Visibility

Visibility is a subject that is simple in principle but has complex subtleties. The simple idea is that any class has public and private elements. Public elements can be used by any other class; private elements can be used only by the owning class. However, each language makes its own rules. Although many languages use such terms as public, private, and protected, they mean different things in different languages. These differences are small, but they lead to confusion, especially for those of us who use more than one language.

The UML tries to address this without getting into a horrible tangle. Essentially, within the UML, you can tag any attribute or operation with a visibility indicator. You can use any marker you like, and its meaning is language dependent. However, the UML provides four abbreviations for visibility: + (public), ?/span> (private), ~ (package), and # (protected). These four levels are used within the UML meta-model and are defined within it, but their definitions vary subtly from those in other languages.

When you are using visibility, use the rules of the language in which you are working. When you are looking at a UML model from elsewhere, be wary of the meanings of the visibility markers, and be aware of how those meanings can change from language to language.

Most of the time, I don't draw visibility markers in diagrams; I use them only if I need to highlight the differences in visibility of certain features. Even then, I can mostly get away with + and ? which at least are easy to remember.