前期回顾：
JDK1.8新特性(一)：JDK1.8究竟有哪些新特性呢

JDK1.8新特性(二)：为什么要关注JDK1.8

JDK1.8新特性(三)：Lambda表达式，让你爱不释手


上一篇 JDK1.8新特性(三)：Lambda表达式，让你爱不释手 ，只是简单的讲到Lambda表达式的语法、使用，使得你对它产生了好感，而Lambda表达式是如何实现、定义，你可能不太清楚。本篇将会详细介绍 函数式接口 ，让你在使用JDK新特性时，做到心中有数，自信满满。


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 (); 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>. * <p> * 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. * <p> * 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. * <p> * 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. * * <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); }; } }
（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。 /** * 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); }; } 举例如下：
/** * 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. * * <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 () ; } 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. * * <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); } }
（1）抽象方法：test
抽象方法 boolean test(T t) ,用于条件判断。 /** * 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) ; 举例如下：
/** * 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 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); }
（3）方法：negate
方法 Predicate<T> negate() ，用于取反判断。 /** * 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。 /** * 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); }
4. Function接口 java.util.function.Function 接口，是一个函数型接口，用来根据一个类型的数据得到另外一个类型的数据。 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; } }
（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 。 /** * 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)); }
（3）方法：andThen
方法 <V> Function<T, V> andThen(Function<? super R, ? extends V> after) ,用来进行组合操作，即：”先做什么，再做什么“的场景。 /** * 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)); }


三、函数式编程 函数式编程并不是Java提出的新概念，它将计算机运算看作是函数的计算。 函数式编程最重要的基础是λ演算，而且λ演算的函数是可以接受函数当作输入（参数）和输出（返回值）的。 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。 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 JDK1.8新特性(三)：Lambda表达式，让你爱不释手 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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