Java 8 基础教程 - Predicate

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