Java8新特性04 内置功能接口

内置功能接口借鉴了Google Guava库的实践。Java8的内置功能接口在java.util.function包下，主要有四大核心的功能性接口：谓词（predicate）、函数（function）、生产者（supplier）、消费者（consumer）

一. 谓词（Predicate)

这里是谓词的源码：

/*
 * Copyright (c) 2010, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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package java.util.function;

import java.util.Objects;

/**
 * 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);
    }
}

说明：谓词是单参数的返回布尔值的函数式接口，输入一个函数，返回true或者false。同时该接口也包含多个默认方法或静态方法使用谓词转换成复杂的逻辑表达式（与、或、非等）

举例：

package com.test.a;

import java.util.function.Predicate;

public class Test {
    public static void main(String args[]) {
        
       String str="hello";
       
       //1.测试抽象方法
       Predicate<String> predicate1=(String s)->s.length()>6;
       boolean result1=predicate1.test(str);
       System.out.println(result1);//false
       
       //2.使用 default Predicate<T> and(Predicate<? super T> other)
       Predicate<String> predicate2=(String s)->s.length()==5;
       boolean result2=predicate1.and(predicate2).test(str);
       System.out.println(result2);//false
       
       //3.使用 default Predicate<T> or(Predicate<? super T> other) 
       boolean result3=predicate1.or(predicate2).test(str);
       System.out.println(result3);//true
       
       //4.使用default Predicate<T> negate()
       boolean result4=predicate1.negate().test(str);
       System.out.println(result4);//true
       
       //5.使用static <T> Predicate<T> isEqual(Object targetRef)
       Predicate<String> predicate5=Predicate.isEqual("hello2");
       boolean result5=predicate5.test(str);
       System.out.println(result5);//false
 
    }
}

二. 函数（Function）

这是函数的源码：

/*
 * Copyright (c) 2010, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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package java.util.function;

import java.util.Objects;

/**
 * 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;
    }
}

说明：Function接受一个参数，并产生一个结果。默认方法可以将多个函数串在一起（compose，andThen)

default <V> Function<V, R> compose(Function<? super V, ? extends T> before)返回一个先执行before函数对象apply方法，再执行当前函数对象apply方法的函数对象；

default <V> Function<T, V> andThen(Function<? super R, ? extends V> after)返回一个先执行当前函数对象apply方法，再执行after函数对象apply方法的函数对象；

举例：

package com.test.a;

import java.util.function.Function;

public class Test {
    public static void main(String args[]) {
  
        String str="hello";
        Function<String, Integer> function=(String)->2;
        Integer result=function.apply(str);
        System.out.println(result);
        
        Function<Integer, Integer> function2=(Integer i )->i*2;
        Function<Integer, Integer> function3=(Integer j )->j*j;
        int result2=function2.compose(function3).apply(2);//先算function3的结果，再算function2的结果
        System.out.println(result2);
        int result3=function2.andThen(function3).apply(2);//先算function2的结果，再算function3的结果
        System.out.println(result3);
        
    
    }
}

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 三. 生产者（Supplier)

这是生产者的源码：

/*
 * Copyright (c) 2012, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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package java.util.function;

/**
 * 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();
}

说明：Suppliers产生一个给定的泛型类型的结果。与Function不同的是Suppliers不接受输入参数。

举例：

import java.util.function.Supplier;

public class Test {
    public static void main(String args[]) {
  
        Supplier<String> supplier=()->"hello";
        System.out.println(supplier.get());
    
    }
}


hello

note:上面的String可以改成具体的对象，比如person对象，从而调用对象的set和get方法等操作

四. 消费者（Consumer）

这是消费者的源码：

/*
 * Copyright (c) 2010, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
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 */
package java.util.function;

import java.util.Objects;

/**
 * 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); };
    }
}

说明：Consumers代表一个单一的输入参数上执行操作。Consumer的操作可能会更改输入参数的内部状态。

举例：

package com.test.a;

import java.util.function.Consumer;

public class Test {
    public static void main(String args[]) {

        Consumer<Person> consumer = (Person person) -> person.setAge(23);
        Person person = new Person();
        person.setAge(25);
        consumer.accept(person);
        System.out.println(person.getAge());
    }
}


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