Add javadoc and inline comments to HashMap.java

[rbshealy]

The file as it is now is pretty bare and needs to be well-commented. Adding these comments will make the code more readable and understandable to the user.

requirements:
Add a header to all public classes and methods
Add @return and @param where appropriate
Add a description of what the method/class does and why it is implemented
Follow oracle javadoc programming standards
Keep descriptions concise and to the point

optional:
free to add inline comments to clarify certain statements
free to add visual representation of the hash table with example cases

[stale]

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[stale]

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