Java8 Streams 例子

在本教程中，我们将向你展示一些java 8个例子来演示的流filter()，collect()使用，findany()和orelse().

1. Streams filter() and collect()

1.1 Java 8之前, 过滤一 List 如下所示 :

BeforeJava8.java

1. package com.mkyong.java8;

2.  

3. import java.util.ArrayList;

4. import java.util.Arrays;

5. import java.util.List;

6.  

7. public class BeforeJava8 {

8.  

9. public static void main(String[] args) {

10.  

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

12. List<String> result = getFilterOutput(lines, "mkyong");

13. for (String temp : result) {

14. System.out.println(temp); //output : spring, node

15. }

16.  

17. }

18.  

19. private static List<String> getFilterOutput(List<String> lines, String filter) {

20. List<String> result = new ArrayList<>();

21. for (String line : lines) {

22. if (!"mkyong".equals(line)) { // we dont like mkyong

23. result.add(line);

24. }

25. }

26. return result;

27. }

28.  

29. }

Output

1. spring

2. node

1.2 在java 8，流等效的例子。filter()过滤列表，和collect()一起转换流到一个List中。

NowJava8.java

1. package com.mkyong.java8;

2.  

3. import java.util.Arrays;

4. import java.util.List;

5. import java.util.stream.Collectors;

6.  

7. public class NowJava8 {

8.  

9. public static void main(String[] args) {

10.  

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

12.  

13. List<String> result = lines.stream() // convert list to stream

14. .filter(line -> !"mkyong".equals(line)) // we dont like mkyong

15. .collect(Collectors.toList()); // collect the output and convert streams to a List

16.  

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

18.  

19. }

20.  

21. }

Output

1. spring

2. node

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

Person.java

1. package com.mkyong.java8;

2.  

3. public class Person {

4.  

5. private String name;

6. private int age;

7.  

8. public Person(String name, int age) {

9. this.name = name;

10. this.age = age;

11. }

12.  

13. //gettersm setters, toString

14. }

2.1 在java 8之前, 你会像下面示例的样子获取一个Person 对象 :

BeforeJava8.java

1. package com.mkyong.java8;

2.  

3. import java.util.Arrays;

4. import java.util.List;

5.  

6. public class BeforeJava8 {

7.  

8. public static void main(String[] args) {

9.  

10. List<Person> persons = Arrays.asList(

11. new Person("mkyong", 30),

12. new Person("jack", 20),

13. new Person("lawrence", 40)

14. );

15.  

16. Person result = getStudentByName(persons, "jack");

17. System.out.println(result);

18.  

19. }

20.  

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

22.  

23. Person result = null;

24. for (Person temp : persons) {

25. if (name.equals(temp.getName())) {

26. result = temp;

27. }

28. }

29. return result;

30. }

31. }

Output

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

2.2  在java 8使用流等效的例子， 用stream.filter() 过滤List .findAny().orElse (null) 返回一个对象的条件。

NowJava8.java

1. package com.mkyong.java8;

2.  

3. import java.util.Arrays;

4. import java.util.List;

5.  

6. public class NowJava8 {

7.  

8. public static void main(String[] args) {

9.  

10. List<Person> persons = Arrays.asList(

11. new Person("mkyong", 30),

12. new Person("jack", 20),

13. new Person("lawrence", 40)

14. );

15.  

16. Person result1 = persons.stream() // Convert to steam

17. .filter(x -> "jack".equals(x.getName())) // we want "jack" only

18. .findAny() // If 'findAny' then return found

19. .orElse(null); // If not found, return null

20.  

21. System.out.println(result1);

22.  

23. Person result2 = persons.stream()

24. .filter(x -> "ahmook".equals(x.getName()))

25. .findAny()

26. .orElse(null);

27.  

28. System.out.println(result2);

29.  

30. }

31.  

32. }

Output

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

2. null

2.3 多个条件.

NowJava8.java

1. package com.mkyong.java8;

2.  

3. import java.util.Arrays;

4. import java.util.List;

5.  

6. public class NowJava8 {

7.  

8. public static void main(String[] args) {

9.  

10. List<Person> persons = Arrays.asList(

11. new Person("mkyong", 30),

12. new Person("jack", 20),

13. new Person("lawrence", 40)

14. );

15.  

16. Person result1 = persons.stream()

17. .filter((p) -> "jack".equals(p.getName()) && 20 == p.getAge())

18. .findAny()

19. .orElse(null);

20.  

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

22.  

23. //or like this

24. Person result2 = persons.stream()

25. .filter(p -> {

26. if ("jack".equals(p.getName()) && 20 == p.getAge()) {

27. return true;

28. }

29. return false;

30. }).findAny()

31. .orElse(null);

32.  

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

34.  

35. }

36.  
37.  

38. }

Output

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

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

3. Streams filter() and map()

NowJava8.java

1. package com.mkyong.java8;

2.  

3. import java.util.Arrays;

4. import java.util.List;

5. import java.util.stream.Collectors;

6.  

7. public class NowJava8 {

8.  

9. public static void main(String[] args) {

10.  

11. List<Person> persons = Arrays.asList(

12. new Person("mkyong", 30),

13. new Person("jack", 20),

14. new Person("lawrence", 40)

15. );

16.  

17. String name = persons.stream()

18. .filter(x -> "jack".equals(x.getName()))

19. .map(Person::getName) //convert stream to String

20. .findAny()

21. .orElse("");

22.  

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

24.  

25. List<String> collect = persons.stream()

26. .map(Person::getName)

27. .collect(Collectors.toList());

28.  

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

30.  

31. }

32.  

33. }

Output

1. name : jack

2.  

3. mkyong

4. jack

5. lawrence