Java 8 Stream Filter Example

Java 8 Filter

This tutorial will show you how to use Java 8 stream API‘s filter() and map() methods. The filter() method in Java 8 or onward is used to produce a stream containing only a certain type of objects into the result output and map() method is used to transform a stream from one type to another type into the result output.

The filter() method of Java stream API is an intermediate operation that reads data from a stream and returns a new stream after transforming data based on the given condition. Filter allows you to filter elements of a stream that match a given Predicate. So filter() is an example of Predicate.

So filter returns a stream consisting of the elements of this stream that match the given predicate.

The signature of filter() method in Java’s Stream interface is given below:

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

The filter() returns a stream consisting of the elements of this stream that match the given predicate.

Prerequisites

Java 8+

Java 8 Stream’s Filter Example

I will create a main class and apply filter() method various ways with examples.

I define a list of string values on which I will apply filter() method in different ways.

The first filter() method checks if the string is equal to roytuts or soumitra then build a result list with these values. So it outputs roytuts and soumitra.

Next I am returning any string based on roytuts. So it outputs soumitra which is any string from the list. If desired value is not found then it will return null.

Next if the found value is compatible with string then transform it into string and return otherwise return null. So it outputs roytuts.

public class JavaStreamFilter {

	public static void main(String[] args) {
		// define a list of string values
		List<String> list = Arrays.asList("roytuts", "soumitra", "abc", "java", "bekar");

		// produce a result list which has only 'roytuts' or 'soumitra'
		// first filter the values
		// second collect again those values into a list
		List<String> resultList = list.stream().filter(s -> "roytuts".equals(s) || "soumitra".equals(s))
				.collect(Collectors.toList());

		resultList.forEach(s -> System.out.println(s));
		
		System.out.println("---");

		// find any value in the list of string using findAny() and if the desired value
		// is not found then return null
		String findAny = list.stream().filter(s -> "roytuts".equals(s)).findAny().orElse(null);

		System.out.println(findAny);
		System.out.println("---");

		// transform the found value into string if it is compatible
		String mapToString = list.stream().filter(s -> "roytuts".equals(s)).map(String::toString).findAny()
				.orElse(null);

		System.out.println(mapToString);
	}

}

Testing Filter Example

When you run the above program you will see below output in the console:

roytuts
soumitra
---
roytuts
---
roytuts

Source Code

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