Java 8 基础教程 - Predicate

时间:2022-03-30 07:47:46

在Java 8中,Predicate是一个函数式接口,可以被应用于lambda表达式和方法引用。其抽象方法非常简单:

/**
* Evaluates this predicate on the given argument.
*
* @param t the input argument
* @return {@code true} if the input argument matches the predicate,
* otherwise {@code false}
*/
boolean test(T t);

即对t进行断言,返回true或者false。 例如:在filter中 就会接收一个Predicate

/**
* Returns a stream consisting of the elements of this stream that match
* the given predicate.
*
* <p>This is an <a href="package-summary.html#StreamOps">intermediate
* operation</a>.
*
* @param predicate a non-interfering stateless predicate to apply to each element to determine if it
* should be included in the new returned stream.
* @return the new stream
*/
Stream<T> filter(Predicate<? super T> predicate);

下面来演示一下如何使用Predicate

package predicateExample;

public class Employee {

   public Employee(Integer id, Integer age, String gender, String fName, String lName){
this.id = id;
this.age = age;
this.gender = gender;
this.firstName = fName;
this.lastName = lName;
} private Integer id;
private Integer age;
private String gender;
private String firstName;
private String lastName; //Please generate Getter and Setters @Override
public String toString() {
return this.id.toString()+" - "+this.age.toString(); //To change body of generated methods, choose Tools | Templates.
}
public static Predicate<Employee> isAdultMale() {
return p -> p.getAge() > 21 && p.getGender().equalsIgnoreCase("M");
}
public static Predicate<Employee> isAdultFemale() {
return p -> p.getAge() > 18 && p.getGender().equalsIgnoreCase("F");
}
public static Predicate<Employee> isAgeMoreThan(Integer age) {
return p -> p.getAge() > age;
}
}

上面的代码定义了多个Predicate,分别对应多个筛选条件,下面开始使用这些断言:

package predicateExample;

import java.util.List;
import java.util.function.Predicate;
import java.util.stream.Collectors; public class EmployeePredicates
{
public static Predicate<Employee> isAdultMale() {
return p -> p.getAge() > 21 && p.getGender().equalsIgnoreCase("M");
} public static Predicate<Employee> isAdultFemale() {
return p -> p.getAge() > 18 && p.getGender().equalsIgnoreCase("F");
} public static Predicate<Employee> isAgeMoreThan(Integer age) {
return p -> p.getAge() > age;
} public static List<Employee> filterEmployees (List<Employee> employees, Predicate<Employee> predicate) {
return employees.stream().filter( predicate ).collect(Collectors.<Employee>toList());
}
}
package predicateExample;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import static predicateExample.EmployeePredicates.*; public class TestEmployeePredicates {
public static void main(String[] args){
Employee e1 = new Employee(1,23,"M","Rick","Beethovan");
Employee e2 = new Employee(2,13,"F","Martina","Hengis");
Employee e3 = new Employee(3,43,"M","Ricky","Martin");
Employee e4 = new Employee(4,26,"M","Jon","Lowman");
Employee e5 = new Employee(5,19,"F","Cristine","Maria");
Employee e6 = new Employee(6,15,"M","David","Feezor");
Employee e7 = new Employee(7,68,"F","Melissa","Roy");
Employee e8 = new Employee(8,79,"M","Alex","Gussin");
Employee e9 = new Employee(9,15,"F","Neetu","Singh");
Employee e10 = new Employee(10,45,"M","Naveen","Jain"); List<Employee> employees = new ArrayList<Employee>();
employees.addAll(Arrays.asList(new Employee[]{e1,e2,e3,e4,e5,e6,e7,e8,e9,e10})); System.out.println(filterEmployees(employees, isAdultMale())); System.out.println(filterEmployees(employees, isAdultFemale())); System.out.println(filterEmployees(employees, isAgeMoreThan(35))); //Employees other than above collection of "isAgeMoreThan(35)" can be get using negate()
System.out.println(filterEmployees(employees, isAgeMoreThan(35).negate()));
}
}

输出:

[1 - 23, 3 - 43, 4 - 26, 8 - 79, 10 - 45]
[5 - 19, 7 - 68]
[3 - 43, 7 - 68, 8 - 79, 10 - 45]
[1 - 23, 2 - 13, 4 - 26, 5 - 19, 6 - 15, 9 - 15]
正则表达式表示为Predicate

可以通过Pattern.compile().asPredicate()将正则表达式转换为Predicate。 在Java 8之前,从一个数组中找出符合正则规则的字符串的方法是

public static void main(String[] args)
{ Pattern pattern = Pattern.compile("^(.+)@example.com$"); // Input list
List<String> emails = Arrays.asList("alex@example.com", "bob@yahoo.com",
"cat@google.com", "david@example.com"); for(String email : emails)
{
Matcher matcher = pattern.matcher(email); if(matcher.matches())
{
System.out.println(email);
}
}
}

转换为Predicat为:

import java.util.Arrays;
import java.util.List;
import java.util.function.Predicate;
import java.util.regex.Pattern;
import java.util.stream.Collectors; public class RegexPredicateExample {
public static void main(String[] args) {
// Compile regex as predicate
Predicate<String> emailFilter = Pattern
.compile("^(.+)@example.com$")
.asPredicate(); // Input list
List<String> emails = Arrays.asList("alex@example.com", "bob@yahoo.com",
"cat@google.com", "david@example.com"); // Apply predicate filter
List<String> desiredEmails = emails
.stream()
.filter(emailFilter)
.collect(Collectors.<String>toList()); // Now perform desired operation
desiredEmails.forEach(System.out::println);
}
}
原文连接:https://www.codemore.top/cates/Backend/post/2018-05-06/predicate