When content assist is invoked on an existing identifier, it can either replace the identifier with the chosen completion or do an insert.
The default behavior is to insert. You can toggle this behavior while inside the content assist selection dialog by pressing and holding the Ctrl key while selecting the completion. The highlighted text is overwritten.
If you wish the completion to overwrite without pressing the Ctrl key then you can select 'Preferences > Java > Editor > Content Assist > Completion overwrites'.