浅谈Java8新特性Predicate接口

一、前言

Java 8中引入了Predicate功能接口。

Java Predicate表示一个参数的谓词。

Predicate是一个布尔值的函数。

Java Predicate是一个功能接口，属于java.util.function包。

Predicate的功能方法是test(T t)。

Predicate的其他方法是test、isEqual、and、or、negate和not。

not方法在Java 11中被引入。

在本文章，我们将提供Predicate的例子及其所有方法。

二、test(T t)

boolean test(T t) 

test是Predicate的功能方法。它在给定的参数上评估这个谓词。

例1：

PredicateTestDemo1.java

import java.util.function.Predicate;
public class PredicateTestDemo1 {
  public static void main(String[] args) {
	// Is username valid
	Predicate<String> isUserNameValid = u -> u != null && u.length() > 5 && u.length() < 10;
	System.out.println(isUserNameValid.test("Mahesh")); //true

	// Is password valid
	Predicate<String> isPasswordValid = p -> p != null && p.length() > 8 && p.length() < 15;
	System.out.println(isPasswordValid.test("Mahesh123")); //true

	// Word match
	Predicate<String> isWordMatched = s -> s.startsWith("Mr.");
	System.out.println(isWordMatched.test("Mr. Mahesh")); //true

	//Odd numbers
	Predicate<Integer> isEven = n -> n % 2 == 0;
	for(int i = 0 ; i < 5 ; i++) {
	  System.out.println("Is "+ i + " even: " + isEven.test(i));
	}
  }
}

输出结果

true
true
true
Is 0 even: true
Is 1 even: false
Is 2 even: true
Is 3 even: false
Is 4 even: true

例2：

PredicateTestDemo2.java

import java.util.function.Function;
import java.util.function.Predicate;
public class PredicateTestDemo2 {
  public static void main(String[] args){
    Predicate<Student> maleStudent = s-> s.getAge() >= 20 && "male".equals(s.getGender());
    Predicate<Student> femaleStudent = s-> s.getAge() > 18 && "female".equals(s.getGender());

    Function<Student,String> maleStyle = s-> "Hi, You are male and age "+s.getAge();
    Function<Student,String> femaleStyle = s-> "Hi, You are female and age "+ s.getAge();

    Student s1 = new Student("Gauri", 20,"female");
    if(maleStudent.test(s1)){
        System.out.println(s1.customShow(maleStyle));
    }else if(femaleStudent.test(s1)){
        System.out.println(s1.customShow(femaleStyle));
    }
  }

Student.java

import java.util.function.Function;
public class Student {
  private String name;
  private int age;
  private String gender;
  private int marks;
  public Student(String name, int age, String gender){
    this.name = name;
    this.age = age;
    this.gender = gender;
  }
  public Student(String name, int age, String gender, int marks){
    this.name = name;
    this.age = age;
    this.gender = gender;
    this.marks = marks;
  }
  public String getName() {
    return name;
  }
  public int getAge() {
    return age;
  }
  public String getGender() {
    return gender;
  }
  public int getMarks() {
    return marks;
  }
  public  String customShow(Function<Student,String> fun){
    return fun.apply(this);
  }
  public String toString(){
    return name+" - "+ age +" - "+ gender + " - "+ marks;
  }
}

输出结果

Hi, You are female and age 20

三、and(Predicate<? super T> other)

default Predicate<T> and(Predicate<? super T> other) 

and是Predicate的默认方法，它返回一个组成的谓词，表示这个谓词和其他谓词的短路逻辑AND。在评估组成的谓词时，如果这个谓词是假的，那么其他谓词将不会被评估。在错误的情况下，如果此谓词抛出错误，那么其他谓词将不会被评估。

PredicateAndDemo.java

import java.util.function.Predicate;
public class PredicateAndDemo {
  public static void main(String[] args) {
	Predicate<Student> isMaleStudent = s -> s.getAge() >= 20 && "male".equals(s.getGender());
	Predicate<Student> isFemaleStudent = s -> s.getAge() > 18 && "female".equals(s.getGender());
	Predicate<Student> isStudentPassed = s -> s.getMarks() >= 33;

