자바 의 함수 식 인터페이스 @ Functional 인터페이스 상세 설명 (소스 코드 첨부)

자바 의 함수 식 인터페이스 @ Functional 인터페이스 상세 설명
함수 식 인터페이스의 정의
자바 8 에서 아래 의 임의의 조건 을 만족 시 키 는 인 터 페 이 스 는 모두 함수 식 인터페이스 입 니 다. 1. @ FunctionalInterface 에 의 해 주석 되 는 인터페이스 로 @ FunctionalInterface 주석 에 대한 제약 을 만족 시 킵 니 다.2. @ FunctionalInterface 에 주석 이 달 린 인터페이스 가 없 지만 @ FunctionalInterface 주석 에 대한 제약 을 만족 시 킵 니 다 @ FunctionalInterface 주석 에 대한 제약: 1. 인터페이스 가 있 고 추상 적 인 방법 만 있 을 수 있 습 니 다. 방법 만 정의 할 수 있 습 니 다. 방법 체 2. 인터페이스 에 Object 류 중의 Public 방법 을 복사 하 는 것 은 함수 식 인터페이스 방법 이 아 닙 니 다.
예 를 들 면:

@FunctionalInterface
interface FunctionalInterfaceTest {
    String getInfo(String input);
}

함수 식 인터페이스의 실례

lambda 표현 식

방법의 인용

이미 구조 기 나 방법의 인용

public class Main {

    public static void main(String[] args)
            throws ClassNotFoundException,
            IllegalAccessException,
            InstantiationException,
            NoSuchMethodException,
            InvocationTargetException, NoSuchFieldException {
        /**
         * 1、lambda   
         *         ，lambda   ，    String     ，    String     。
         *          FunctionInterfaceTest   
         */
        FunctionalInterfaceTest fiTest1 = str -> str + " copy";
        /**
         * 2、Main     functionalInterfaceTestMethod        ，      。       
         * FunctionInterfaceTest   。
         *                 （    String     ，    String     ），
         *                        。    ， java                
         *      。    ，             ，               。
         */
        FunctionalInterfaceTest fiTest2 = Main::functionalInterfaceTestMethod;
        /**
         * 3、      
         *        ：      ，        。               。
         *     “      ” “      ” FunctionInterfaceTest        ，
         *       FunctionInterfaceTest     ，  ，String       
         * new String(str)     ，        。
         *              ，JDK                    ，  String       。
         *        String::new，             ，          
         */
        FunctionalInterfaceTest fiTest3 = String::new;

        System.out.println(useFunctionalInterface("Hello World!", fiTest1));
        System.out.println(useFunctionalInterface("Hello World!", fiTest2));
        System.out.println(useFunctionalInterface("Hello World!", fiTest3));
        System.out.println(useFunctionalInterface("Hello World!", str -> str + " created by lambda in the context"));
        /**
          ：
                Hello World! copy
                        Hello World! copy 2 by reference
            Hello World!
                        Hello World! created by lambda in the context
        */
    }
    
    public static String functionalInterfaceTestMethod(String str) {
        return str + " copy 2 by reference";
    }

    public static String useFunctionalInterface(String str, FunctionalInterfaceTest fiT) {
        return fiT.getInfo(str);
    }
}

상용 봉 인 된 함수 식 인터페이스
각각 Function, Cosumer, Predicate, Supplier

/**
 *                ，                  。               。
 Function
     T     ，    R     

 Consumer
     T     ，    

 Predicate
     T     ，    boolean     

 Supplier
      ，    T     
 */
Function add_postfix = str -> str + "postfix";
Consumer print_string = System.out::println;
Predicate judge_positive = n -> n > 0;
Supplier supplier = () -> "supply";
List list = Arrays.asList("adfsg", "sdafef", "", "s", "231243", "hgjrepjrg");
list.stream()
        .map(str -> str + "1")
        .filter(str -> str.length() > 2)
        .sorted((str1, str2) -> str2.compareTo(str1))
        .forEach(System.out::println);
/**
      ：
            sdafef1
            hgjrepjrg1
            adfsg1
            2312431
            1234dfgh
*/

이 밖 에 다 중 인자 의 경우 자바 가 BiFunction, BiConsumer, BiPredicate 를 봉인 했다.

//   java        ，    BiSupplier
        BiFunction combine_string = (str1, str2) -> str1 + str2;
        BiConsumer print_two_string = (str1, str2) -> System.out.println(str1 + str2);
        BiPredicate str_equal = String::equals;
        int bif_result = biFunctionTestMethod("abs", "pdf", (str1, str2) -> str1.length() + str2.length());
        biConsumerTestMethod("1234", "dfgh", (str1, str2) -> System.out.println(str1 + str2));
        boolean bip_result_1 = biPredictTestMethod("abc", "abc", str_equal),
                bip_result_2 = biPredictTestMethod("abc", "def", str_equal);
        System.out.println(bif_result);
        System.out.println(bip_result_1);
        System.out.println(bip_result_2);
/*
          ：
                6
                true
                false
*/

그 밖 에 compose 와 andThen 방법 도 있 습 니 다. 그 본질은 수학 에서 함수 에 부합 되 는 것 입 니 다. 유일한 차이 점 은 함수 \ (f (x), g (x) \), compose 등 가 는 \ (f (g (x) \), andThen 등 가 는 \ (g (f (x) \) 와 같 습 니 다. 실행 순서 가 다 를 뿐 입 니 다.

