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浅谈Java8新特性Predicate接口

admin 2022-11-25 4人围观 ,发现0个评论

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testisEqualandornegatenot

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) 

testPredicate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 isUserNameValid = u -> u != null && u.length() > 5 && u.length() <10; 	System.out.println(isUserNameValid.test("Mahesh")); //true  	// Is password valid 	Predicate isPasswordValid = p -> p != null && p.length() > 8 && p.length() <15; 	System.out.println(isPasswordValid.test("Mahesh123")); //true 	 	// Word match 	Predicate isWordMatched = s -> s.startsWith("Mr."); 	System.out.println(isWordMatched.test("Mr. Mahesh")); //true 	 	//Odd numbers 	Predicate 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 maleStudent = s-> s.getAge() >= 20 && "male".equals(s.getGender());     Predicate femaleStudent = s-> s.getAge() > 18 && "female".equals(s.getGender());          Function maleStyle = s-> "Hi, You are male and age "+s.getAge();     Function 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 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<&#63; super T> other)

 default Predicate and(Predicate<&#63; super T> other) 

andPredicate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 isMaleStudent = s -> s.getAge() >= 20 && "male".equals(s.getGender()); 	Predicate isFemaleStudent = s -> s.getAge() > 18 && "female".equals(s.getGender()); 	Predicate 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<&#63; super T> other)

 default Predicate or(Predicate<&#63; super T> other)

orPredicate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 isMaleStudent = s -> s.getAge() >= 20 && "male".equals(s.getGender()); 	Predicate isFemaleStudent = s -> s.getAge() > 18 && "female".equals(s.getGender()); 	Predicate 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 negate() 

negatePredicate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 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 isValidName = s -> s.length() > 5 && s.length() <15;        System.out.println(isValidName.negate().test("Krishna")); //false                Predicate isLessThan50 = n -> n <50;        System.out.println(isLessThan50.negate().test(60)); //true                Predicate 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  Predicate isEqual(Object targetRef) 

isEqualPredicate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 isHelloMsg = Predicate.isEqual("Hello"); Predicate 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 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 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<&#63; super T> target)

 static  Predicate not(Predicate<&#63; super T> target) 

notJava 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 isOdd = n -> n % 2 == 1;     Predicate isEven = Predicate.not(isOdd);     System.out.println(isEven.test(10)); //true          Predicate 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 list = new ArrayList<>();	 	list.add("Vijay"); 	list.add("Ramesh"); 	list.add("Mahesh"); 	 	Predicate 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 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 isScoreGt40 = std -> std.getMarks() > 40; 	filterStudent(isScoreGt40, list).forEach(s -> System.out.println(s)); 	 	System.out.println("--- All Male Students ---"); 	Predicate isMaleStudent = std -> "male".equals(std.getGender()); 	filterStudent(isMaleStudent, list).forEach(s -> System.out.println(s)); 	 	System.out.println("--- All Female Students ---"); 	Predicate 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 filterStudent(Predicate predicate, List 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

到此这篇关于浅谈Java8新特性Predicate接口的文章就介绍到这了,更多相关Java Predicate内容请搜索以前的文章或继续浏览下面的相关文章希望大家以后多多支持!

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