JDK1.8新特性(四)：函数式接口

缺乏、安全感 2023-02-11 14:26 121阅读 0赞

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前期回顾：  
[JDK1.8新特性(一)：JDK1.8究竟有哪些新特性呢][JDK1.8_JDK1.8]  
[JDK1.8新特性(二)：为什么要关注JDK1.8][JDK1.8_JDK1.8 1]  
[JDK1.8新特性(三)：Lambda表达式，让你爱不释手][JDK1.8_Lambda]

上一篇[《Lambda表达式，让你爱不释手》][JDK1.8_Lambda]，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
 */
 @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：

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。

/**
 * 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.
 *
 * 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 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);
     */
    @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.
 *
 * 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
 */
 @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。

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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？

*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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[JDK1.8_JDK1.8]: https://blog.csdn.net/xcbeyond/article/details/103652701
[JDK1.8_JDK1.8 1]: https://blog.csdn.net/xcbeyond/article/details/104175960
[JDK1.8_Lambda]: https://blog.csdn.net/xcbeyond/article/details/106166683