Java 8 Streamsフィルターの例
このチュートリアルでは、ストリームfilter()、collect()、findAny()、およびorElse()の使用法を示すJava8の例をいくつか示します。
1. Streams filter（）およびcollect（）
1.1 Before Java 8, filter a List like this :
BeforeJava8.java
package com.example.java8;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
public class BeforeJava8 {
public static void main(String[] args) {
List lines = Arrays.asList("spring", "node", "example");
List result = getFilterOutput(lines, "example");
for (String temp : result) {
System.out.println(temp); //output : spring, node
}
}
private static List getFilterOutput(List lines, String filter) {
List result = new ArrayList<>();
for (String line : lines) {
if (!"example".equals(line)) { // we dont like example
result.add(line);
}
}
return result;
}
}
出力
spring node
1.2 The equivalent example in Java 8, stream.filter() to filter a List, and collect() to convert a stream into a List.
NowJava8.java
package com.example.java8;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
public class NowJava8 {
public static void main(String[] args) {
List lines = Arrays.asList("spring", "node", "example");
List result = lines.stream() // convert list to stream
.filter(line -> !"example".equals(line)) // we dont like example
.collect(Collectors.toList()); // collect the output and convert streams to a List
result.forEach(System.out::println); //output : spring, node
}
}
出力
spring node
2. Streams filter（）、findAny（）およびorElse（）
Person.java
package com.example.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 Before Java 8, you get a Person by name like this :
BeforeJava8.java
package com.example.java8;
import java.util.Arrays;
import java.util.List;
public class BeforeJava8 {
public static void main(String[] args) {
List persons = Arrays.asList(
new Person("example", 30),
new Person("jack", 20),
new Person("lawrence", 40)
);
Person result = getStudentByName(persons, "jack");
System.out.println(result);
}
private static Person getStudentByName(List persons, String name) {
Person result = null;
for (Person temp : persons) {
if (name.equals(temp.getName())) {
result = temp;
}
}
return result;
}
}
出力
Person{name='jack', age=20}
2.2 The equivalent example in Java 8, use stream.filter() to filter a List, and .findAny().orElse (null) to return an object conditional.
NowJava8.java
package com.example.java8;
import java.util.Arrays;
import java.util.List;
public class NowJava8 {
public static void main(String[] args) {
List persons = Arrays.asList(
new Person("example", 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);
}
}
出力
Person{name='jack', age=20}
null
2.3 For multiple condition.
NowJava8.java
package com.example.java8;
import java.util.Arrays;
import java.util.List;
public class NowJava8 {
public static void main(String[] args) {
List persons = Arrays.asList(
new Person("example", 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);
}
}
出力
result 1 :Person{name='jack', age=20}
result 2 :Person{name='jack', age=20}
3. Streams filter（）およびmap（）
NowJava8.java
package com.example.java8;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
public class NowJava8 {
public static void main(String[] args) {
List persons = Arrays.asList(
new Person("example", 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 collect = persons.stream()
.map(Person::getName)
.collect(Collectors.toList());
collect.forEach(System.out::println);
}
}
出力
name : jack example jack lawrence
Note
このStreamsチュートリアルを強くお勧めします–Processing Data with Java SE 8 Streams