Stream的Filter示例

Stream的Filter示例

该篇将向你展示几个示例，用来演示Stream的filter()、collect()、findAny()和orElse的使用。

Stream的filter()和collect()

在Java 8之前的版本，想下面这样过滤List列表：

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class BeforeJava8 {

    public static void main(String[] args) {

        List<String> lines = Arrays.asList("spring", "node", "mkyong");
        List<String> result = getFilterOutput(lines, "mkyong");
        for (String temp : result) {
            System.out.println(temp);    //output : spring, node
        }

    }

    private static List<String> getFilterOutput(List<String> lines, String filter) {
        List<String> result = new ArrayList<>();
        for (String line : lines) {
            if (!"mkyong".equals(line)) { // we dont like mkyong
                result.add(line);
            }
        }
        return result;
    }

}

输出

spring
node

在Java 8中，stream.filter()过滤List列表，collect()将流转换为List列表

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class NowJava8 {

    public static void main(String[] args) {

        List<String> lines = Arrays.asList("spring", "node", "mkyong");

        List<String> result = lines.stream()                // convert list to stream
                .filter(line -> !"mkyong".equals(line))     // we dont like mkyong
                .collect(Collectors.toList());              // collect the output and convert streams to a List

        result.forEach(System.out::println);                //output : spring, node

    }

}

输出

spring
node

Stream的filter()、findAny()和orElse()

Person.java

public class Person {

    private String name;
    private int age;

    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    //gettersm setters, toString
}

在Java 8以前的版本，你可以像这面这样通过名字来获取Person

import java.util.Arrays;
import java.util.List;

public class BeforeJava8 {

    public static void main(String[] args) {

        List<Person> persons = Arrays.asList(
                new Person("mkyong", 30),
                new Person("jack", 20),
                new Person("lawrence", 40)
        );

        Person result = getStudentByName(persons, "jack");
        System.out.println(result);

    }

    private static Person getStudentByName(List<Person> persons, String name) {

        Person result = null;
        for (Person temp : persons) {
            if (name.equals(temp.getName())) {
                result = temp;
            }
        }
        return result;
    }
}

输出

Person{name='jack', age=20}

java8中，stream.filter()过滤List列表，findAny()、orElse(null)返回满足条件的对象

import java.util.Arrays;
import java.util.List;

public class NowJava8 {

    public static void main(String[] args) {

        List<Person> persons = Arrays.asList(
                new Person("mkyong", 30),
                new Person("jack", 20),
                new Person("lawrence", 40)
        );

        Person result1 = persons.stream()                        // Convert to steam
                .filter(x -> "jack".equals(x.getName()))        // we want "jack" only
                .findAny()                                      // If 'findAny' then return found
                .orElse(null);                                  // If not found, return null

        System.out.println(result1);
        
        Person result2 = persons.stream()
                .filter(x -> "ahmook".equals(x.getName()))
                .findAny()
                .orElse(null);

        System.out.println(result2);

    }

}

输出

Person{name='jack', age=20}
null

多条件

import java.util.Arrays;
import java.util.List;

public class NowJava8 {

    public static void main(String[] args) {

        List<Person> persons = Arrays.asList(
                new Person("mkyong", 30),
                new Person("jack", 20),
                new Person("lawrence", 40)
        );

        Person result1 = persons.stream()
                .filter((p) -> "jack".equals(p.getName()) && 20 == p.getAge())
                .findAny()
                .orElse(null);

        System.out.println("result 1 :" + result1);

        //or like this
        Person result2 = persons.stream()
                .filter(p -> {
                    if ("jack".equals(p.getName()) && 20 == p.getAge()) {
                        return true;
                    }
                    return false;
                }).findAny()
                .orElse(null);

        System.out.println("result 2 :" + result2);
    }
}

输出

result 1 :Person{name='jack', age=20}
result 2 :Person{name='jack', age=20}

Stream的filter()和map()

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class NowJava8 {

    public static void main(String[] args) {

        List<Person> persons = Arrays.asList(
                new Person("mkyong", 30),
                new Person("jack", 20),
                new Person("lawrence", 40)
        );

        String name = persons.stream()
                .filter(x -> "jack".equals(x.getName()))
                .map(Person::getName)                        //convert stream to String
                .findAny()
                .orElse("");

        System.out.println("name : " + name);

        List<String> collect = persons.stream()
                .map(Person::getName)
                .collect(Collectors.toList());

        collect.forEach(System.out::println);
    }
}

输出

name : jack

mkyong
jack
lawrence