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函数式接口

作者：xcbeyond
疯狂源自梦想，技术成就辉煌！微信公众号：《程序猿技术大咖》号主，专注后端开发多年，拥有丰富的研发经验，乐于技术输出、分享，现阶段从事微服务架构项目的研发工作，涉及架构设计、技术选型、业务研发等工作。对于Java、微服务、数据库、Docker有深入了解，并有大量的调优经验。

一、函数式接口
函数式接口（functional Interface），有且仅有一个抽象方法的接口，但可以有多个非抽象的方法。

适用于Lambda表达式使用的接口。如创建线程：

new Thread(() -> System.out.println(Thread.currentThread().getName())).start();
1
其中，Lambda表达式代替了new Runnable()，这里的Runable接口就属于函数式接口，最直观的体现是使用了
@FunctionalInterface注解，而且使用了一个抽象方法(有且仅有一个)，如下：

package java.lang;

/**
* The <code>Runnable</code> interface should be implemented by any
* class whose instances are intended to be executed by a thread. The
* class must define a method of no arguments called <code>run</code>.
* 
* This interface is designed to provide a common protocol for objects that
* wish to execute code while they are active. For example,
* <code>Runnable</code> is implemented by class <code>Thread</code>.
* Being active simply means that a thread has been started and has not
* yet been stopped.
* 
* In addition, <code>Runnable</code> provides the means for a class to be
* active while not subclassing <code>Thread</code>. A class that implements
* <code>Runnable</code> can run without subclassing <code>Thread</code>
* by instantiating a <code>Thread</code> instance and passing itself in
* as the target. In most cases, the <code>Runnable</code> interface should
* be used if you are only planning to override the <code>run()</code>
* method and no other <code>Thread</code> methods.
* This is important because classes should not be subclassed
* unless the programmer intends on modifying or enhancing the fundamental
* behavior of the class.
*
* @author Arthur van Hoff
* @see java.lang.Thread
* @see java.util.concurrent.Callable
* @since JDK1.0
*/
@FunctionalInterface
public interface Runnable {
/**
* When an object implementing interface <code>Runnable</code> is used
* to create a thread, starting the thread causes the object's
* <code>run</code> method to be called in that separately executing
* thread.
* 
* The general contract of the method <code>run</code> is that it may
* take any action whatsoever.
*
* @see java.lang.Thread#run()
*/
public abstract void run();
}

1. 格式
修饰符 interface 接口名 {
public abstract 返回值类型方法名 (可选参数列表)；

}

注：public abstract可以省略（因为默认修饰为public abstract）

如：

public interface MyFunctionalInterface {
public abstract void method();
}

2. 注解@FunctionalInterface
@FunctionalInterface，是JDK1.8中新引入的一个注解，专门指代函数式接口，用于一个接口的定义上。

和@Override注解的作用类似，@FunctionalInterface注解可以用来检测接口是否是函数式接口。如果是函数式接口，则编译成功，否则编译失败（接口中没有抽象方法或者抽象方法的个数多余1个）。

package com.xcbeyond.study.jdk8.functional;

/**
* 函数式接口
* @Auther: xcbeyond
* @Date: 2020/5/17 0017 0:26
*/
@FunctionalInterface
public interface MyFunctionalInterface {
public abstract void method();

// 如果存在多个抽象方法，则编译失败，即：@FunctionalInterface飘红
// public abstract void method1();
}

3. 实例
函数式接口：

package com.xcbeyond.study.jdk8.functional;

/**
* 函数式接口
* @Auther: xcbeyond
* @Date: 2020/5/17 0017 0:26
*/
@FunctionalInterface
public interface MyFunctionalInterface {
public abstract void method();

// 如果存在多个抽象方法，则编译失败，即：@FunctionalInterface飘红
// public abstract void method1();
}

测试：

package com.xcbeyond.study.jdk8.functional;

/**
* 测试函数式接口
* @Auther: xcbeyond
* @Date: 2020/5/17 0017 0:47
*/
public class MyFunctionalInterfaceTest {

public static void main(String[] args) {
// 调用show方法，参数中有函数式接口MyFunctionalInterface，所以可以使用Lambda表达式，来完成接口的实现
show("hello xcbeyond!", msg -> System.out.printf(msg));
}

