You can enable spell-checking support in the Java editor on the Java > Editor > Spelling preference page. Spelling errors are displayed in the Java editor and corresponding Quick Fixes are available:
You can make the dictionary also available to the content assist. However, there is currently no dictionary included in Eclipse. The required format is just a list of words separated by newlines and the Quick Fixes allow you to add new words to the dictionary on-the-fly. Contributions of dictionaries would be welcome.