Java Predicate及Consumer接口函数代码实现解析

Z时代
2024-01-10
分类：综合

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
  }
}