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JDK1.8新特性(四)：函数式接口

2022-06-25 173
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简介： 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。

image.png

上一篇《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：


newThread(() ->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：


packagejava.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*/@FunctionalInterfacepublicinterfaceRunnable {
/*** 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()*/publicabstractvoidrun();
}


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：


publicinterfaceMyFunctionalInterface {
publicabstractvoidmethod();
}


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）。


packagecom.xcbeyond.study.jdk8.functional;
/*** 函数式接口* @Auther: xcbeyond* @Date: 2020/5/17 0017 0:26*/@FunctionalInterfacepublicinterfaceMyFunctionalInterface {
publicabstractvoidmethod();
// 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：


packagecom.xcbeyond.study.jdk8.functional;
/*** 函数式接口* @Auther: xcbeyond* @Date: 2020/5/17 0017 0:26*/@FunctionalInterfacepublicinterfaceMyFunctionalInterface {
publicabstractvoidmethod();
// 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：


packagecom.xcbeyond.study.jdk8.functional;
/*** 测试函数式接口* @Auther: xcbeyond* @Date: 2020/5/17 0017 0:47*/publicclassMyFunctionalInterfaceTest {
publicstaticvoidmain(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*/publicstaticvoidshow(Stringmessage, MyFunctionalInterfacemyFunctionalInterface) {
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。


packagejava.util.function;
importjava.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*/@FunctionalInterfacepublicinterfaceConsumer<T> {
/*** Performs this operation on the given argument.** @param t the input argument*/voidaccept(Tt);
/*** 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*/defaultConsumer<T>andThen(Consumer<?superT>after) {
Objects.requireNonNull(after);
return (Tt) -> { 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)*/@TestpublicvoidacceptMethodTest() {
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Stringreverse=newStringBuffer(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*/publicvoidacceptMethod(Stringmessage, 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*/defaultConsumer<T>andThen(Consumer<?superT>after) {
Objects.requireNonNull(after);
return (Tt) -> { accept(t); after.accept(t); };
}


举例如下：


/*** 测试Consumer<T> andThen(Consumer<? super T> after)* 输出结果：*  XCBEYOND*  xcbeyond*/@TestpublicvoidandThenMethodTest() {
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*/publicvoidandThenMethod(Stringmessage, 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。


packagejava.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*/@FunctionalInterfacepublicinterfaceSupplier<T> {
/*** Gets a result.** @return a result*/Tget();
}


举例如下：


@Testpublicvoidtest() {
Stringstr=getMethod(() ->"hello world!");
System.out.println(str);
}
publicStringgetMethod(Supplier<String>supplier) {
returnsupplier.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）。


packagejava.util.function;
importjava.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*/@FunctionalInterfacepublicinterfacePredicate<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}*/booleantest(Tt);
/*** 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*/defaultPredicate<T>and(Predicate<?superT>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*/defaultPredicate<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*/defaultPredicate<T>or(Predicate<?superT>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(ObjecttargetRef) {
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}*/booleantest(Tt);


举例如下：


/*** 测试boolean test(T t);*/@TestpublicvoidtestMethodTest() {
Stringstr="xcbey0nd";
booleanresult=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*/publicbooleantestMethod(Stringstr, Predicatepredicate) {
returnpredicate.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*/defaultPredicate<T>and(Predicate<?superT>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*/defaultPredicate<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*/defaultPredicate<T>or(Predicate<?superT>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。


packagejava.util.function;
importjava.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*/@FunctionalInterfacepublicinterfaceFunction<T, R> {
/*** Applies this function to the given argument.** @param t the function argument* @return the function result*/Rapply(Tt);
/*** 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<?superV, ?extendsT>before) {
Objects.requireNonNull(before);
return (Vv) ->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<?superR, ?extendsV>after) {
Objects.requireNonNull(after);
return (Tt) ->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() {
returnt->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。


packagejava.util.function;
importjava.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*/@FunctionalInterfacepublicinterfaceFunction<T, R> {
/*** Applies this function to the given argument.** @param t the function argument* @return the function result*/Rapply(Tt);
/*** 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<?superV, ?extendsT>before) {
Objects.requireNonNull(before);
return (Vv) ->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<?superR, ?extendsV>after) {
Objects.requireNonNull(after);
return (Tt) ->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() {
returnt->t;
    }
}


举例如下：


/*** 测试R apply(T t)，完成字符串整数的转换*/@TestpublicvoidapplyMethodTest() {
// 字符串类型的整数StringnumStr="123456";
Integernum=applyMethod(numStr, n->Integer.parseInt(n));
System.out.println(num);
}
publicIntegerapplyMethod(Stringstr, Function<String, Integer>function) {
returnfunction.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<?superV, ?extendsT>before) {
Objects.requireNonNull(before);
return (Vv) ->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<?superR, ?extendsV>after) {
Objects.requireNonNull(after);
return (Tt) ->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






文章标签：
Java
测试技术
自然语言处理
关键词：
jdk接口
jdk函数式接口
jdk特性
jdk特性接口
jdk特性函数式接口
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