LongToDoubleFunction Interface in Java with Examples

The LongToDoubleFunction is a functional interface from the java.util.function package, introduced in Java 8. It accepts a primitive long value and returns a primitive double result.

• Its single abstract method is applyAsDouble().
• It avoids boxing and unboxing by working directly with primitive values.

Example: Program to demonstrate LongToDoubleFunction by converting a distance from meters to kilometers.

import java.util.function.LongToDoubleFunction;

public class GFG {
    public static void main(String[] args) {

        LongToDoubleFunction metersToKilometers =
            meters -> meters / 1000.0;

        System.out.println(
            metersToKilometers.applyAsDouble(5000L)
        );
    }
}

5.0

Explanation: The lambda expression receives 5000L meters and divides it by 1000.0. The resulting value, 5.0, is returned as a primitive double.

Syntax

The LongToDoubleFunction interface can be represented as:

@FunctionalInterface
public interface LongToDoubleFunction {
double applyAsDouble(long value);
}

Here, the function takes a primitive long as input and returns a primitive double.

Method Of LongToDoubleFunction

The LongToDoubleFunction interface provides one abstract method, applyAsDouble(), which accepts a primitive long value and returns a primitive double result.

applyAsDouble()

The applyAsDouble() method receives a primitive long value, applies the function logic, and returns the computed result as a primitive double.

Syntax

double applyAsDouble(long value);

• Parameter: value – the primitive long value supplied to the function.
• Return Type: Returns the calculated result as a primitive double.

Example: Program to use applyAsDouble() to calculate the average of a given value and 10.

import java.util.function.LongToDoubleFunction;

public class GFG {
    public static void main(String[] args) {

        LongToDoubleFunction operation =
            value -> (value + 10) / 2.0;

        System.out.println(operation.applyAsDouble(20L));
    }
}

15.0

Explanation: The lambda receives 20L, adds 10, and divides the result by 2.0. The expression produces 15.0, which is returned as a primitive double.

Why Use LongToDoubleFunction?

LongToDoubleFunction is useful when an operation takes a primitive long value and produces a primitive double result. It is especially suitable for numerical calculations and unit conversions where decimal results are required. Unlike Function<Long, Double>, it works directly with primitive values instead of wrapper types.

LongToDoubleFunction vs Function<Long, Double>