Java 8 Streams filter examples

1. Streams filter() and collect()

1.1 Before Java 8, filter a List like this :

BeforeJava8.java

package com.favtuts.java8;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class Java8StreamFilter {

    public static void main(String[] args) {
        List<String> lines = Arrays.asList("spring", "node", "favtuts");
        List<String> result = getFilterOutput(lines, "favtuts");
        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 (!"favtuts".equals(line)) {   // we dont like favtuts
                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.favtuts.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", "favtuts");

        List<String> result = lines.stream()                // convert list to stream
                .filter(line -> !"favtuts".equals(line))    // we dont like favtuts
                .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.favtuts.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
    @Override
    public String toString() {
        return "Person{" +
            " name='" + getName() + "'" +
            ", age='" + getAge() + "'" +
            "}";
    }

    public String getName() {
        return this.name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return this.age;
    }

    public void setAge(int age) {
        this.age = age;
    }
}

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

BeforeJava8.java

package com.favtuts.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("favtuts", 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.favtuts.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("favtuts", 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.favtuts.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("favtuts", 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.favtuts.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("favtuts", 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

favtuts
jack
lawrence

Note

Highly recommend this Streams tutorial – Processing Data with Java SE 8 Streams

Download Source Code

$ git clone https://github.com/favtuts/java-core-tutorials-examples

$ cd java-basic/java8

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