在本教程中，我們將向你展示一些java 8個例子來演示的流filter()，collect()使用，findany()和orelse().

1. Streams filter() and collect()

1.1 Java 8之前, 過濾一 List 如下所示 :

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

Output

spring

node

1.2 在java 8，流等效的例子。filter()過濾列表，和collect()一起轉換流到一個List中。

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()                // 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
 
    }
 
}
 

2. Streams filter(), findAny() and 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之前, 你會像下面示例的樣子獲取一個Person 物件 :

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使用流等效的例子， 用stream.filter() 過濾List .findAny().orElse (null) 返回一個物件的條件。

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()                        // 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);
 
    }
 
}

Output

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

null

2.3 多個條件.

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) -> "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);
 
    }
 
 
}

Output

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

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

​​​

3. Streams filter() and 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 -> "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);
 
    }
 
}

Output

name : jack


mkyong

jack

lawrence

注
強烈推薦這–處理資料流教程  – 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
5. Processing Data with Java SE 8 Streams

轉自： https://blog.csdn.net/wangmuming/article/details/72630250