Java 8 Streams filter

石奈子發表於2017-07-30
JavaFilter

在這篇教程中，我們將用少量的例子向你展示java8中的Streams filter(), collect(), findAny() and orElse().

  • Streams filter() 和 collect()

    • Java 8之前, 過濾一個集合是這樣的 :

      BeforeJava8.java
package com.ricky.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", "ricky");
        List<String> result = getFilterOutput(lines, "ricky");
        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 (!"ricky".equals(line)) { // we dont like ricky
                result.add(line);
            }
        }
        return result;
    }

}

    • 輸出

      spring
node

    • 1.2 java8中等價的例子，使用stream.filter()去過濾集合，使用collect()將stream轉化為一個集合。

    NowJava8.java
package com.ricky.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", "ricky");

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

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

    }

}

    -輸出

    spring
node

  • Streams filter(), findAny() and orElse()

Person.java
package com.ricky.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
}
  • Java 8之前, 你獲取Person的name是這樣的 :
 BeforeJava8.java
        package com.ricky.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("ricky", 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;
            }
        }
輸出

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

```

  • 等價的java8列子，使用stream.filter()過濾集合， .findAny().orElse (null)返回一個符合條件的物件。 

    NowJava8.java
    package com.ricky.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("ricky", 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);

        }

    }

    輸出

    Person{name='jack', age=20}
    null
    • 多種條件 
      NowJava8.java
    package com.ricky.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("ricky", 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);

        }


    }

  • 輸出

    result 1 :Person{name='jack', age=20}
    result 2 :Person{name='jack', age=20}
    • Streams filter() and map()
NowJava8.java
package com.ricky.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("ricky", 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);

    }

}

  • 輸出

    name : jack

ricky
jack
lawrence

翻譯

ricky 20170728

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