java 8之函数式接口

Java 8中函数式接口是一大新特性。本文就来说说Java 8 中的几种函数式接口。
一、Function接口
Function接口就是将传入的参数，通过一个或多个方法的迭代，输出和参数类型一样或者不一样的结果。
示例：

// Function<T, R>
//toInteger的Function中的T类型必须与toString的Function中的T类型相同，
Function<String, Integer> toInteger = s -> {
    System.out.println("******");
    return Integer.valueOf(s);
};
Function<String, Integer> toString = toInteger.andThen(Integer::valueOf);
//toObj的Function中的R类型必须与toString的Function中的R类型相同，compose方法中的参数类型必须与toString的Function中的T类型相同
Function<String, Integer> toObj = toString.compose(String::valueOf);
Integer a = toInteger.apply("3");
Integer b = toString.apply("123");
System.out.println(a);
System.out.println(b);
System.out.println(toObj.apply("123"));

Function接口源码：

@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;
    }
}

从源码中可以看出，先执行 Function<T, R> apply(t) = R，然后再执行Function<R, V> apply(R) = V。忽略过程，传入的参数为T，输出的结果为V。
我的理解为：假设n个andThen()迭代，那么就是不断用输出结果继续作为输入进行操作，最后一个出口的结果作为最终结果。

从源码上可以看出，先执行Function<V, T> apply(v) = T，然后再执行Function<T, R> apply(T) = R。忽略过程，初始传入的参数为T，输出的结果为R。
我的理解为：假设n个compose()迭代，那么就是为了得到R,前面的apply()不断输出，直至得到T。

static <T> Function<T, T> identity()：返回一个输入和输出都为T的Function对象

二、Predicate接口
Predicate接口是起判断的作用，
示例：

Predicate<String> predicate = s -> s.length() > 0;
Predicate<String> predicate1 = s -> s.length() > 1;
boolean flag = predicate1.and(predicate).test("ss");
boolean flag1 = predicate.negate().test("ss");
System.out.println(flag);
System.out.println(flag1);

源码分析：

@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);
    }
}

test()方法相当于判断输入参数是否满足我们所约定的条件，返回true或false。
and()方法是要求同时满足多个条件。
or()方法是要求满足其中一个条件。
negate()方法是对test()方法的结果取反。

三、Supplier接口
Supplier接口主要用来返回一个任意范型的值，和Function接口不同的是该接口没有任何参数。
示例：

Supplier<Car> redCar = Car::new;
System.out.println(redCar.get());

四、Consumer接口
Consumer接口表示一个接受单个输入参数并且没有返回值的操作，可能会更改输入参数的内部状态
示例：

Car car = new Car();
Consumer<Car> blueCar = s -> s.setColor("blue");
blueCar.accept(car);
System.out.println(car.getColor());

返回结果：blue

java 8之函数式接口

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