浅谈Java8新特性Predicate接口

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 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 &RFSrwympqlt; 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));http://

}else if(femaleStudenthttp://.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 super T> other)

default Predicate 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 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 super T> other)

default Predicate 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 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()

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 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)

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 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 super T> target)

static Predicate 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 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

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