Java Stream filter()

Learn to use Stream.filter(Predicate) method to traverse all the elements and filter all elements which match a given condition through Predicate argument.

1. Stream filter() Method

The stream() method syntax is as follows:

Stream<T> filter(Predicate<? super T> condition)  

Predicate is a functional interface and represents the condition to filter out the non-matching elements from the stream.

• filter() is a intermediate Stream operation.
• It returns a Stream consisting of the elements of the given stream that match the given predicate.
• The filter() argument should be stateless predicate which is applied to each element in the stream to determine if it should be included or not.
• Predicate is a functional interface. So, we can also pass lambda expression also.
• It returns a new Stream so we can use other operations applicable to any stream.

2. Java Stream filter() Example

2.1. Filtering a Stream using Lambda Expression

In this Java example, we are iterating over a stream of numbers. We filter all even numbers from the Stream and print them into Console.

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

public class Main 
{
	public static void main(String[] args) 
	{
		List<Integer> list = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);

		list.stream()
			.filter(n -> n % 2 == 0)
			.forEach(System.out::println);
	}
}

Program output.

2
4
6
8
10

2.2. Filtering a Stream using Predicate

Java example to iterate over a stream of integers and print only even numbers. This example uses Predicate class in place of the lambda expression, though both are the same things.

import java.util.Arrays;
import java.util.List;
import java.util.function.Predicate;

public class Main 
{
	public static void main(String[] args) 
	{
		List<Integer> list = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
		
		Predicate<Integer> condition = new Predicate<Integer>() 
		{
			@Override
			public boolean test(Integer n) {
				if (n % 2 == 0) {
					return true;
				}
				return false;
			}
		};

		list.stream().filter(condition).forEach(System.out::println);
	}
}

Program output.

2
4
6
8
10

2.3. Filtering a Stream and Collecting into a List

We can use the collect(Collectors.toList()) method to collect the stream of filtered elements into a List.

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class Main 
{
	public static void main(String[] args) 
	{
		List<Integer> list = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);

		List<Integer> evenNumbers = list.stream()
					.filter(n -> n % 2 == 0)
					.collect(Collectors.toList());
		
		System.out.println(evenNumbers);
	}
}

Program output.

[2, 4, 6, 8, 10]

2.4. Java Stream filter() and map() Example

We can use the map() method to collect the stream elements and then convert each number to its square before collecting it to the List.

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class Main 
{
	public static void main(String[] args) 
	{
		List<Integer> list = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);

		List<Integer> evenNumbers = list.stream()
					.filter(n -> n % 2 == 0)
					.map(n -> n * n)
					.collect(Collectors.toList());
		
		System.out.println(evenNumbers);
	}
}

Program output.

[4, 16, 36, 64, 100]

Happy Learning !!