22.12. Custom Functional Interface Example

@FunctionalInterface interface MathOperation { int operate(int a, int b); }

In Java, a functional interface is an interface that contains exactly one abstract method. This example defines a custom functional interface named MathOperation. By using the @FunctionalInterface annotation, we indicate that this interface is intended to be used with lambda expressions or method references. The method operate(int a, int b) allows subclasses or lambda expressions to implement the behavior of mathematical operations, which, in this case, will require two integer parameters.

Imagine you have a single button that performs different mathematical operations based on how it is used. A MathOperation interface acts like that button—it accepts different lambda expressions as behaviors but only allows one method to execute at a time.

public class LambdaExample { public static void main(String[] args) { MathOperation addition = (a, b) -> a + b; MathOperation multiply = (a, b) -> a * b; System.out.println("Addition: " + addition.operate(5, 3)); System.out.println("Multiplication: " + multiply.operate(5, 3)); } }

In the main method of the LambdaExample class, two instances of the MathOperation interface are created using lambda expressions. The first instance addition defines a lambda that takes two integers and returns their sum. The second instance multiply takes two integers and returns their product. The results of both operations are printed to the console by calling the operate method on each instance with the integers 5 and 3.

Consider a restaurant where chefs can create different dishes based on the same ingredients. Here, the ingredients are the integers (5 and 3), and the dishes are the operations: addition or multiplication. Each chef (lambda expression) takes the same ingredients and creates different outcomes, just like the addition and multiply operations produce different results using the same numbers.

System.out.println("Addition: " + addition.operate(5, 3)); System.out.println("Multiplication: " + multiply.operate(5, 3));

The two System.out.println statements display the results of the operations on the integers 5 and 3. The first line outputs the result of the addition operation, while the second one outputs the result of the multiplication operation. Based on the lambda expressions defined earlier, the output will be: 'Addition: 8' for the addition and 'Multiplication: 15' for the multiplication.

If you think of a simple calculator, each operation yields a different result. In this scenario, the calculator performs two operations: one addition and one multiplication. When you input 5 and 3, the calculator shows you 8 for addition and 15 for multiplication, illustrating how different operations can yield distinct results from the same inputs.

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