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("[email protected]", "[email protected]", "[email protected]", "[email protected]"); 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("[email protected]", "[email protected]", "[email protected]", "[email protected]"); // 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