[Java/Android] Java8 Lambda 사용법

Java8 Lambda

Java8에서 Lambda 표현식을 지원합니다.

이전 버전에서는 익명 클래스로 구현해야했습니다.

build.gradle 설정

compileOptions {
    sourceCompatibility JavaVersion.VERSION_1_8
    targetCompatibility JavaVersion.VERSION_1_8
}

예를들어 버튼에 클릭리스너를 달아준다고 했을 때 보통 아래처럼 사용합니다.

btn.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {
        
    }
});

Lambda를 쓴다면?

btn.setOnClickListener( 
        v -> {
            
        }
);

btn.setOnClickListener(
        (View v) -> {

        }
);

Runnable runnable = () -> {
    System.out.println("Hello Runnable");
};
new Thread(runnable).start();

new Thread(
        () -> System.out.println("Hello Runnable2")
).start();

new Thread(
        () -> { System.out.println("Hello Runnable3"); }
).start();

위와 같이 Lambda 표현법은 여러가지가 있습니다.

( ) -> { }

( a ) -> { }

( int a ) -> { }

a -> { }

() -> { System.out.println("Hello"); }

() -> System.out.println("Hello")

하지만 Lambda 표현식을 쓰기 위해서 제약 조건이 있습니다.

1개의 메소드를 가진 Interface 이어야 합니다.

RxJava2 + Lambda 여러가지 사용 예시

public void emit() {
    Observable<String> source = Observable.just("RED", "GREEN", "YELLOW");

    Disposable d = source.subscribe(
            v -> System.out.println("onNext() : v : " + v),
            err -> System.err.println("onError() : err : " + err.getMessage()),
            () -> System.out.println("onComplete()")
    ) ;

    System.out.println("isDisposed() : " + d.isDisposed());
}

public void emit2() {
    /*ObservableOnSubscribe observableOnSubscribe = new ObservableOnSubscribe() {
        @Override
        public void subscribe(ObservableEmitter emitter) throws Exception {

        }
    };*/

    Observable<Integer> source = Observable.create(
            emitter -> {
                emitter.onNext(100);
                emitter.onNext(200);
                emitter.onNext(300);
                emitter.onNext(400);
                emitter.onComplete();
            }
    );

    Disposable d = source.subscribe(
            v -> System.out.println("onNext() : " + v)
    );
}

public void emit3() {
    int[] arr = {100, 200, 300};
    Observable.fromArray(toIntegerArray(arr))
            .subscribe(data -> System.out.println(data));
}

private static Integer[] toIntegerArray(int[] intArray) {
    Integer[] integerArray = new Integer[intArray.length];
    for(int i = 0; i < intArray.length; i++) {
        integerArray[i] = intArray[i];
    }
    return integerArray;

}

public void emit4() {
    List<String> names = new ArrayList<>();
    names.add("Jerry");
    names.add("Kim");
    names.add("Joy");

    Observable.fromIterable(names)
            .subscribe(System.out::println);
}

public void emit5() {
    Callable<String> callable = () -> {
        Thread.sleep(1000);
        return "Hello Callable";
    };

    Observable.fromCallable(callable)
            .doOnComplete( () -> System.out.println("Hello Callable complete!"))
            .subscribe(System.out::println);
}

public void emit6() {
    Observable.fromCallable(() -> {return "Hello Callable";})
            .subscribe(System.out::println);
}

public void emit8() {
    Future<String> future = Executors.newSingleThreadExecutor().submit(
            () -> {
                Thread.sleep(1000);
                return "Hello Future";
            }
    );
    Observable.fromFuture(future)
            .subscribe(System.out::println);
}

public void emit9() {

    Observable.fromFuture(Executors.newSingleThreadExecutor().submit( () -> {
                return "Hello Futrue2";
            })).subscribe(System.out::println);
}

public void emit10() {
    /*Publisher<String> publisher = new Publisher<String>() {
        @Override
        public void subscribe(Subscriber<? super String> s) {
            s.onNext("Hello Observable.fromPublisher()");
            s.onComplete();
        }
    }*/
    
    Publisher<String> publisher = (Subscriber<? super String> s) -> {
        s.onNext("Hello Observable.fromPublisher()");
        s.onComplete();
    };
    Observable<String> source = Observable.fromPublisher(publisher);
    source.subscribe(System.out::println);
}

참고

Lambda 와 RetroLambda by 정상욱님