Well if you want to rename all instances of a variable in a file you can use a quick assist to do it. Just place the cursor at the variable and press Ctrl + 1 to see a list of Quick Assists available. Quick Fixes are similar but help resolve common errors.
I use the ‘rename in file’ quick assist all the time. But pressing Ctrl + 1 and then selecting rename in file is not very intuitive. Since Eclipse 3.2 there is an easier way of doing this. You can assign keyboard shortcuts to these Quick Fixes. I have assigned Ctrl + Alt + R to rename in file. This is a lot easier.