Java Predicate及Consumer接口函数代码实现解析
predicate函数编程
predicate功能判断输入的对象是否符合某个条件。官方文档解释到:determines if the input object matches some criteria.
了解predicate接口作用后,在学习predicate函数编程前,先看一下java 8关于predicate的源码:
@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); default predicate<t> and(predicate<? super t> other) { objects.requirenonnull(other); return (t) -> test(t) && other.test(t); } default predicate<t> negate() { return (t) -> !test(t); } default predicate<t> or(predicate<? super t> other) { objects.requirenonnull(other); return (t) -> test(t) || other.test(t); } static <t> predicate<t> isequal(object targetref) { return (null == targetref) ? objects::isnull : object -> targetref.equals(object); } }
从上面代码可以发现,java 8新增了接口的默认(default)方法和(static)静态方法。在java 8以前,接口里的方法要求全部是抽象方法。但是静态(static)方法只能通过接口名调用,不可以通过实现类的类名或者实现类的对象调用;默认(default)方法只能通过接口实现类的对象来调用。
接下来主要来使用接口方法test,可以使用匿名内部类提供test()方法的实现,也可以使用lambda表达式实现test()。
体验一下predicate的函数式编程,使用lambda实现。其测试代码如下:
@test public void testpredicate(){ java.util.function.predicate<integer> boolvalue = x -> x > 5; system.out.println(boolvalue.test(1));//false system.out.println(boolvalue.test(6));//true }
第1行代码:定义一个predicate实现,入参为integer,返回传入参数与5做比较。
第2,3行代码调用第一行,传入相关参数。
consumer函数编程
consumer接口的文档声明如下:
an operation which accepts a single input argument and returns no result. unlike most other functional interfaces, consumer is expected to operate via side-effects.
即接口表示一个接受单个输入参数并且没有返回值的操作。不像其它函数式接口,consumer接口期望执行带有副作用的操作(consumer的操作可能会更改输入参数的内部状态)。
同样,在了解consumer函数编程前,看一下consumer源代码,其源代码如下:
@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); }; } }
从上面代码可以看出,consumer使用了java 8接口新特性——接口默认(default)方法。接下来使用接口方法accept,体验一下consumer函数编程。其测试代码如下:
@test public void testconsumer(){ user user = new user("zm"); //接受一个参数 consumer<user> userconsumer = user1 -> user1.setname("zmchange"); userconsumer.accept(user); system.out.println(user.getname());//zmchange }
在java 8之前的实现如下:
@test public void test(){ user user = new user("zm"); this.change(user); system.out.println(user.getname());//输出zmchange } private void change(user user){ user.setname("zmchange"); }
predicate和consumer综合应用
为了详细说明predicate和consumer接口,通过一个学生例子:student类包含姓名、分数以及待付费用,每个学生可根据分数获得不同程度的费用折扣。
student类源代码:
public class student { string firstname; string lastname; double grade; double feediscount = 0.0; double basefee = 2000.0; public student(string firstname, string lastname, double grade) { this.firstname = firstname; this.lastname = lastname; this.grade = grade; } public void printfee(){ double newfee = basefee - ((basefee * feediscount)/100); system.out.println("the fee after discount: " + newfee); } }
然后分别声明一个接受student对象的predicate接口以及consumer接口的实现类。本例子使用predicate接口实现类的test()方法判断输入的student对象是否拥有费用打折的资格,然后使用consumer接口的实现类更新输入的student对象的折扣。
public class predicateconsumerdemo { public static student updatestudentfee(student student, predicate<student> predicate, consumer<student> consumer){ if (predicate.test(student)){ consumer.accept(student); } return student; } }
predicate和consumer接口的test()和accept()方法都接受一个泛型参数。不同的是test()方法进行某些逻辑判断并返回一个boolean值,而accept()接受并改变某个对象的内部值。updatestudentfee方法的调用如下所示:
public class test { public static void main(string[] args) { student student1 = new student("ashok","kumar", 9.5); student1 = updatestudentfee(student1, //lambda expression for predicate interface student -> student.grade > 8.5, //lambda expression for consumer inerface student -> student.feediscount = 30.0); student1.printfee(); //the fee after discount: 1400.0 student student2 = new student("rajat","verma", 8.0); student2 = updatestudentfee(student2, //lambda expression for predicate interface student -> student.grade >= 8, //lambda expression for consumer inerface student -> student.feediscount = 20.0); student2.printfee();//the fee after discount: 1600.0 } }
通过简单对predicate接口和consumer接口进行应用,对函数式编程有了一个直观认识。
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