Java Swing ActionMap keys

Question

I'm coming up the learning curve for Swing and am reading about InputMap and ActionMap. The O'Reilly book on Swing says the keys for ActionMap can be any Object, but by convention are Strings.

Is there any reason to use an object other than a String? (e.g. an enum)

I would think it's bad practice to use String literals, as they can be mistyped without producing any compile errors. So I'm guessing it's good practice to store the ActionMap key as an immutable static final class variable defined once (e.g. MyAction.NAME).

edit: Just to clarify, the thought that came to mind when I was reading the docs was to use an enum rather than a String. Is there a reason I should use a String?

Answer 1

I'm not sure why you wouldn't use an enum directly (haven't looked at the O'Reilly book), but in cases like this, I'd probably rather use an Enum.toString() method, as in

MyActions.NAME.toString()

This saves the overhead of defining so many static variables, let's you extend the functionality of the MyActions enum if you wish to store further information about the state, and it guarantees a unique string per enum. All with compile-time errors on typos.

Edit: Swing predates Enum. Can that just be our convenient catch-all answer? Look at the api for card layout. If there was ever a design that begged for enums, it's a card layout (allows cycling through a list of components to be displayed in the same place). I suppose there are use-cases where strings might be preferred (user added material, for example), but come on: most of the time I'm going to be using this with some sort of predefined state pattern.

Answer 2

Actually, what you propose is what Swing itself does (sometimes), so you are right.

Check the javax.swing.DefaultEditorKit class. All these static final strings are used by Swing to create the ActionMap.

However, if you check their initialization for some of the look and feels, such as WindowsLookAndFeel, they also have hardcoded string literals in the source code.