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<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
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
false
true
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 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<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
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")); //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
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)); //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
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