Functional Interfaces & Descriptor

What

A functional descriptor is just the abstract method signature of a functional interface.

• Expresses what data are the parameters and what is the return type
  • 단어 그대로 함수가 어떤 입력값을 받고 어떤 반환값을 주는지에 대한 설명을 람다 표현식 문법으로 표현한 것
• A functional interface is any interface in Java that has exactly one abstract method

Functional Interface	Functional Descriptor (추상 메서드가 어떤 역할을 하는지 간략하게 설명)
Predicate	T → boolean
Consumer	T → void
Function<T,R>	T → R
Supplier	( ) → T
BiPredicate<L, R>	(L, R) → boolean
BiConsumer<L, R>	(T, U) → void
BiFunction<T,U,R>	(T, U) → R
Runnable	() → void

• 출처: https://inpa.tistory.com/entry/🚀-Function-Descriptor [Inpa Dev 👨‍💻:티스토리]
• Function Descriptor
• using generics
  • generics을 잘 쓴 코드는 품질이 높고, 나중에 좋은 회사 갈지 말지 판단하는 기준이 됨
• Use if you want to force the type you want to use
• 기본적으로 만들어진 애들은 편의로 만들어 놓은거임
  • 우리가 똑같은거 또 만들어도 됨

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Example

• They are both: (T, U) -> R, which can be implemented by BiFunction<T,U,R>

@FunctionalInterface
public interface Calculator {
    int compute(int a, int b); 
    // must match Math.max(int, int)
}
 
// same thing with above except written differently
@FunctionalInterface
public interface Calculator<T,U,R>
{
	R calculate(T a, U b);
}
 

Rules

• A lambda expression must match the functional interface’s descriptor — which means it must implement the interface’s abstract method.
• To assign a lambda to a functional interface, the method signature (i.e. parameter types and return type) must match exactly.

With method reference

• Method Reference

import java.util.function.BiFunction;
 
public class Test {
    public static void main(String[] args) {
        // Using custom functional interface
        Calculator c = Math::max;
        System.out.println("Custom Calculator: " + c.compute(10, 20));
 
        // Using built-in BiFunction
        BiFunction<Integer, Integer, Integer> f = Math::max;
        System.out.println("Built-in BiFunction: " + f.apply(10, 20));
    }
}

• With constructors
  • Function<String,Member>로 자동 유추 가능함
  • Image