java8 functionalInterface Function

Java 8 FunctionalInterface 1/4 Function

Function 예제

/**
 *  Function, The Transformer
 *  function consumer predicate supplier
 */
/*
//abstract class instance
Function<String, Integer> toInt = new Function<String, Integer>(){
    @Override
    public Integer apply(String value){
        return Integer.parseInt(value);
    }
};
int result = toInt.apply("100");
System.out.println(result);
*/


//mapper, String -> Integer
Function<String, Integer> toInt = value->Integer.parseInt(value);   //Lambda Expression
int result = toInt.apply("123");
System.out.println(result);//output : 123


//같은 Type을 return 하면 Identity라고 한다.
/**
 *  함수형프로그래밍에서는 함수라는것 자체가 First Class Citizen이기 때문에
 *  parameter로만 받는게 아니라 return 값으로도 함수를 쓸 수 있다.
 */
//Function<Integer, Integer> identity = Function.identity();    //java.util.function에 있는 기본함수
Function<Integer, Integer> identity = t -> t;                   //Lambda를 이용해 구현한 identity function
System.out.println(identity.apply(200));//output:200

Function.java 내용

@FunctionalInterface
public interface Function<T, R> {

    /**
     * Applies this function to the given argument.
     *
     * @param t the function argument
     * @return the function result
     */
    R apply(T t);

    /**
     * Returns a composed function that first applies the {@code before}
     * function to its input, and then applies this function to the result.
     * If evaluation of either function throws an exception, it is relayed to
     * the caller of the composed function.
     *
     * @param <V> the type of input to the {@code before} function, and to the
     *           composed function
     * @param before the function to apply before this function is applied
     * @return a composed function that first applies the {@code before}
     * function and then applies this function
     * @throws NullPointerException if before is null
     *
     * @see #andThen(Function)
     */
    default <V> Function<V, R> compose(Function<? super V, ? extends T> before) {
        Objects.requireNonNull(before);
        return (V v) -> apply(before.apply(v));
    }

    /**
     * Returns a composed function that first applies this function to
     * its input, and then applies the {@code after} function to the result.
     * If evaluation of either function throws an exception, it is relayed to
     * the caller of the composed function.
     *
     * @param <V> the type of output of the {@code after} function, and of the
     *           composed function
     * @param after the function to apply after this function is applied
     * @return a composed function that first applies this function and then
     * applies the {@code after} function
     * @throws NullPointerException if after is null
     *
     * @see #compose(Function)
     */
    default <V> Function<T, V> andThen(Function<? super R, ? extends V> after) {
        Objects.requireNonNull(after);
        return (T t) -> after.apply(apply(t));
    }

    /**
     * Returns a function that always returns its input argument.
     *
     * @param <T> the type of the input and output objects to the function
     * @return a function that always returns its input argument
     */
    static <T> Function<T, T> identity() {
        return t -> t;
    }

예제파일 : https://github.com/sungjaeHong/Java8-example