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 validPredicate<String> isUserNameValid = u -> u != null && u.length() > 5 && u.length() < 10;System.out.println(isUserNameValid.test('Mahesh')); //true// Is password validPredicate<String> isPasswordValid = p -> p != null && p.length() > 8 && p.length() < 15;System.out.println(isPasswordValid.test('Mahesh123')); //true// Word matchPredicate<String> isWordMatched = s -> s.startsWith('Mr.');System.out.println(isWordMatched.test('Mr. Mahesh')); //true//Odd numbersPredicate<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

truetruetrueIs 0 even: trueIs 1 even: falseIs 2 even: trueIs 3 even: falseIs 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

falsetrue

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 studentBoolean result = isMaleStudent.or(isFemaleStudent).test(student1);System.out.println(result); //true//Is student passed, tooresult = isMaleStudent.or(isFemaleStudent).and(isStudentPassed).test(student1);System.out.println(result); //true }}

输出

truetrue

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

falsetruefalsetruefalsefalse

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')); //falseSystem.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---truefalse---Testing Mahabharat book---truefalse

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

输出

truetruefalse

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

RameshMahesh

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

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