Add ReservoirSampling algorithm to randomized module

[cureprotocols]

Algorithm Overview:

• Efficient for selecting k random elements from a stream of unknown size
• Commonly used in streaming systems, big data pipelines, and memory-limited environments
• Time Complexity: O(n)
• Space Complexity: O(k)

---

Implementation Details:

• Class: ReservoirSampling
• Package: com.thealgorithms.randomized
• JavaDoc included for class and method
• Demonstration included in main() method
• File name and class name follow PascalCase
• Fully formatted using clang-format

---

Reference:

• Reservoir Sampling - Wikipedia

Author: Michael Alexander Montoya (@cureprotocols)

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized it.
• All filenames are in PascalCase.
• All functions and variable names follow Java naming conventions.
• All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• All new code is formatted with clang-format -i --style=file path/to/your/file.java

[DenizAltunkapan]

What if sampleSize > stream.length? Perhaps there should be error handling for invalid input, the rest lgtm

[cureprotocols]

✅ Added input validation for sampleSize > stream.length as requested — thanks for the suggestion, @DenizAltunkapan!

Let me know if anything else is needed — happy to improve further 🙌

[siriak]

The code looks good, could you please add some JUnit tests and remove main? You could check that the correct number of elements is returned, that they are all from the initial set, maybe other properties of the algorithm (see https://github.com/TheAlgorithms/Java/tree/master/src/test/java/com/thealgorithms)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 84.61538% with 2 lines in your changes missing coverage. Please review.

Project coverage is 73.78%. Comparing base (2570a99) to head (5341847).

Files with missing lines	Patch %	Lines
...om/thealgorithms/randomized/ReservoirSampling.java	84.61%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6204      +/-   ##
============================================
- Coverage     73.78%   73.78%   -0.01%     
- Complexity     5299     5304       +5     
============================================
  Files           671      672       +1     
  Lines         18344    18357      +13     
  Branches       3546     3549       +3     
============================================
+ Hits          13536    13545       +9     
- Misses         4262     4265       +3     
- Partials        546      547       +1     

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[cureprotocols]

✅ Applied clang-format and added JUnit test to src/test/java/com/thealgorithms/randomized/.

All requested changes completed. Ready for final review 💪

[siriak]

Code looks good, please fix PR checks and it's ready to merge. Thank you for patience, I'm very busy at the moment so response times are high :)

[cureprotocols]

✅ Final test file now formatted with clang-format --style=file
✅ Verified formatting with --dry-run --Werror
✅ Test class is correctly placed under:
src/test/java/com/thealgorithms/randomized/ReservoirSamplingTest.java

CI will re-run shortly. Appreciate all the support — ready for merge when you are 💪

[siriak]

Error: /home/runner/work/Java/Java/src/main/java/com/thealgorithms/randomized/ReservoirSampling.java:19:1: Utility classes should not have a public or default constructor. [HideUtilityClassConstructor]
Error: /home/runner/work/Java/Java/src/test/java/com/thealgorithms/randomized/ReservoirSamplingTest.java:3:47: Using the '.' form of import should be avoided - org.junit.jupiter.api.Assertions.. [AvoidStarImport]

[cureprotocols]

✅ All requested changes are now complete:

• ✅ Converted ReservoirSampling to final class with private constructor
• ✅ No wildcard imports (org.junit.jupiter.api.Assertions.* removed)
• ✅ JUnit test placed in correct directory: src/test/java/com/thealgorithms/randomized
• ✅ Verified both files pass clang-format --style=file
• ✅ Confirmed formatting via --dry-run --Werror

Thanks again for your review and feedback. Ready for merge when you are.