Java Stream API Example in Java

Introduction

The Stream API (Java 8+) supports declarative processing of collections — filtering, transforming and aggregating data in a pipeline without explicit loops.

Example Program

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

public class StreamApiDemo {
    public static void main(String[] args) {
        List<Integer> numbers = Arrays.asList(3, 8, 1, 10, 4, 6, 15);

        // Filter evens, square them, sort, collect
        List<Integer> result = numbers.stream()
                .filter(n -> n % 2 == 0)
                .map(n -> n * n)
                .sorted()
                .collect(Collectors.toList());

        System.out.println("Original : " + numbers);
        System.out.println("Pipeline : " + result);

        // String pipeline
        List<String> words = Arrays.asList("java", "stream", "api", "example");
        long count = words.stream()
                .filter(w -> w.length() > 4)
                .count();

        System.out.println("Words with length > 4: " + count);
    }
}

Sample Output

Original : [3, 8, 1, 10, 4, 6, 15]
Pipeline : [16, 36, 64, 100]
Words with length > 4: 2

Common Stream Operations

Best Practices

• Use streams for readability when the pipeline is short and clear.
• Prefer traditional loops for complex branching or when performance tuning on hot paths.
• Call stream() on the collection; do not reuse a consumed stream.

Common Mistakes

• Forgetting that intermediate operations are lazy until a terminal operation runs.
• Using parallel streams on small datasets where overhead exceeds benefit.