Java Streams are a powerful way to process data declaratively. But many developers don’t go beyond basic map and filter. Here's a complete list of Stream API methods you should practice, from beginner to advanced — with real-world use cases.
🔹 1. map()
What it does: Transforms each element in a stream.
Use case: Extract names from a list of employees.
🔹2. filter()
What it does: Removes elements based on a condition.
Use case: Select developers with salary > ₹1,00,000.
🔹3. flatMap()
What it does: Flattens nested collections into a single stream.
Use case: Combine all skills from a list of developers (each with their own list).
🔹 4. collect()
What it does: Terminal operation to accumulate stream results.
Common collectors:
1.toList(), toSet(), toMap()
2.joining()
3.groupingBy(), partitioningBy()
4.reducing(), summarizingDouble()
🔹5. toMap()
What it does: Creates a map from stream data.
Use case: Map developer names to their salaries.
Handles duplicate keys with merge logic.
🔹 6. groupingBy()
What it does: Groups elements by a classifier function.
Use case: Group employees by department or projects by domain.
Can be combined with counting(), mapping(), maxBy(), averagingDouble().
🔹7. partitioningBy()
What it does: Splits elements into two groups (true/false).
Use case: Partition users into active and inactive.
🔹 8. reduce()
What it does: Combines elements into a single result.
Use case: Calculate total salary or find the longest name.
🔹9. distinct()
What it does: Removes duplicates.
Use case: Get unique technologies used in all projects.
🔹10. sorted()
What it does: Sorts the stream.
Use case: Sort developers by salary or age.
🔹 11. limit() / skip()
What it does: Pagination-style control.
Use case: Get top 5 earners, skip first 10 results.
🔹 12. anyMatch(), allMatch(), noneMatch()
What it does: Test if elements match a condition.
Use case: Check if any developer uses “Docker”.
🔹 13. findFirst() / findAny()
What it does: Terminal operations to get an optional element.
Use case: Find the first active developer.
🔹 14. peek()
What it does: Debug intermediary steps (not recommended for production).
Use case: Log elements mid-pipeline.
🔹15. count()
What it does: Counts elements in the stream.
Use case: Count how many developers are assigned to a project.
✅ Practice Ideas
Use a single object model like:
Developer → with skills, salary, projects, orders
Project → with domain
Department → for grouping
You can practice all of the above with these relationships. Realistic and reusable!
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