[Java8 Lambda] Functional Interface

🤹‍♂️Java Brains 강의🤹‍♂️

https://www.youtube.com/playlist?list=PLqq-6Pq4lTTa9YGfyhyW2CqdtW9RtY-I3

 

✨ Lambda vs. Interface Implementations

· Interface

public interface Greeting {
    public void perform();
}

 

· Interface Implementation

public class GreetingImpl implements Greeting{
    @Override
    public void perform() {
        System.out.println("Hello Java8");
    }
}

 

· Main

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

        // Interface Implementation 사용
        GreetingImpl greeting = new GreetingImpl();
        greeting.perform();

        // Lambda 사용
        // 이렇게 쓰려면 interface 가 only one method 이어야 함
        Greeting GreetingFunction = () -> System.out.println("Hello Java8");
    }
}

➡️ By calling the interface method on it, just as if it were as an instance of a class

 

✨ Type Inference

☑️ 컴파일러가 타입을 추론하는 것

public class Main {
    public static void main(String[] args) {
        printLambda(s -> s.length());
    }

    public static void printLambda(StringLengthLambda l) {
        System.out.println(l.getLength("Hello Lambda"));
    }

    interface StringLengthLambda {
        int getLength(String s);
    }
}

 

✨ Functional Interface

☑️ an interface which has only one abstract method

☑️ use for declaring lambda types

☑️ @FunctionalInterface (Optional)

 

· Classic Case

public class Main {
    public static void main(String[] args) {
    
        // Anonymous inner class
        Thread myThread = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("Printed inside Runnable");
            }
        });
        myThread.run();

        // Lambda 의 Functional Interface
        Thread myLambdaThread = new Thread(() -> System.out.println("Printed inside Lambda Runnable"));
        myLambdaThread.run();
    }
}

출력 값

➡️ Lambda behaves like the interface that it expects thread

➡️ Remember, this works because Runnable has a single method

 

The advantage of using an interface is we can use lambdas in place of all those anonymous inner classes and all the other methods signatures which accept the interface you don't have to rewrite them for the new function path
→ a huge advantage when it comes to backward compatibility