/**
* 定义一个方法，参数使用函数式接口MyFunctionalInterface
* @param myFunctionalInterface
*/
public static void show(String message, MyFunctionalInterface myFunctionalInterface) {
myFunctionalInterface.method(message);
}
}

函数式接口，用起来是不是更加的灵活，可以在具体调用处进行接口的实现。

函数式接口，可以很友好地支持Lambda表达式。

二、常用的函数式接口
在JDK1.8之前已经有了大量的函数式接口，最熟悉的就是java.lang.Runnable接口了。

JDK 1.8 之前已有的函数式接口:

java.lang.Runnable
java.util.concurrent.Callable
java.security.PrivilegedAction
java.util.Comparator
java.io.FileFilter
java.nio.file.PathMatcher
java.lang.reflect.InvocationHandler
java.beans.PropertyChangeListener
java.awt.event.ActionListener
javax.swing.event.ChangeListener
而在JDK1.8新增了java.util.function包下的很多函数式接口，用来支持Java的函数式编程，从而丰富了Lambda表达式的使用场景。

这里主要介绍四大核心函数式接口：

java.util.function.Consumer：消费型接口
java.util.function.Supplier：供给型接口
java.util.function.Predicate：断定型接口
java.util.function.Function：函数型接口
1. Consumer接口
java.util.function.Consumer接口，是一个消费型的接口，消费数据类型由泛型决定。

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.
*
* 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); };
}
}

（1）抽象方法：accept
Consumer接口中的抽象方法void accept(T t)，用于消费一个指定泛型T的数据。

举例如下：

/**
* 测试void accept(T t)
*/
@Test
public void acceptMethodTest() {
acceptMethod("xcbeyond", message -> {
// 完成字符串的处理，即：通过Consumer接口的accept方法进行对应数据类型(泛型)的消费
String reverse = new StringBuffer(message).reverse().toString();
System.out.printf(reverse);
});
}

/**
* 定义一个方法，用于消费message字符串
* @param message
* @param consumer
*/
public void acceptMethod(String message, Consumer<String> consumer) {
consumer.accept(message);
}

（2）方法：andThen
方法andThen，可以用来将多个Consumer接口连接到一起，完成数据消费。

/**
* 测试Consumer<T> andThen(Consumer<? super T> after)
* 输出结果：
* XCBEYOND
* xcbeyond
*/
@Test
public void andThenMethodTest() {
andThenMethod("XCbeyond", t -> {
// 转换为大小输出
System.out.println(t.toUpperCase());
}, t -> {
// 转换为小写输出
System.out.println(t.toLowerCase());
});
}

/**
* 定义一个方法，将两个Consumer接口连接到一起，进行消费
* @param message
* @param consumer1
* @param consumer2
*/
public void andThenMethod(String message, Consumer<String> consumer1, Consumer<String> consumer2) {
consumer1.andThen(consumer2).accept(message);
}

2. Supplier接口
java.util.function.Supplier接口，是一个供给型接口，即：生产型接口。只包含一个无参方法：T get()，用来获取一个泛型参数指定类型的数据。

package java.util.function;

/**
* 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 <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();
}

举例如下：

@Test
public void test() {
String str = getMethod(() -> "hello world!");
System.out.println(str);
}

public String getMethod(Supplier<String> supplier) {
return supplier.get();
}

3. Predicate接口
java.util.function.Predicate接口，是一个断定型接口，用于对指定类型的数据进行判断，从而得到一个判断结果（boolean类型的值）。

package java.util.function;

import java.util.Objects;

/**
* Represents a predicate (boolean-valued function) of one argument.
*
* 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.
*
* 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 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<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);
}
}

（1）抽象方法：test
抽象方法boolean test(T t),用于条件判断。

/**
* 测试boolean test(T t);
*/
@Test
public void testMethodTest() {
String str = "xcbey0nd";
boolean result = testMethod(str, s -> s.equals("xcbeyond"));
System.out.println(result);
}

