Android : Enums Vs Magic Constants

I was recently introduced to IntDef and StringDef interfaces by a co worker. I was using enums to maintain a list of constants for some purpose and he suggested me to use these interfaces. I went ahead to read more on that and would be sharing here what I learned.

So, it turns out that enums often require more than twice as much memory as static constants (quoted from andev docs). They are class objects, so :

• A single reference to an enum constant occupies 4 bytes
• Every enum occupies space that is the sum of all its fields’ sizes and overhead of an object
• the class itself occupies some space.

Whereas constants require fixed space, 4 bytes in case of an int constant and (num of chars) * 1 bytes in a string constant.

That said, we can replace enums with constants only in cases where we are just using them as constants. In cases where we are

• storing more (or complex data) data in enums, or
• using method overloading (every enum constant can have it’s own implementation of a method)

we’ll have to go with enums.

Now, I’ll be explaining how to use these interfaces.

We declare them as following :

    @StringDef({NotificationAction.DISPLAYED, NotificationAction.CONTENT_CLICKED,
        NotificationAction.ACTION_CLICKED, NotificationAction.DISMISSED})
    @Retention(RetentionPolicy.SOURCE)
    public @interface NotificationAction {
        String DISPLAYED = "Displayed";
        String CONTENT_CLICKED = "ContentClicked";
        String ACTION_CLICKED = "ActionClicked";
        String DISMISSED = "Dismissed";
    }

That’s it ! Now you can directly use them for annotating fields or method parameters in your code.

Fields :

@NotificationAction String action;

action = "dummy" // doesn't work
action = NotificationAction.DISPLAYED; // works

Methods :

public void actOnAction(@NotificationAction action){}

actOnAction("dummy") // doesn't work
actOnAction(NotificationAction.DISMISSED) // works

This way, it also makes it convenient to allow only certain values at these places.

It won’t improve your app’s performance by a significant bit but yeah, small things like this do count.

Let me know in case of any doubts.

Cheers !

Ref : http://stackoverflow.com/questions/29183904/should-i-strictly-avoid-using-enums-on-android