ToDoubleBiFunction Interface in Java with Examples

The ToDoubleBiFunction is a functional interface from the java.util.function package introduced in Java 8. It represents a function that accepts two input arguments and returns a primitive double value. It is useful when an operation requires two inputs and the result specifically needs to be a double.

Example: Following example uses ToDoubleBiFunction to multiply the second input by 3.2 and add the result to the first input.

import java.util.function.ToDoubleBiFunction;

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

        ToDoubleBiFunction<Integer, String> operation =
                (a, b) -> a + Integer.parseInt(b) * 3.2;

        double result = operation.applyAsDouble(3, "10");

        System.out.println(result);
    }
}

35.0

Explanation: The function accepts an Integer and a String. The lambda expression converts "10" to an integer, multiplies it by 3.2, and adds 3, producing 35.0.

Syntax

@FunctionalInterface
public interface ToDoubleBiFunction<T, U> {
double applyAsDouble(T t, U u);
}

Here:

• T represents the type of the first input argument.
• U represents the type of the second input argument.
• applyAsDouble() applies the defined operation to both arguments and returns a primitive double.

How ToDoubleBiFunction Works

ToDoubleBiFunction takes two input values, applies the defined operation, and returns the result as a primitive double.

• Define Input Types: T represents the first input type, while U represents the second input type.
• Create the Function: A lambda expression or method reference defines the operation performed on both inputs.
• Call applyAsDouble(): The method receives the two input values and executes the defined operation.
• Return the Result: The operation produces and returns a primitive double value.

For example:

ToDoubleBiFunction<Double, Integer> totalPrice =
(price, quantity) -> price * quantity;

double result = totalPrice.applyAsDouble(99.5, 4);
System.out.println(result);

Output:

398.0

Here, the function accepts a Double representing the price and an Integer representing the quantity. It multiplies the two values and returns 398.0 as a primitive double.

Method Used By ToDoubleBiFunction

applyAsDouble()

The applyAsDouble() method applies the defined operation to two input arguments and returns the resulting double value.

Syntax

double applyAsDouble(T t, U u)

Parameters:

• t – the first input argument of type T.
• u – the second input argument of type U.

Returns: A double result produced by the function.

import java.util.function.ToDoubleBiFunction;

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

        ToDoubleBiFunction<Integer, Double> calculateCost =
                (quantity, price) -> quantity * price;

        double result = calculateCost.applyAsDouble(5, 49.5);

        System.out.println(result);
    }
}

247.5

Explanation: The function takes 5 as the quantity and 49.5 as the price. The lambda expression multiplies these values, and applyAsDouble() returns 247.5.

ToDoubleBiFunction Vs BiFunction

ToDoubleBiFunction<T, U> is similar to BiFunction<T, U, Double>, but ToDoubleBiFunction directly returns the primitive double type.