Predicate和Consumer接口– Java 8中java.util.function包下的接口

//Returns a predicate which evaluates to true only if this predicate  //and the provided predicate both evaluate to true.  and(Predicate<? super T> p)     //Returns a predicate which negates the result of this predicate.  negate()     //Returns a predicate which evaluates to true if either  //this predicate or the provided predicate evaluates to true  or(Predicate<? super T> p)     //Returns true if the input object matches some criteria  test(T t)     //Returns a predicate that evaluates to true if both or neither  //of the component predicates evaluate to true  xor(Predicate<? super T> p)

除了test()方法是抽象方法以外，其他方法都是默认方法（译者注：在Java 8中，接口可以包含带有实现代码的方法，这些方法称为default方法）。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()。

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。不像其他函数式接口，Consumer接口期望执行带有副作用的操作（译者注：Consumer的操作可能会更改输入参数的内部状态）。

Consumer接口中有2个方法，有且只有一个声明为accept(T t)的方法，接收一个输入参数并且没有返回值。为了详细说明Predicate和Consumer接口，我们来考虑一下学生的例子：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。

class Student{        String firstName;        String lastName;        Double grade;        Double feeDiscount = 0.0;        Double baseFee = 20000.0;        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);        }    }

public class PreidcateConsumerDemo {       public static Student updateStudentFee(Student student, Predicate<Student> predicate, Consumer<Student> consumer){            //Use the predicate to decide when to update the discount.            if ( predicate.test(student)){                //Use the consumer to update the discount value.                consumer.accept(student);          }            return student;        }    }

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();        Student student2 = new Student("Rajat","Verma", 8.0);        student2 = updateStudentFee(student2,                                  student -> student.grade >= 8,                                  student -> student.feeDiscount = 20.0);        student2.printFee();    }