Java之函数式接口@FunctionalInterface详解（附源码）

Java之函数式接口@FunctionalInterface详解

函数式接口的定义

在java8中，满足下面任意一个条件的接口都是函数式接口：

1、被@FunctionalInterface注释的接口，满足@FunctionalInterface注释的约束。

2、没有被@FunctionalInterface注释的接口，但是满足@FunctionalInterface注释的约束

@FunctionalInterface注释的约束：

1、接口有且只能有个一个抽象方法，只有方法定义，没有方法体

2、在接口中覆写Object类中的public方法，不算是函数式接口的方法。

比如：

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

函数式接口的实例

1. lambda表达式
2. 方法的引用
3. 已有构造器或方法的引用

public class Main {

    public static void main(String[] args)
            throws ClassNotFoundException,
            IllegalAccessException,
            InstantiationException,
            NoSuchMethodException,
            InvocationTargetException, NoSuchFieldException {
        /**
         * 1、lambda表达式
         * 这种形式最为直观，lambda表达式，接收一个String类型的参数，返回一个String类型的结果。
         * 完全符合函数式接口FunctionInterfaceTest的定义
         */
        FunctionalInterfaceTest fiTest1 = str -> str + " copy";
        /**
         * 2、Main方法当中的functionalInterfaceTestMethod方法接收一个参数，返回一个结果。符合函数式接口
         * FunctionInterfaceTest的定义。
         * 函数式接口只是定义了个方法的约定（接收一个String类型的参数，返回一个String类型的结果），
         * 而对于方法内部进行何种操作则并没有做任何的限制。在这点上，跟java以前的版本中的实现类与接口之间的
         * 关系很类似。不同的是，函数式接口更偏重于计算过程，约束了一个计算过程的输入和输出。
         */
        FunctionalInterfaceTest fiTest2 = Main::functionalInterfaceTestMethod;
        /**
         * 3、构造方法引用
         * 构造函数的结构：接收输入参数，然后返回一个对象。这种约束跟函数式接口的约束很像。
         * 所以只要“输入参数类型”与“输出参数类型”跟FunctionInterfaceTest中的方法约束相同，
         * 就可以创建出FunctionInterfaceTest接口的实例，如下，String的构造方法中有
         * new String(str)的构造方法，所以可以得到实例。
         * 这里存在一个类型推断的问题，JDK的编译器已经帮我们自动找到了只有一个参数，且是String类型的构造方法。
         * 这就是我们直接String::new，没有指定使用哪一个构造方法，却可以创建实例的原因
         */
        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"));
        /**
        输出：
        		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);
    }
}

常用的封装好的函数式接口

分别为Function<T, R>, Cosumer<T>, Predicate<T>, Supplier<T>

/**
 * 常用的函数式接口主要有四种类型，是通过其输入和输出的参数来进行区分的。定义了编码过程中主要的使用场景。
 Function<T,R>
 接收一个T类型的参数，返回一个R类型的结果

 Consumer<T>
 接收一个T类型的参数，不返回值

 Predicate<T>
 接收一个T类型的参数，返回一个boolean类型的结果

 Supplier<T>
 不接受参数，返回一个T类型的结果
 */
Function<String, String> add_postfix = str -> str + "postfix";
Consumer<String> print_string = System.out::println;
Predicate<Integer> judge_positive = n -> n > 0;
Supplier<String> supplier = () -> "supply";
List<String> 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);
/**
	输出：
			sdafef1
			hgjrepjrg1
			adfsg1
			2312431
			1234dfgh
*/

此外，对于多参数的情况，Java还封装了BiFunction<T, U, R>, BiConsumer<T, U>, BiPredicate<T, U>。

// 由于java不能返回多个参数，所以没有BiSupplier
        BiFunction<String, String, String> combine_string = (str1, str2) -> str1 + str2;
        BiConsumer<String, String> print_two_string = (str1, str2) -> System.out.println(str1 + str2);
        BiPredicate<String, String> 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);
/*
		输出：
				6
				true
				false
*/

在此之外，还有compose和andThen方法，其本质就是数学当中的符合函数，唯一的区别：对于函数\(f(x),g(x)\)，compose等价于\(f(g(x))\)，andThen等价于\(g(f(x))\)，就是执行顺序不同而已。

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

// BiPredicate, Predicate 的 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

源码

Function.java

/**
 * Represents a function that accepts one argument and produces a result.
 *
 * <p>This is a <a href="package-summary.html">functional interface</a>
 * whose functional method is {@link #apply(Object)}.
 *
 * @param <T> the type of the input to the function
 * @param <R> the type of the result of the function
 *
 * @since 1.8
 */
@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;
    }
}

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.
 *
 * <p>This is a <a href="package-summary.html">functional interface</a>
 * whose functional method is {@link #accept(Object)}.
 *
 * @param <T> the type of the input to the operation
 *
 * @since 1.8
 */
@FunctionalInterface
public interface Consumer<T> {

    /**
     * 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<T> 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.
 *
 * <p>This is a <a href="package-summary.html">functional interface</a>
 * whose functional method is {@link #test(Object)}.
 *
 * @param <T> the type of the input to the predicate
 *
 * @since 1.8
 */
@FunctionalInterface
public interface Predicate<T> {

    /**
     * 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.
     *
     * <p>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<T> 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<T> 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.
     *
     * <p>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<T> 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 <T> 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 <T> Predicate<T> 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 <T>     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 <T> Predicate<T> not(Predicate<? super T> target) {
        Objects.requireNonNull(target);
        return (Predicate<T>)target.negate();
    }
}

Supplier.java

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

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