【Java 8 新特性】Java Predicate示例_猫巳_java predicate

irpas 02-07 2005

Java Predicate示例 1. test(T t)2. and(Predicate<? super T> other)3. or(Predicate<? super T> other)4. negate()5. isEqual(Object targetRef)6. not(Predicate<? super T> target)在Stream中使用Predicate参考文献

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。

1. 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 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 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 4. 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 5. 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 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 参考文献

【1】Interface Predicate 【2】Java Predicate Example