LongFunction Interface in Java with Examples

The LongFunction interface is a functional interface from the java.util.function package, introduced in Java 8. It represents a function that accepts a long value and produces a result of type R.

• Uses the generic type R to define the return type.
• Useful when the input is a primitive long and the operation produces a result of another type.
• Similar to Function<Long, R>, but LongFunction directly accepts a long value.

Example: The following program, uses a lambda expression with LongFunction to divide a long value by 2.

import java.util.function.LongFunction;

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

        LongFunction<Double> operation = value -> value / 2.0;

        System.out.println(operation.apply(3L));
    }
}

1.5

Explanation: The lambda expression divides the input value by 2.0. The apply() method executes the operation for 3L and returns 1.5.

Syntax

@FunctionalInterface
public interface LongFunction<R> {
R apply(long value);
}

Note: LongFunction<R> is a specialization of Function<Long, R> that accepts a primitive long, avoiding boxing of the input value into Long.

Working of LongFunction

The LongFunction interface separates the input type from the result type. It always accepts a primitive long value, while the generic type R determines the type of result produced by the operation.

The working process is as follows:

• A lambda expression is assigned to a LongFunction object.
• The lambda expression provides the implementation of the apply() method.
• When apply() is called, the supplied primitive long value is passed to the lambda expression.
• The operation defined in the lambda expression is performed.
• The resulting value is returned as type R.

Example: Program to demonstrates how LongFunction converts a primitive long value into a String.

import java.util.function.LongFunction;

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

        LongFunction<String> operation =
            value -> "Number is " + value;

        String result = operation.apply(100L);

        System.out.println(result);
    }
}

Number is 100

Explanation: The value 100L is supplied to apply(). The lambda converts the value into a String and returns "Number is 100".

apply() Method

The apply() method is the single abstract method of the LongFunction interface. It accepts a primitive long value, performs the operation defined by the function, and returns a result of type R.

Syntax

R apply(long value)

• Parameter: value: The primitive long value passed to the function.
• Return Value: The method returns the result produced by the function, of type R.

Example: Program to demonstrates how the apply() method executes a lambda expression that calculates the square of a given long value.

import java.util.function.LongFunction;

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

        LongFunction<String> square =
                value -> "Square is " + (value * value);

        System.out.println(square.apply(5L));
    }
}

Square is 25

Explanation: The apply(5L) method passes 5L to the lambda expression. The expression calculates the square of the value and returns the result as a string.