Using composition or inheritance to enhance this linked list
Considering that…


You have full access to the source code of the original SList class
To make inheritance work, you had to edit SList.java to change head and size from private to protected access
The assignment asks you to "modify the representation of SList"
The resulting class performs the same task as the original code, adding no new functionality (just a large performance optimization for insertEnd() with a negligible cost to insertFront())


I see no reason to use either inheritance or composition.  Just edit SList.java directly to modify its implementation.  The resulting SList class should be fully substitutable with the original, just with better performance characteristics.