Question

Why is it a bad idea to use global variables?

I want to make an array of variables available for all of the classes to use. It will store key states, so it can’t be set as const (final).

Could you show me some other (proper) way of how to do this?

Answer 1

Why is it bad idea to use global variables?

Because it breaks encapsulation. All code can access them and modify them.

I want to make array of variables avaiable for all the classes to use

If there is only one instance to be used by all your code, you can use a singleton:

public class MyGlobalKeys {
    private static final MyGlobalKeys globals = new MyGlobalKeys();

    // disable construction outside of this class
    private MyGlobalKeys() {}

    public void getInstance() {
       return globals;
    }

    public void addKey(String key, bool state) { ... }
    public bool hasKey(String key) { ... }
    // and so on ...
}

Note that this sample is particularly not thread-safe – for improved implementation approaches, see Java Singleton Design Pattern