JDK1.8新特性(四)：函数式接口

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>. * * 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 */@FunctionalInterfacepublic 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）。

语言方法

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package com.xcbeyond.study.jdk8.functional;/** * 函数式接口 * @Auther: xcbeyond * @Date: 2020/5/17 0017 0:26 */@FunctionalInterfacepublic 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：

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 */@FunctionalInterfacepublic 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) */@Testpublic 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 */@Testpublic 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 */@FunctionalInterfacepublic interface Supplier<T> { /** * Gets a result. * * @return a result */ T get();}

举例如下：

@Testpublic 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 */@FunctionalInterfacepublic 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 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. * * 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); */@Testpublic 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. * * 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. * * 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. * * 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 */@FunctionalInterfacepublic 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 */@Testpublic 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。

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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？

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