ADDITIONAL SYSTEM INFORMATION :
Operating System: darwin 24.6.0 (macOS)
JDK Version: openjdk version "21" 2023-09-19 LTS
Build: OpenJDK Runtime Environment (build 21+35-LTS)
Architecture: x86_64

Additional Information:
- Working on OpenJDK JDK repository
- Testing with jtreg test framework

A DESCRIPTION OF THE PROBLEM :
I would like to contribute to OpenJDK by adding explicit null
checks using Objects.requireNonNull to improve defensive
programming and provide clearer error messages.

Motivation:
Some public API methods currently throw NullPointerException
when null is passed, but the error message is not clear. Using
Objects.requireNonNull provides a clearer error message and
makes the API contract explicit.

Proposed Changes:
- Add Objects.requireNonNull check in DataOutputStream.writeBytes(String s)
- This will provide a clear error message when null is passed

Target File:
src/java.base/share/classes/java/io/DataOutputStream.java

Testing:
I will add test cases to verify that null checks work correctly
and provide appropriate error messages.

This is my first contribution to OpenJDK. I'm ready to submit
a pull request once a JBS issue is created.