/**
* 定义一个方法，用于字符串的判断。
* @param str
* @param predicate
* @return
*/
public boolean testMethod(String str, Predicate predicate) {
return predicate.test(str);
}

（2）方法：and
方法Predicate<T> and(Predicate<? super T> other)，用于将两个Predicate进行逻辑”与“判断。

/**
* 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);
}

（4）方法：or
方法Predicate<T> or(Predicate<? super T> other)，用于两个Predicate的逻辑”或“判断。

package java.util.function;

import java.util.Objects;

/**
* Represents a function that accepts one argument and produces a result.
*
* 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;
}
}

（1）抽象方法：apply
抽象方法R apply(T t)，根据类型T的参数获取类型R的结果。

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

举例如下：

/**
* 测试R apply(T t)，完成字符串整数的转换
*/
@Test
public void applyMethodTest() {
// 字符串类型的整数
String numStr = "123456";
Integer num = applyMethod(numStr, n -> Integer.parseInt(n));
System.out.println(num);
}

public Integer applyMethod(String str, Function<String, Integer> function) {
return function.apply(str);
}

（2）方法：compose
方法<V> Function<V, R> compose(Function<? super V, ? extends T> before),获取apply的function。

和指令式编程相比，函数式编程强调函数的计算比指令的执行重要。

和过程化编程相比，函数式编程里函数的计算可随时调用。

当然，Java大家都知道是面向对象的编程语言，一切都是基于对象的特性（抽象、封装、继承、多态）。在JDK1.8出现之前，我们关注的往往是某一对象应该具有什么样的属性，当然这也就是面向对象的核心——对数据进行抽象。但JDK1.8出现以后，这一点开始出现变化，似乎在某种场景下，更加关注某一类共有的行为（有点类似接口），这也就是JDK1.8提出函数式编程的目的。如下图所示，展示了面向对象编程到函数式编程的变化。

[外链图片转存失败,源站可能有防盗链机制,建议将图片保存下来直接上传(img-cOkAh3K9-1590250788080)(./4.面向对象编程到函数式编程的变化.png)]

Lambda表达式就是更好的体现了函数式编程，而为了支持Lambda表达式，才有了函数式接口。

另外，为了在面对大型数据集合时，为了能够更加高效的开发，编写的代码更加易于维护，更加容易运行在多核CPU上，java在语言层面增加了Lambda表达式。在上一节中，我们已经知道Lambda表达式是多么的好用了。

在JDK1.8中，函数式编程随处可见，在你使用过程中简直很爽，例如：Stream流。

函数式编程的优点，也很多，如下：

1. 代码简洁，开发快速
函数式编程大量使用函数，减少了代码的重复，因此程序比较短，开发速度较快。

2. 接近自然语言，易于理解
函数式编程的自由度很高，可以写出很接近自然语言的代码。

例如，两数只差，可以写成(x, y) -> x – y

3. 更方便的代码管理
函数式编程不依赖、也不会改变外界的状态，只要给定输入参数，返回的结果必定相同。因此，每一个函数都可以被看做独立单元，很有利于进行单元测试（unit testing）和除错（debugging），以及模块化组合。

4. 易于"并发编程"
函数式编程不需要考虑"死锁"，因为它不修改变量，所以根本不存在"锁"线程的问题。不必担心一个线程的数据，被另一个线程修改，所以可以很放心地把工作分摊到多个线程，部署"并发编程"。

5. 代码的热升级
函数式编程没有副作用，只要保证接口不变，内部实现是外部无关的。所以，可以在运行状态下直接升级代码，不需要重启，也不需要停机。

四、总结
在JDK1.8中，函数式接口/编程将会随处可见，也有有助于你更好的理解JDK1.8中的一些新特性。关于函数式接口，在接下来具体特性、用法中将会体现的淋漓尽致。

JDK1.8提出的函数式接口，你是否赞同呢？

参考资料：

1.https://www.cnblogs.com/Dorae/p/7769868.html

2.https://blog.csdn.net/stormkai/article/details/94364233

3.https://baike.baidu.com/item/函数式编程

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