How to Use Stream Filter in Java: Simple Guide with Examples

In Java, you use stream().filter() to select elements from a collection that match a condition. The filter method takes a condition as a lambda expression and returns a new stream with only the elements that satisfy that condition.

Syntax

The filter method is used on a stream and takes a Predicate (a condition) as an argument. It returns a new stream containing only elements that pass the condition.

• stream(): Converts a collection into a stream.
• filter(condition): Keeps elements where condition is true.
• collect(): Gathers the filtered elements back into a collection.

java

collection.stream()
          .filter(element -> condition)
          .collect(Collectors.toList());

Example

This example shows how to filter a list of numbers to keep only even numbers using stream().filter().

java

import java.util.*;
import java.util.stream.*;

public class StreamFilterExample {
    public static void main(String[] args) {
        List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6);
        List<Integer> evenNumbers = numbers.stream()
            .filter(n -> n % 2 == 0)
            .collect(Collectors.toList());
        System.out.println(evenNumbers);
    }
}

Output

[2, 4, 6]

Common Pitfalls

One common mistake is forgetting to collect the filtered stream back into a collection, which means no output is produced. Another is using a filter condition that always returns true or false, which defeats the purpose.

Also, streams are lazy; without a terminal operation like collect() or forEach(), the filter won't run.

java

import java.util.*;
import java.util.stream.*;

public class FilterPitfall {
    public static void main(String[] args) {
        List<String> names = Arrays.asList("Anna", "Bob", "Alice");

        // Wrong: No terminal operation, so nothing happens
        names.stream()
            .filter(name -> name.startsWith("A"));

        // Right: Collect results to a list
        List<String> filtered = names.stream()
            .filter(name -> name.startsWith("A"))
            .collect(Collectors.toList());
        System.out.println(filtered);
    }
}

Output

[Anna, Alice]

Quick Reference

Remember these tips when using stream().filter():

• Always use a terminal operation like collect() or forEach() to execute the stream.
• The filter condition is a lambda that returns true for elements to keep.
• Streams do not change the original collection.
• Filter can be chained with other stream operations like map() or sorted().

Key Takeaways

Use stream().filter() to select elements matching a condition from a collection.

Always end the stream with a terminal operation like collect() to get results.

The filter condition is a lambda expression returning true for elements to keep.

Streams do not modify the original collection; they create new filtered streams.

Common mistakes include missing terminal operations and incorrect filter conditions.