Filtering Operations in Stream

This lesson discusses filtering operations in streams.

We'll cover the following

The filtering operations filters the given stream and returns a new stream, which contains only those elements that are required for the next operation.

filter() method

The Stream interface has a filter() method to filter a stream. This is an intermediate operation. Below is the method definition of filter() method.

Stream filter(Predicate<? super T> predicate)

Parameter -> A predicate to apply to each element to determine if it should be included.

Return Type -> It returns a stream consisting of the elements of this stream that match the given predicate.

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import java.util.ArrayList;
import java.util.List;
import java.util.stream.Stream;
public class StreamDemo {
    public static void main(String[] args) {
        //Created a list of integers
        List<Integer> list = new ArrayList<>();
        list.add(1);
        list.add(12);
        list.add(23);
        list.add(45);
        list.add(6);
        list.stream()                           // Created a stream from the list
                .filter(num -> num > 10)        //filter operation to get only numbers greater than 10
                .forEach(System.out::println);  // Printing each number in the list after filtering.
        //Again printing the elements of List to show that the original list is not modified.
        System.out.println("Original list is not modified");
        list.stream() 
                .forEach(System.out::println);
    }
}

In the above example, we created a list of integers. We followed the below steps:

  1. Create a stream from our list.
  2. Apply a filter() operation on this stream. We want to print only those numbers which are greater than 10, so we add a filter.

Please note that the filter operation does not modify the original List.

filter() with custom object

Let’s look at another example of filter() with a custom object.

In the below example, we are using multiple conditions in the filter method.

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import java.util.ArrayList;

import java.util.List;



public class StreamDemo {



    public static void main(String[] args) {

        //Created a list of Person object.

        List<Person> list = new ArrayList<>();

        list.add(new Person("Dave", 23));

        list.add(new Person("Joe", 18));

        list.add(new Person("Ryan", 54));

        list.add(new Person("Iyan", 5));

        list.add(new Person("Ray", 63));



        // We are filtering out those persons whose age is more than 18 and less than 60

        list.stream()

                .filter(person -> person.getAge() > 18 && person.getAge() < 60)

                .forEach(System.out::println);



    }

}



class Person {

    String name;

    int age;



    Person(String name, int age) {

        this.name = name;

        this.age = age;

    }



    public String getName() {

        return name;

    }



    public int getAge() {

        return age;

    }



    @Override

    public String toString() {

        return "Person{" +

                "name='" + name + '\'' +

                ", age=" + age +

                '}';

    }



}

In the above example, we used multiple conditions inside our filter.

filter() chaining

In the above example, we wrote all the conditions in a single filter.

We can also chain the filter method to make the code more readable.

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import java.util.ArrayList;

import java.util.List;



public class StreamDemo {



    public static void main(String[] args) {

        List<Person> list = new ArrayList<>();

        list.add(new Person("Dave", 23));

        list.add(new Person("Joe", 18));

        list.add(new Person("Ryan", 54));

        list.add(new Person("Iyan", 5));

        list.add(new Person("Ray", 63));



        list.stream()

                .filter(person -> person.getName() != null ) // Filtering the object where name is not null

                .filter(person -> person.getAge() > 18 ) // Filtering the objects where age is greater than 18

                .filter(person -> person.getAge() < 60) // Filtering the objects where age is less than 60

                .forEach(System.out::println);



    }

}



class Person {

    String name;

    int age;



    Person(String name, int age) {

        this.name = name;

        this.age = age;

    }



    public String getName() {

        return name;

    }



    public int getAge() {

        return age;

    }



    @Override

    public String toString() {

        return "Person{" +

                "name='" + name + '\'' +

                ", age=" + age +

                '}';

    }



}

Complete the following quiz to test what you’ve learned this lesson.

1

Which of the following is true about the filter() method? Choose all that apply.

A)

It is an intermediate operation.

B)

It takes a Function as a parameter.

C)

It takes a Predicate as a parameter.

D)

None of the above.

Question 1 of 20 attempted

In the next lesson, we will discuss the mapping operations in Stream.

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