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I followed the advice given here in several posts on how to declare global contants:

public class Constants {

    public static final int i1 = 1;
    public static final int i2 = 2; 
    }

I just include this class in my project and refer to the constants like this:

in any other class...
    GlobalsVars.gi1 = Constants.i1;

(ps I hope this is OK and do not need to do anything to the Constants class like initializing or anything.)

But as I found out here: assigning int to Integer using static global variables is not a good idea. My app crashes sometimes when accessing the constants.

Though I find it really weird, since my app is rather small, but may be the Constants class - not an activity - is really removed from the memory in certain cases, though I access its constants in all my activities. That's why I would think it should not be removed from memory anyway.

But for sure, my app crashes in certain cases when accessing the Constants.i1 value.

What would be the best way just to declare some constants in a reliable way. (In c-Derivatives there are the easy to use macros) But there is nothing like this in Android.

-> all I need are "reliable" constants in Java...

EDIT:

declaration of GlobalVars class added

public class GlobalVars {
    public static Integer gi1;
    public static Integer gi2;
}

Many thanks

EDIT: added crash log

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.xxxx.xxxx/com.xxxx.xxxx.screens.One_screen}: java.lang.NullPointerException at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1830) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1851) at android.app.ActivityThread.access$1500(ActivityThread.java:132) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1038) at android.os.Handler.dispatchMessage(Handler.java:99) at android.os.Looper.loop(Looper.java:150) at android.app.ActivityThread.main(ActivityThread.java:4293) at java.lang.reflect.Method.invokeNative(Native Method) at java.lang.reflect.Method.invoke(Method.java:507) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:849) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:607) at dalvik.system.NativeStart.main(Native Method) Caused by: java.lang.NullPointerException at com.xxxx.xxxx.screens.Settings_screen.presentOnScreen(One_screen.java:172) at com.xxxx.xxxx.screens.Settings_screen.onCreate(One_screen.java:49) at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1072) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1794) ... 11 more

and the line 172 in One_screen is:

if (GlobalVars.gi1 == Constants.i1){
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user387184
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    if your variable gi1 is not a static variable , so you can't do like this NameOfYourClass.gi1 , this is only applied for statics variables ; example : YourClassName.staticVar = Constants.i1; or : this.gi1 = Constants.i1; – Houcine Nov 04 '11 at 13:51
  • I added the declaration of GlobalVars. There I declare the variables as static as you pointed out. – user387184 Nov 04 '11 at 13:56
  • It is very weird. If you decompile your code, you'll see that all the constant you refered are converted into values. Like this. Original) int a = Const.A; // value A = 1 Compiled) int a = 1; // referenced constant is converted into value itself. What about check your compiled code? – kingori Nov 04 '11 at 13:58

4 Answers4

3

The more general problem you are encountering is how to save state across several Activities and all parts of your application. A static variable (for instance, a singleton) is a common Java way of achieving this. I have found however, that a more elegant way in Android is to associate your state with the Application context.

As you know, each Activity is also a Context, which is information about its execution environment in the broadest sense. Your application also has a context, and Android guarantees that it will exist as a single instance across your application.

The way to do this is to create your own subclass of android.app.Application, and then specify that class in the application tag in your manifest. Now Android will automatically create an instance of that class and make it available for your entire application. You can access it from any context using the Context.getApplicationContext() method (Activity also provides a method getApplication() which has the exact same effect):

class MyApp extends Application {

  private String myState;

  public String getState(){
    return myState;
  }
  public void setState(String s){
    myState = s;
  }
}

class Blah extends Activity {

  @Override
  public void onCreate(Bundle b){
    ...
    MyApp appState = ((MyApp)getApplicationContext());
    String state = appState.getState();
    ...
  }
}

This has essentially the same effect as using a static variable or singleton, but integrates quite well into the existing Android framework. Note that this will not work across processes (should your app be one of the rare ones that has multiple processes).

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You don't need static in Java to make them constant. You can make them final and public and access them relatively the same way. The difference though, is you'll have to create an instance of the class with a reference ever single time you want to access the constants which consumes memory for no real reason. You can do the address this issue by following the Singleton pattern which will create a single instance of the class which you can access through the static getInstance() method.

