The LongToDoubleFunction is a functional interface in Java that belongs to the java.util.function package. It represents a function that accepts a long value as an argument and produces a result of type double. This functional interface is typically used in scenarios where we want to perform an operation that converts a long input into a double output without having to box or unbox objects.
Here’s how we can use the LongToDoubleFunction interface:
1. Structure of the Interface
The LongToDoubleFunction interface has a single abstract method:
@FunctionalInterface
public interface LongToDoubleFunction {
double applyAsDouble(long value);
}
This method, applyAsDouble, takes a long as input and returns a double.
2. Example Usage
Example 1: Using a Lambda Expression
We can use a lambda expression to implement the applyAsDouble method:
package org.kodejava.util.function;
import java.util.function.LongToDoubleFunction;
public class LongToDoubleFunctionExample {
public static void main(String[] args) {
// Example 1: Convert a long to a double (e.g., divide by 2.5)
LongToDoubleFunction longToDouble = (value) -> value / 2.5;
long input = 10L;
double result = longToDouble.applyAsDouble(input);
System.out.println("Input: " + input);
System.out.println("Result: " + result);
}
}
Output:
Input: 10
Result: 4.0
Example 2: Referencing a Method
We can also use a method reference if we have a method that matches the signature of applyAsDouble.
package org.kodejava.util.function;
import java.util.function.LongToDoubleFunction;
public class LongToDoubleFunctionExample2 {
public static void main(String[] args) {
LongToDoubleFunction convertToDouble = LongToDoubleFunctionExample2::convert;
long input = 15L;
double result = convertToDouble.applyAsDouble(input);
System.out.println("Input: " + input);
System.out.println("Result: " + result);
}
public static double convert(long value) {
return value * 1.2; // Example logic
}
}
Output:
Input: 15
Result: 18.0
3. Practical Use Cases
- Mathematical Calculations: Converting
longvalues (like IDs or timestamps) into floating-point representations for calculations. - Unit Conversion: Transforming a value in one unit (e.g., seconds as a
long) to another (e.g., minutes as adouble). - Stream API: We can use
LongToDoubleFunctionwith streams, such asLongStream, for functional programming.
Example 3: Using with Streams
package org.kodejava.util.function;
import java.util.function.LongToDoubleFunction;
import java.util.stream.LongStream;
public class LongToDoubleFunctionExample3 {
public static void main(String[] args) {
LongToDoubleFunction divideByPi = (value) -> value / Math.PI;
LongStream.of(10L, 20L, 30L)
.mapToDouble(divideByPi) // Map each long to a double
.forEach(System.out::println); // Print each result
}
}
Output:
3.183098861837907
6.366197723675814
9.549296585513721
4. Key Points
- It is one of the specialized functional interfaces in Java (
LongToIntFunction,DoubleToLongFunction, etc.) to avoid boxing overhead. - It is a functional interface, so we can use it with lambda expressions, method references, or anonymous classes.
- Commonly used with streams for functional-style processing of numbers.
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