java separate file for global variables

Question

I have a newbie question. If I have some global variables that are shared by two classes or more how can I have them in a separate file so that any class can read and update them. Is this possible without using Interfaces?

Answer 1

Yes, since interfaces variables are all implicitly static, so each of these variables has only one instance in the jvm.

However, a better way to do it [in my opinion] would probably be having them declared in some singleton class and using it.

Answer 2

The best way to do this is to have your shared application state accessible via interface methods, then have an implementing class that holds the variables, and pass this instance of the class to your other classes during construction (which they accept as an instance of the interface).

This is better than using a static class or singleton since it allows you to mock out the functionality of the shared state for testing, improves general code reusability, and allows you to change the implementation and configuration of the shared state without impacting any of the code using it.

E.g.

// Session interface for all application shared state.
public interface ApplicationSession
{
  public int getMaxUserLimit();
}

// A backing for the interface (simple in memory version, maybe future versions use a database, who knows).
public class SomeApplicationSession implements ApplicationSession
{
  private volatile int maxUserLimit = 0;
  public void setMaxUserLimit(int limit) { this.maxUserLimit = limit; }
  public int getMaxUserLimit() { return maxUserLimit; }
}


// ClassA uses the supplied session.
public class MyClassA
{
  private ApplicationSession session;
  public myClassA(ApplicationSession session)
  {
    this.session = session;
  }
}

// usage...
public class MyMain
{
  public static void main(String[] args)
  {
    // Create / get session (ultimately possibly from a factory).
    ApplicationSession session = new SomeApplicationSession();

    ClassA myClassA = new ClassA(session);
    // do stuff..
  }
}