// compose   andThen
Function compose_function = ((Function) (str -> str + "abc")).compose((Function) (str -> str + str.length()));
System.out.println("Compose function: " + compose_function.apply("Hello World! "));
Function andThen_function = ((Function) (str -> str + "abc")).andThen((Function) (str -> str + str.length()));
System.out.println("AndThen function: " + andThen_function.apply("Hello World! "));
// Bicosumer, cosumer, bifunction       

// BiPredicate, Predicate   and, or, negate
System.out.println(str_equal.negate().test("a", "a")); // false
System.out.println(judge_positive.and(n -> n > 2).test(5)); // true
System.out.println(judge_positive.or(n -> n < -1).test(-10)); // true

소스 코드
Function.java

/**
 * Represents a function that accepts one argument and produces a result.
 *
 * 
This is a functional interface
 * whose functional method is {@link #apply(Object)}.
 *
 * @param  the type of the input to the function
 * @param  the type of the result of the function
 *
 * @since 1.8
 */
@FunctionalInterface
public interface Function {

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

Consumer.java

/**
 * Represents an operation that accepts a single input argument and returns no
 * result. Unlike most other functional interfaces, {@code Consumer} is expected
 * to operate via side-effects.
 *
 * 
This is a functional interface
 * whose functional method is {@link #accept(Object)}.
 *
 * @param  the type of the input to the operation
 *
 * @since 1.8
 */
@FunctionalInterface
public interface Consumer {

    /**
     * Performs this operation on the given argument.
     *
     * @param t the input argument
     */
    void accept(T t);

    /**
     * Returns a composed {@code Consumer} that performs, in sequence, this
     * operation followed by the {@code after} operation. If performing either
     * operation throws an exception, it is relayed to the caller of the
     * composed operation.  If performing this operation throws an exception,
     * the {@code after} operation will not be performed.
     *
     * @param after the operation to perform after this operation
     * @return a composed {@code Consumer} that performs in sequence this
     * operation followed by the {@code after} operation
     * @throws NullPointerException if {@code after} is null
     */
    default Consumer andThen(Consumer super T> after) {
        Objects.requireNonNull(after);
        return (T t) -> { accept(t); after.accept(t); };
    }
}

Predicate.java

/**
 * Represents a predicate (boolean-valued function) of one argument.
 *
 * 
This is a functional interface
 * whose functional method is {@link #test(Object)}.
 *
 * @param  the type of the input to the predicate
 *
 * @since 1.8
 */
@FunctionalInterface
public interface Predicate {

    /**
     * Evaluates this predicate on the given argument.
     *
     * @param t the input argument
     * @return {@code true} if the input argument matches the predicate,
     * otherwise {@code false}
     */
    boolean test(T t);

    /**
     * Returns a composed predicate that represents a short-circuiting logical
     * AND of this predicate and another.  When evaluating the composed
     * predicate, if this predicate is {@code false}, then the {@code other}
     * predicate is not evaluated.
     *
     * 
Any exceptions thrown during evaluation of either predicate are relayed
     * to the caller; if evaluation of this predicate throws an exception, the
     * {@code other} predicate will not be evaluated.
     *
     * @param other a predicate that will be logically-ANDed with this
     *              predicate
     * @return a composed predicate that represents the short-circuiting logical
     * AND of this predicate and the {@code other} predicate
     * @throws NullPointerException if other is null
     */
    default Predicate and(Predicate super T> other) {
        Objects.requireNonNull(other);
        return (t) -> test(t) && other.test(t);
    }

    /**
     * Returns a predicate that represents the logical negation of this
     * predicate.
     *
     * @return a predicate that represents the logical negation of this
     * predicate
     */
    default Predicate negate() {
        return (t) -> !test(t);
    }

    /**
     * Returns a composed predicate that represents a short-circuiting logical
     * OR of this predicate and another.  When evaluating the composed
     * predicate, if this predicate is {@code true}, then the {@code other}
     * predicate is not evaluated.
     *
     * 
Any exceptions thrown during evaluation of either predicate are relayed
     * to the caller; if evaluation of this predicate throws an exception, the
     * {@code other} predicate will not be evaluated.
     *
     * @param other a predicate that will be logically-ORed with this
     *              predicate
     * @return a composed predicate that represents the short-circuiting logical
     * OR of this predicate and the {@code other} predicate
     * @throws NullPointerException if other is null
     */
    default Predicate or(Predicate super T> other) {
        Objects.requireNonNull(other);
        return (t) -> test(t) || other.test(t);
    }

    /**
     * Returns a predicate that tests if two arguments are equal according
     * to {@link Objects#equals(Object, Object)}.
     *
     * @param  the type of arguments to the predicate
     * @param targetRef the object reference with which to compare for equality,
     *               which may be {@code null}
     * @return a predicate that tests if two arguments are equal according
     * to {@link Objects#equals(Object, Object)}
     */
    static  Predicate isEqual(Object targetRef) {
        return (null == targetRef)
                ? Objects::isNull
                : object -> targetRef.equals(object);
    }

    /**
     * Returns a predicate that is the negation of the supplied predicate.
     * This is accomplished by returning result of the calling
     * {@code target.negate()}.
     *
     * @param      the type of arguments to the specified predicate
     * @param target  predicate to negate
     *
     * @return a predicate that negates the results of the supplied
     *         predicate
     *
     * @throws NullPointerException if target is null
     *
     * @since 11
     */
    @SuppressWarnings("unchecked")
    static  Predicate not(Predicate super T> target) {
        Objects.requireNonNull(target);
        return (Predicate)target.negate();
    }
}

Supplier.java

/**
 * Represents a supplier of results.
 *
 * 
There is no requirement that a new or distinct result be returned each
 * time the supplier is invoked.
 *
 * 
This is a functional interface
 * whose functional method is {@link #get()}.
 *
 * @param  the type of results supplied by this supplier
 *
 * @since 1.8
 */
@FunctionalInterface
public interface Supplier {

    /**
     * Gets a result.
     *
     * @return a result
     */
    T get();
}