	// Testing if male student passed.
	Student student1 = new Student("Mahesh", 22, "male", 30);
	Boolean result = isMaleStudent.and(isStudentPassed).test(student1);
	System.out.println(result); //false

	// Testing if female student passed.
	Student student2 = new Student("Gauri", 19, "female", 40);
	result = isFemaleStudent.and(isStudentPassed).test(student2);
	System.out.println(result); //true
  }
}

输出

false
true

四、or(Predicate<? super T> other)

default Predicate<T> or(Predicate<? super T> other)

or是Predicate的默认方法，它返回一个组成的谓词，表示此谓词和其他谓词的短路逻辑OR。在评估组成的谓词时，如果此谓词为真，那么其他谓词将不会被评估。在错误的情况下，如果此谓词抛出错误，那么其他谓词将不会被评估。

PredicateOrDemo.java

import java.util.function.Predicate;
public class PredicateOrDemo {
  public static void main(String[] args) {
	Predicate<Student> isMaleStudent = s -> s.getAge() >= 20 && "male".equals(s.getGender());
	Predicate<Student> isFemaleStudent = s -> s.getAge() > 18 && "female".equals(s.getGender());
	Predicate<Student> isStudentPassed = s -> s.getMarks() >= 33;

	Student student1 = new Student("Mahesh", 22, "male", 35);
	//Test either male or female student
	Boolean result = isMaleStudent.or(isFemaleStudent).test(student1);
	System.out.println(result); //true
	//Is student passed, too
	result = isMaleStudent.or(isFemaleStudent).and(isStudentPassed).test(student1);
	System.out.println(result); //true
  }
}

输出

true
true

五、negate()

default Predicate<T> negate() 

negate是Predicate的默认方法，它返回一个表示该谓词的逻辑否定的谓词。如果评估的结果是真的，negate将使它变成假的，如果评估的结果是假的，negate将使它变成真的。

PredicateNegateDemo.java

import java.util.function.Predicate;
public class PredicateNegateDemo {
  public static void main(String[] args) {
       Predicate<Integer> isNumberMatched = n -> n > 10 && n < 20;
       //With negate()
       Boolean result = isNumberMatched.negate().test(15);
       System.out.println(result); //false

       //Without negate()
       result = isNumberMatched.test(15);
       System.out.println(result); //true

       Predicate<String> isValidName = s -> s.length() > 5 && s.length() < 15;
       System.out.println(isValidName.negate().test("Krishna")); //false

       Predicate<Integer> isLessThan50 = n -> n < 50;
       System.out.println(isLessThan50.negate().test(60)); //true

       Predicate<Integer> isGreaterThan20 = n -> n > 20;
       System.out.println(isGreaterThan20.negate().test(30));  //false   

       result = isLessThan50.and(isGreaterThan20).negate().test(25);
       System.out.println(result); //false
  }
}

输出

false
true
false
true
false
false

六、isEqual(Object targetRef)

static <T> Predicate<T> isEqual(Object targetRef) 

isEqual是Predicate的静态方法，它返回测试两个参数是否等于Objects.equals(Object, Object)的谓词。我们使用Predicate.isEqual创建predicate，如下所示。

Predicate<String> isHelloMsg = Predicate.isEqual("Hello");
Predicate<Book> isMahabharatBook = Predicate.isEqual(new Book("Mahabharat", "Vyas"));

PredicateIsEqualDemo.java

import java.util.function.Predicate;
public class PredicateIsEqualDemo {
  public static void main(String[] args) {
    System.out.println("---Testing Hello message---");
    Predicate<String> isHelloMsg = Predicate.isEqual("Hello");
    System.out.println(isHelloMsg.test("Hello")); //true
    System.out.println(isHelloMsg.test("Hi"));  //false

    System.out.println("---Testing Mahabharat book---");
    Book mahabharatBook = new Book("Mahabharat", "Vyas");
    Predicate<Book> isMahabharatBook = Predicate.isEqual(mahabharatBook);
    System.out.println(isMahabharatBook.test(new Book("Mahabharat", "Vyas"))); //true
    System.out.println(isMahabharatBook.test(new Book("Ramayan", "Valmiki"))); //false
  }
}

class Book {
  private String name;
  private String writer;
  public Book(String name, String writer) {
	this.name = name;
	this.writer = writer;
  }
  public String getName() {
    return name;
  }
  public String getWriter() {
    return writer;
  }
  public boolean equals(final Object obj) {
    if (obj == null) {
        return false;
    }
    final Book b = (Book) obj;
    if (this == b) {
        return true;
    } else {
        return (this.name.equals(b.name) && (this.writer == b.writer));
    }
  }
}

