Java Predicate函数接口介绍及使用示例

转x33g5p2x  于2022-09-23 转载在 Java  
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Predicate 函数式接口是在 Java 8 中引入。Java Predicate 表示一个参数的Predicate接口。 Predicate 是一个布尔值函数。 Java Predicate 是一个函数式接口，属于 java.util.function 包。 Predicate的函数方法是test(T t)。 Predicate 的其他方法是 test、isEqual、and、or、negate 和 not。 Java 11 中引入了 not 方法。在本文中，我们将提供 Predicate 及其所有方法的示例。

1. test(T t)

boolean test(T t)

test 是 Predicate 的函数方法。它根据给定的参数评估这个Predicate接口。
示例 1：
PredicateTestDemo1.java

package com.concretepage;
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

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

示例 2：
PredicateTestDemo2.java

package com.concretepage;
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

package com.concretepage;
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

Hi, You are female and age 20

2.and(Predicate<?super T> other)

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

and 是 Predicate 的默认方法，它返回一个组合Predicate接口，该Predicate接口表示该Predicate接口与另一个Predicate接口的短路逻辑与。在评估组合Predicate接口时，如果此Predicate接口是 false，则不会评估其他Predicate接口。如果发生错误，如果此Predicate接口抛出错误，则不会评估其他Predicate接口。
PredicateAndDemo.java

package com.concretepage;
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

3. or(Predicate<? super T> other)

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

or 是 Predicate 的默认方法，它返回一个组合Predicate接口，该Predicate接口表示该Predicate接口和另一个Predicate接口的短路逻辑 OR。在评估组合Predicate接口时，如果此Predicate接口是 true，则不会评估其他Predicate接口。如果发生错误，如果此Predicate接口抛出错误，则不会评估其他Predicate接口。
PredicateOrDemo.java

package com.concretepage;
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

4. negate（）

default Predicate<T> negate()

negate 是 Predicate 的默认方法，它返回一个表示该Predicate接口的逻辑否定的Predicate接口。如果评估结果为 true，则 negate 将变为 false，如果评估结果为 false，则 negate 将变为 true。
PredicateNegateDemo.java

package com.concretepage;
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

5. isEqual(Object targetRef)

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

isEqual 是 Predicate 的静态方法，它返回根据 Objects.equals(Object, Object) 测试两个参数是否相等的Predicate接口。我们使用 Predicate.isEqual 创建Predicate接口，如下所示。

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

例子如下：
PredicateIsEqualDemo.java

package com.concretepage;
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"))); //true
  }
}

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

6. 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Predicate接口，即提供的Predicate接口的否定。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

package com.concretepage;
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

带有流的Predicate接口

我们将提供使用 Predicate 和 Stream 的示例。我们将使用 Stream.filter 过滤列表。 filter() 的语法如下。

filter(Predicate predicate)

filter() 返回 Stream 的实例，该实例包含处理给定 Predicate 后的过滤数据。
示例 1：
PredicateStreamDemo1.java

package com.concretepage;
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

示例 2：
PredicateStreamDemo2.java

package com.concretepage;
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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原文链接 : https://www.concretepage.com/java/java-8/java-predicate

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