I would ignore advice on commenting. Things should be clear, not commented. Docstrings are nice if you are going to generate some documentation for end users; googling for "string isempty java" is easier than opening up java/lang/String.java and searching for "empty" in your text editor. (But both techniques have their advantages and disadvantages.)

Tests are a great way to show what your library can do. I recommend adding those before you worry about documentation. (How do you know your code, as posted, works?)