DeeV
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  • thanks very much for the link - I checked but I am unclear, since the singleton which is assigned to a static variable could also be removed from memory - wouldn't this also cause that its values are lost then? since it's static I mean? – user387184 Nov 04 '11 at 14:06
  • A new one is created if it's null, so even if it's gone or deleted it will come back the next time you try to get it. – DeeV Nov 04 '11 at 14:09
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Please post the logs of the crash. After reading your other post (the one you linked to) I'm certain the problem lies elsewhere. For instance, in your other question you mention in a comment to an answer:

I got a crash report of my app in a place where it compares if (GlobalVars.gi1 == Constants.i1)

Autoboxing in Java5+ supports this type of comparison. The code given works universally.

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Pedantic
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  • Ok, so the only way this would fail is if GlobalVars.gi1 is null at the moment. Can't compare a null to a primitive. Without seeing more code about how or in what context this is used, I can't be any more help. – Pedantic Nov 04 '11 at 14:43
  • when the app starts I get the value from a DB like this: GlobalVars.gi1 = appCursor.getInt(appCursor.getColumnIndexOrThrow(MyDBAdapter.KEY1)); if (GlobalVars.gi1 != null) { ...some stuff here } else { GlobalVars.gi1 = Constants.i1; // ensure != null } - that's it. The value of GlobalVars.gi1 is then accessed in this activity without problems. Switching to the next activity I access the value again and this is where it crashes - but very very rarely - not even during my testing - only in one crash report I got. That's why I am even more confused... – user387184 Nov 04 '11 at 14:49
  • I check the DB value and ensure that GlobalVars.gi1 != null by setting it to Constants.i1 in case I should get null from the DB - please see my previous comment that I wrote – user387184 Nov 04 '11 at 14:53
  • Chris is right. The problem is elsewhere. Those values will not be reclaimed unless the entire application process is killed. – Che Jami Nov 04 '11 at 14:57
  • what do you mean when you write "those values will not be reclaimed" ? I don't quite understand – user387184 Nov 04 '11 at 14:59
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Your NullPointerException is occurring because GlobalVars.gi1 is null, not Constants.i1. You should always be able to rely on hard-coded integer values as they are part of the class definition.

If you simply wish to store a small number of long-lived integer variables. I suggest looking into SharedPreferences to store them (instead of GlobalVars).

You can find out more here.

If you wish to pass data only from one Activity to another, look at adding values to the Intent's extras using Intent.putExtra and retrieve the extras Bundle in the next Activity using Intent.getExtras on the Intent retrieved from Activity.getIntent.

Che Jami
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  • Thanks very much for your comment - I guess then whatever happens to my classes Contants or GlobalVars, its because they are removed from memory? I will implement both as singletons. I am also 100% sure that nowhere in my code this GlobalVars.gi1 is set to null - while it is initialized at the very start of the app with an extra verification not to be null. And by the way, accessing the same GlobalVars.gi1 in the previous activity did not cause the error. It must "loose" its value somehow then switching from one to the other activity? Right? – user387184 Nov 04 '11 at 14:41
  • You can't *lose* your `Constants` values because they are always assigned with your hard-coded values! The way you store the data depends on your needs. Roughly in the order of amount and complexity of the data: SharedPreferences -> Serialized Objects/Parcelable Objects -> Singletons -> Database. – Che Jami Nov 04 '11 at 14:44
  • I pass parameter values this way using putExtra, however I need a global Variable holder object that's why I use GlobalVars, so I can access all the values from all activities. Isn't this OK? The GlobalVars Class is imported to all activities and holds some global values? – user387184 Nov 04 '11 at 15:03
  • Yes this is fine. The only thing I can conclude is that your `GlobalVars` value is being set to `null` by your own application code. – Che Jami Nov 04 '11 at 15:11
  • thanks - I will change my classes Constants and GlobalVars to Singletons not using static and re-re-recheck the code. The problem is that I can not get it to crash at all myself... – user387184 Nov 04 '11 at 15:30