输出

---Testing Hello message---
true
false
---Testing Mahabharat book---
true
false

七、not(Predicate<? super T> target)

static <T> Predicate<T> not(Predicate<? super T> target) 

not是Java 11中引入的Predicate的静态方法。

not返回的predicate是对所提供的predicate的否定。

这是通过返回调用target.negate()的结果来完成的。

找到Predicate.not的例子。

PredicateNotDemo.java

import java.util.function.Predicate;
public class PredicateNotDemo {
  public static void main(String[] args) {
    Predicate<Integer> isOdd = n -> n % 2 == 1;
    Predicate<Integer> isEven = Predicate.not(isOdd);
    System.out.println(isEven.test(10)); //true

    Predicate<String> isNotHelloMsg = Predicate.not(Predicate.isEqual("Hello"));
    System.out.println(isNotHelloMsg.test("Hi")); //true
    System.out.println(isNotHelloMsg.test("Hello")); //false
  }
}

输出

true
true
false

在Stream中使用Predicate

我们将提供例子在Stream中使用Predicate。

我们将使用Stream.filter来过滤列表。

filter()的语法如下。

filter(Predicate predicate)

filter()在处理给定的Predicate后，返回由过滤后的数据组成的Stream实例。

例1：

PredicateStreamDemo1.java

import java.util.ArrayList;
import java.util.List;
import java.util.function.Predicate;
public class PredicateStreamDemo1 {
  public static void main(String[] args) {
	List<String> list = new ArrayList<>();
	list.add("Vijay");
	list.add("Ramesh");
	list.add("Mahesh");

	Predicate<String> isNameEndsWithSh = s -> s.endsWith("sh");

	list.stream().filter(isNameEndsWithSh)
	  .forEach(s -> System.out.println(s));
  }
}

输出

Ramesh
Mahesh

例2：

PredicateStreamDemo2.java

import java.util.ArrayList;
import java.util.List;
import java.util.function.Predicate;
import java.util.stream.Collectors;
public class PredicateStreamDemo2 {
  public static void main(String[] args) {
	List<Student> list = new ArrayList<>();
	list.add(new Student("Mahesh", 20, "male", 38));
	list.add(new Student("Gauri", 21, "female", 45));
	list.add(new Student("Krishna", 19, "male", 42));
	list.add(new Student("Radha", 20, "female", 35));

	System.out.println("--- All students scoring marks > 40 ---");
	Predicate<Student> isScoreGt40 = std -> std.getMarks() > 40;
	filterStudent(isScoreGt40, list).forEach(s -> System.out.println(s));

	System.out.println("--- All Male Students ---");
	Predicate<Student> isMaleStudent = std -> "male".equals(std.getGender());
	filterStudent(isMaleStudent, list).forEach(s -> System.out.println(s));

	System.out.println("--- All Female Students ---");
	Predicate<Student> isFemaleStudent = std -> "female".equals(std.getGender());
	filterStudent(isFemaleStudent, list).forEach(s -> System.out.println(s));	

	System.out.println("--- All Female Students scoring > 40 ---");
	filterStudent(isFemaleStudent.and(isScoreGt40), list).forEach(s -> System.out.println(s));
  }

  static List<Student> filterStudent(Predicate<Student> predicate, List<Student> list) {
	return list.stream().filter(predicate).collect(Collectors.toList());
  }
}

输出

--- All students scoring marks > 40 ---
Gauri - 21 - female - 45
Krishna - 19 - male - 42
--- All Male Students ---
Mahesh - 20 - male - 38
Krishna - 19 - male - 42
--- All Female Students ---
Gauri - 21 - female - 45
Radha - 20 - female - 35
--- All Female Students scoring > 40 ---
Gauri - 21 - female - 45

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