Java8 functional interface

Functional Interface

• Abstract Method가 하나인 인터페이스
• 람다식 사용이 가능
• FunctionalInterface annotation을 안붙여도 되지만, 붙이는 것을 권장

Java SDK 8의 java.util.function 패키지에는 수많은 Functional Interface들이 등록있다.

대표적인 finctional interface를 구현해본 코드

 

Cunsumer

import java.util.function.Consumer;

public class ConsumerFunction {

    private static void consumerExamples() {

        final Consumer<String> print = value -> System.out.println(value);
        print.accept("Hello");

        final Consumer<String> greetings = value -> System.out.println("Hello " + value);
        greetings.accept("World");
        greetings.accept("JungHoon");
    }

    public static void main(final String[] args) {
        consumerExamples();
    }
}

Identify

public class IdentityFunction {
    public static void main(String[] args){
        List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5 , 6);
        System.out.println("map (numbers, i -> i * 2)");
        System.out.println(map(numbers, i -> i * 2));

        // map 기능이 필요 없을 때 null로 구분하고 싶다
        System.out.println("map (numbers, null)");
        System.out.println(map(numbers, i -> i));
        System.out.println(map(numbers, Function.identity()));
    }

    private static <T, R> List<R> map(final List<T> list, final Function<T, R> mapper){
        List<R> result = new ArrayList<>();
        for(final T t : list){
            result.add(mapper.apply(t));
        }
        return result;
    }
}

 

 

Supplier

public class LazyEvaluationSupplier {
    public static void main(String[] args){
        long startTime = System.currentTimeMillis();
        printIfValidIndex(0, getVeryExpensiveValue());
        printIfValidIndex(-1, getVeryExpensiveValue());
        printIfValidIndex(-3, getVeryExpensiveValue());
        System.out.println("It took " + ((System.currentTimeMillis() - startTime) / 1000));

        startTime = System.currentTimeMillis();
        printIfValidIndexWithSupplier(0, () -> getVeryExpensiveValue());
        printIfValidIndexWithSupplier(-1, () -> getVeryExpensiveValue());
        printIfValidIndexWithSupplier(-3, () -> getVeryExpensiveValue());
        System.out.println("It took " + ((System.currentTimeMillis() - startTime) / 1000));
    }

    private static String getVeryExpensiveValue() {
        try{
            TimeUnit.SECONDS.sleep(3);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        return "PJH";
    }

    //number 조건이 맞지 않아도 getVeryExpensiveValue method를 parameter 로 전달하기 위해 호출함
    private static void printIfValidIndex(int number, String value){
        if(number >= 0)
            System.out.println("value : " + number);
        else
            System.out.println("Invalid");
    }

    //number 조건이 맞지않을때 불필요한 getVeryExpensiveValue method 호출을 하지 않음
    private static void printIfValidIndexWithSupplier(int number, Supplier<String> valueSupplier){
        if(number >= 0)
            System.out.println("value : " + number);
        else
            System.out.println("Invalid");
    }
}

Predicate

public class PredicateFunction {
    public static void main(String[] args){
        Predicate<Integer> lessThan3 =  number -> number < 3;
        Predicate<Integer> moreThan3 =  number -> number > 3;

        List<Integer> IntegerArray = Arrays.asList(1,2,3,4,1,2,3,5,6,3,1,2);

        filter(IntegerArray, lessThan3).forEach( i -> System.out.print(i + " "));
        System.out.println("");
        filter(IntegerArray, moreThan3).forEach( i -> System.out.print(i + " "));
    }

    private static <T> List<T> filter(List<T> list, Predicate<T> filterFunction){
        List<T> resultArrays = new ArrayList<>();
        for(T value : list){
            if(filterFunction.test(value)) {
                resultArrays.add(value);
            }
        }
        return resultArrays;
    }
}

 

git: https://github.com/junghun133/PracticeJava8