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Java 8 Streams filter示例

编辑教程

在这篇教程中我将向你展示 filter(), collect(), findAny()orElse() 的用法

1. Streams filter() 和 collect()

1.1 在 Java 8 以前, 像下面这样过滤一个 List:

BeforeJava8.java

package com.mkyong.java8;

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&lt;&gt;();
        for (String line : lines) {
            if (!"mkyong".equals(line)) { // we dont like mkyong
                result.add(line);
            }
        }
        return result;
    }

}

Output

spring
node

1.2 Java 8里面的等价写法: 使用collect()将一个List转化为steam(流),然后使用stream.filter() 来过滤它.

NowJava8.java

package com.mkyong.java8;

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()                // 转化为一个流
                .filter(line -&gt; !"mkyong".equals(line))     // 排除 'mkyong'
                .collect(Collectors.toList());              // 吧输出流收集回List中

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

    }

}

Output

spring
node

2. Streams filter(), findAny() 和 orElse()

Person.java

package com.mkyong.java8;

public class Person {

    private String name;
    private int age;

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

    //gettersm setters, toString
}

2.1 在Java 8以前, 你想通过name获取一个Person通常像这样 :

BeforeJava8.java

package com.mkyong.java8;

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;
    }
}

Output

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

2.2 在Java 8里面的等价操作是使用 stream.filter()来过滤 List,使用 .findAny().orElse (null) 来返回指定的对象.

NowJava8.java

package com.mkyong.java8;

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()                        // 转化为流
                .filter(x -&gt; "jack".equals(x.getName()))        // 只过滤出"jack"
                .findAny()                                      // 如果找到了就返回
                .orElse(null);                                  // 如果找不到就返回null

        System.out.println(result1);

        Person result2 = persons.stream()
                .filter(x -&gt; "ahmook".equals(x.getName()))
                .findAny()
                .orElse(null);

        System.out.println(result2);

    }

}

Output

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

2.3 多重条件指定.

NowJava8.java

package com.mkyong.java8;

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) -&gt; "jack".equals(p.getName()) &amp;&amp; 20 == p.getAge())
                .findAny()
                .orElse(null);

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

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

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

    }


}

Output

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

3. Streams filter() 和 map()

NowJava8.java

package com.mkyong.java8;

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 -&gt; "jack".equals(x.getName()))
                .map(Person::getName)                        //吧流转化为String
                .findAny()
                .orElse("");

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

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

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

    }

}

Output

name : jack

mkyong
jack
lawrence

注意
Highly recommend this Streams tutorial – Processing Data with Java SE 8 Streams

References

  1. Java 8 cyclic inference in my case
  2. Java 8 Explained: Using Filters, Maps, Streams and Foreach to apply Lambdas to Java Collections!
  3. Java 8 forEach examples
  4. [Java 8 Streams: multiple filters vs. complex condition][Java 8 Streams_ multiple filters vs. complex condition]
  5. Processing Data with Java SE 8 Streams

[Java 8 Streams_ multiple filters vs. complex condition]: http://stackoverflow.com/questions/24054773/java-8-streams-multiple-filters-vs-complex-condition

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