubleBiFunction is a functional interface in Java defined in the java.util.function package. It represents a function that takes two arguments of any types and produces a double result. It can be used wherever we need to take two input arguments and return a double value.

Here is a breakdown of its functional method:

@FunctionalInterface
public interface ToDoubleBiFunction<T, U> {
    double applyAsDouble(T t, U u);
}

How to Use ToDoubleBiFunction

We can use ToDoubleBiFunction with lambda expressions or method references. Below are examples to demonstrate its usage.

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Example 1: Basic Usage with Lambda Expression

This example demonstrates using a ToDoubleBiFunction to calculate the average of two integers.

package org.kodejava.util.function;

import java.util.function.ToDoubleBiFunction;

public class ToDoubleBiFunctionExample {
    public static void main(String[] args) {
        // Create a ToDoubleBiFunction to calculate the average of two integers
        ToDoubleBiFunction<Integer, Integer> average = (a, b) -> (a + b) / 2.0;

        // Apply the function
        double result = average.applyAsDouble(10, 20);

        System.out.println("The average is: " + result);
    }
}

Output:

The average is: 15.0

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Example 2: Using with Custom Classes

If we have custom types as input, we can define a ToDoubleBiFunction to process their fields.

package org.kodejava.util.function;

import java.util.function.ToDoubleBiFunction;

class MyProduct {
    String name;
    double price;

    MyProduct(String name, double price) {
        this.name = name;
        this.price = price;
    }
}

public class ToDoubleBiFunctionExample2 {
    public static void main(String[] args) {
        // Create two Product objects
        MyProduct product1 = new MyProduct("Laptop", 1200.50);
        MyProduct product2 = new MyProduct("Phone", 800.30);

        // Create a ToDoubleBiFunction to find the total price of two products
        ToDoubleBiFunction<MyProduct, MyProduct> totalPrice =
                (p1, p2) -> p1.price + p2.price;

        // Apply the function
        double result = totalPrice.applyAsDouble(product1, product2);

        System.out.println("The total price is: " + result);
    }
}

Output:

The total price is: 2000.8

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Example 3: Using Method References

If we have a method that matches the signature of ToDoubleBiFunction<T, U>, we can use a method reference instead of a lambda expression.

package org.kodejava.util.function;

import java.util.function.ToDoubleBiFunction;

public class ToDoubleBiFunctionExample3 {
    public static void main(String[] args) {
        // Using a static method reference
        ToDoubleBiFunction<Integer, Integer> maxFunction = ToDoubleBiFunctionExample3::findMax;

        // Apply the function
        double max = maxFunction.applyAsDouble(42, 56);

        System.out.println("The maximum value is: " + max);
    }

    // Static method to find the maximum value
    public static double findMax(int a, int b) {
        return Math.max(a, b);
    }
}

Output:

The maximum value is: 56.0

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When to Use ToDoubleBiFunction

• When we need to compute a double result from two input parameters.
• When we want to pass a function that takes two arguments and returns a double.
• When processing tasks with numerical computations involving two objects or values.

Keynotes:

1. It is part of the java.util.function package and was introduced in Java 8.
2. Since it is a functional interface, it can be used in lambda expressions and method references.
3. It is a good choice for reducing boilerplate code by avoiding explicitly writing anonymous classes.