Java 8 基础教程 - Predicate，java基础面试笔试题

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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

 

转载：https://www.cnblogs.com/hitandrew/p/9007451.html

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