You can use the Display View to type in an expression and evaluate it in the context of the current stack frame. (The name Display is not great - see bug 342144).
- Use content assist to type expressions
- Select the expression and use Ctrl+Shift+I (Inspect) to show the result in a pop-up. You can also invoke the Inspect command from the context menu.
- Press Ctrl+Shift+I again to persist the result in the Expressions view.
See also Eclipse help.