Generic Functional Interfaces

Main concepts

• Lambda expressions cannot be generic: You cannot specify type parameters directly in a lambda expression.
• Functional interfaces can be generic: The target type of a lambda expression is determined by the type arguments specified in the functional interface reference.
• Code reuse: Generic interfaces eliminate the need to create multiple functional interfaces for different types of data, allowing you to reuse logic with different types.

Practical example: SomeTest generic functional interface
The SomeTest generic interface can be used to implement lambdas with different types (e.g. Integer, Double, String).

Code

// Interface funcional genérica com dois parâmetros que retorna um boolean
interface SomeTest<T> {
    boolean test(T n, T m);
}

public class GenericFunctionalInterfaceDemo {
    public static void main(String[] args) {
        // Expressão lambda para verificar se um inteiro é fator de outro
        SomeTest<Integer> isFactor = (n, d) -> (n % d) == 0;
        if (isFactor.test(10, 2)) {
            System.out.println("2 is a factor of 10");
        }

        // Expressão lambda para verificar se um Double é fator de outro
        SomeTest<Double> isFactorD = (n, d) -> (n % d) == 0;
        if (isFactorD.test(212.0, 4.0)) {
            System.out.println("4.0 is a factor of 212.0");
        }

        // Expressão lambda para verificar se uma string faz parte de outra
        SomeTest<String> isIn = (a, b) -> a.indexOf(b) != -1;
        String str = "Generic Functional Interface";
        System.out.println("Testing string: " + str);
        if (isIn.test(str, "face")) {
            System.out.println("'face' is found.");
        } else {
            System.out.println("'face' not found.");
        }
    }
}

Exit

2 is a factor of 10
4.0 is a factor of 212.0
Testing string: Generic Functional Interface
'face' is found.

Explanation of the example
Generic interface definition:

interface SomeTest<T> {
    boolean test(T n, T m);
}

T is the generic type that will be replaced by specific types (Integer, Double, String, etc.).

Use of lambda expressions:

• The first lambda (isFactor) checks whether an integer is divisible by another.
• The second lambda (isFactorD) performs the same operation, but with numbers of type Double.
• The third lambda (isIn) checks if a string contains another string.

Reuse of the same interface:
The SomeTest interface is used with different data types (Integer, Double, String), demonstrating the flexibility of generics.

Benefits of Generic Functional Interfaces

• Redundant code reduction: A single generic interface replaces several specific ones.
• Flexibility: Adaptation to different types of data with the same logical behavior.
• Compatibility with lambda expressions: Allows you to build reusable and more concise lambdas.

Suggested exercise
Redesign the previously mentioned NumericFunc functional interface as a generic interface. Use it to perform operations such as calculating the square of a number or checking whether a number is even.

Full Example: Refactoring NumericFunc as Generic

// Interface funcional genérica com um parâmetro e um retorno
interface NumericFunc<T> {
    T apply(T n);
}

public class GenericNumericFuncDemo {
    public static void main(String[] args) {
        // Lambda para calcular o quadrado de um número inteiro
        NumericFunc<Integer> square = (n) -> n * n;
        System.out.println("Quadrado de 5: " + square.apply(5)); // Saída: 25

        // Lambda para calcular o quadrado de um número Double
        NumericFunc<Double> squareDouble = (n) -> n * n;
        System.out.println("Quadrado de 3.14: " + squareDouble.apply(3.14)); // Saída: 9.8596

        // Lambda para verificar se um número é par (retorna "Par" ou "Ímpar")
        NumericFunc<Integer> isEven = (n) -> (n % 2 == 0) ? n  :  -1;
        System.out.println("Resultado Numero pares de : Resultado com multiplos valores "  + isEven.apply(15)) ;

    }
}

Output:

Quadrado de 5: 25
Quadrado de 3.14: 9.8596
Resultado Número pares de : Resultado com múltiplos valores -1

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