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Java 8 Streams filter examples

Java基础 来源:Cadence_D 1次浏览

  ref : https://www.mkyong.com/java8/java-8-streams-filter-examples/

1. Streams filter() and collect()

1.1 Before Java 8, filter a List like this :

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 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.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 } }

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 Before Java 8, you get a Person by name like this :

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 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.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 For multiple condition.

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




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