The DoubleToLongFunction interface in Java, part of the java.util.function package, represents a functional interface with a method that accepts a double-valued argument and produces a long-valued result. It is often used when a computation or conversion needs to be performed from a double to a long.

Functional Interface

The DoubleToLongFunction interface is annotated with @FunctionalInterface, meaning it has exactly one abstract method:

long applyAsLong(double value);

This method takes a double as input and returns a long result.

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How to Use DoubleToLongFunction

1. Using a Lambda Expression

The simplest way to use the DoubleToLongFunction is by implementing its applyAsLong method with a lambda expression.

Example:

package org.kodejava.util.function;

import java.util.function.DoubleToLongFunction;

public class DoubleToLongFunctionExample {
  public static void main(String[] args) {
    // Define a DoubleToLongFunction that rounds a double to the nearest long
    DoubleToLongFunction roundFunction = value -> Math.round(value);

    // Apply the function
    // Outputs: 43
    System.out.println("Rounded value: " + roundFunction.applyAsLong(42.75));
  }
}

Here, the Math.round method is used to convert the double value to a long.

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2. Defining Custom Conversion Logic

We can use the DoubleToLongFunction to implement custom logic for converting a double to a long.

Example:

package org.kodejava.util.function;

import java.util.function.DoubleToLongFunction;

public class CustomConversionExample {
  public static void main(String[] args) {
    // Convert a double value in kilometers to meters and truncate to long
    DoubleToLongFunction kilometersToMeters = kilometers -> (long) (kilometers * 1000);

    // Apply the function
    // Outputs: 42195
    System.out.println("Meters: " + kilometersToMeters.applyAsLong(42.195));
  }
}

This example demonstrates converting a double (representing a value in kilometers) to meters, truncating the result to a long.

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3. Using with Streams

The DoubleToLongFunction is particularly useful with Java Streams, especially when working with streams of primitive types (e.g., DoubleStream).

Example:

package org.kodejava.util.function;

import java.util.stream.DoubleStream;

public class DoubleStreamExample {
  public static void main(String[] args) {
    // Create a DoubleStream
    DoubleStream doubleStream = DoubleStream.of(1.2, 3.4, 5.6);

    // Map each double to a long using a DoubleToLongFunction
    doubleStream.mapToLong(value -> (long) (value * 10))
            .forEach(System.out::println);
    // Outputs:
    // 12
    // 34
    // 56
  }
}

This example scales each double value by 10 and converts it to long before printing the results.

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4. Combining with Other Functional Interfaces

We can also combine the DoubleToLongFunction with other functional interfaces for more advanced processing workflows.

Example:

import java.util.function.DoubleToLongFunction;
import java.util.function.LongConsumer;

public class CombinedFunction {
  public static void main(String[] args) {
    // DoubleToLongFunction to truncate a temperature value from Celsius to Kelvin
    DoubleToLongFunction celsiusToKelvin = celsius -> (long) (celsius + 273.15);

    // LongConsumer to print the result
    LongConsumer printResult = kelvinValue -> System.out.println("Temperature in Kelvin: " + kelvinValue);

    // Combine the function and consumer
    double celsius = 25.0;
    printResult.accept(celsiusToKelvin.applyAsLong(celsius));
    // Output: Temperature in Kelvin: 298
  }
}

In this example, the DoubleToLongFunction converts Celsius to Kelvin, and the LongConsumer processes the resulting value.

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When to Use DoubleToLongFunction

• When working with functional programming scenarios that involve converting or mapping double values to long.
• In scenarios such as:
  • Rounding or truncating computations.
  • Unit conversions where precision is not required beyond a long.
  • Scaling operations (e.g., multiplying or dividing by a constant factor).
• To simplify operations when working with DoubleStream to produce LongStream.

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Summary

The DoubleToLongFunction makes it straightforward to handle transformations from double to long. By leveraging lambda expressions, streams, or custom logic, we can write clean and concise code for numerical computations or transformations.