Public class with only public static members

Question

I have a public class that only contains public static members.

I know this is not the best thing to do but I was just wondering why on my Android, if I pause the application, open a few others and come back to mine, all variables seems to be (null).

Questions:

1. Is it because of some kind of a memory release made by Android?
2. What then could be a better way to keep such variables?
3. Is a class that extends Application a good option?

Here is my code:

public class Session {

public static String ID = null;
public static String UNIQID = null;
public static String TOKEN = null;
public static String SESSIONID = null;
}

Answer 1

As your application process might get destroyed any time, those static instances might get garbage collected indeed.

If you put these static variables in a custom Application object, same will apply, unless you initialise them in the application's onCreate function each time the application gets (re-)created.

You should keep track of persistent data using either SharedPreferences or a SQLite database.

If these variables are too complex to be stored like that then you might want to consider using a singleton (subclassing Application is not as recommended as it used to be).

public class MySingleton {

  public static MySingleton getInstance(Context context) {
    if (instance==null) {
      // Make sure you don't leak an activity by always using the application
      // context for singletons
      instance = new MySingleton(context.getApplicationContext());
    }
    return instance;
  }

  private static MySingleton instance = null;

  private MySingleton(Context context) {
    // init your stuff here...
  }

  private String id = null;
  private String uniqueId= null;
  private String token = null;
  private String sessionId = null;
}

Answer 2

1. Yes, android may collect memory when required

2. May be something like SharedPreferences