Android Global Variables Not Persistent

I created a class that extends the application to store the variables that I want to get from several actions.

public class MyApplication extends Application 
{
private String fbId, firstName;
private long expires;

@Override
public void onCreate() 
{
    super.onCreate();
}

public String getFbId() 
{
    return fbId;
}

public void setFbId(String fbId) 
{
    this.fbId = fbId;
}

public String getFirstName()
{
    return firstName;
}

public void setFirstName(String firstName) 
{
    this.firstName = firstName;
}

I can access these variables and set these variables when I first run the application. As soon as I leave and restart the application, it will set my values ​​to zero, any suggestions as to why this could happen?