The ToLongBiFunction is a functional interface introduced in Java. It is part of the java.util.function package, and it represents a function that accepts two arguments (of generic types) and produces a long result.

The functional method in this interface is:

long applyAsLong(T t, U u);

Here’s how we can effectively use the ToLongBiFunction:

Steps to Use ToLongBiFunction:

• Import the Interface: Ensure to import the specific interface:

   import java.util.function.ToLongBiFunction;

• Define Behavior: Implement the applyAsLong method either through a lambda expression or anonymous class. The method takes two arguments and returns a long.
• Use in Code: Pass it as a lambda/method reference when working with methods that require this interface.

Example Usage

Example 1: Simple Lambda Expression

package org.kodejava.util.function;

import java.util.function.ToLongBiFunction;

public class ToLongBiFunctionExample {
    public static void main(String[] args) {
        // Define a ToLongBiFunction with a lambda
        ToLongBiFunction<Integer, Integer> addAndConvertToLong = (a, b) -> (long) a + b;

        // Apply the function
        long result = addAndConvertToLong.applyAsLong(5, 10);

        // Output the result
        System.out.println("Result: " + result);
        // Output: Result: 15
    }
}

Example 2: Using Method Reference

package org.kodejava.util.function;

import java.util.function.ToLongBiFunction;

public class ToLongBiFunctionExample2 {
    public static void main(String[] args) {
        // Define the method reference
        ToLongBiFunction<String, String> stringLengthSum = ToLongBiFunctionExample2::computeLengths;

        // Apply the function
        long result = stringLengthSum.applyAsLong("Hello", "World");

        // Output the result
        System.out.println("Result: " + result);
        // Output: Result: 10
    }

    // Method to compute combined string lengths
    public static long computeLengths(String str1, String str2) {
        return (long) (str1.length() + str2.length());
    }
}

Example 3: Practical Application in Streams

We can use this interface in more complex scenarios, like computing operations in streams.

package org.kodejava.util.function;

import java.util.function.ToLongBiFunction;

public class ToLongBiFunctionWithStream {
    public static void main(String[] args) {
        // Example: Summing the lengths of two strings
        ToLongBiFunction<String, String> sumLengths = (a, b) -> a.length() + b.length();

        // Example usage
        long lengthSum = sumLengths.applyAsLong("Programming", "Java");

        System.out.println("Length Sum: " + lengthSum);
        // Output: Length Sum: 16
    }
}

Key Points of ToLongBiFunction:

1. It is generic, meaning we can customize the types of the two input arguments (T and U).
2. The result is always of type long.
3. It is often useful when working with APIs such as streams that require functional-style programming.