Question

As a novice Android developer, I’ve faced a bit strange problem. I want to create a class, which methods other classes-activities-whatever could use for working with files in some special way. Let`s say for simplicity we would be logging some stuff. If I do following within an activity (in OnClick listener for example), everything works just fine:

FileOutputStream fOut = openFileOutput("somefile", MODE_PRIVATE);
OutputStreamWriter osw = new OutputStreamWriter(fOut);
osw.write("Very important foobar");
osw.flush();
osw.close();

But when I try to enclose that into some class and create singleton like that:

public class Logger extends BaseActivity {
//BaseActivity is the "init" class which extends Activity

public static final Logger INSTANCE = new Logger();
private Logger() { 
// singleton
}

public boolean doLog (String whatToLog) {
 try {
     FileOutputStream fOut = openFileOutput("somefile", MODE_PRIVATE);
 OutputStreamWriter osw = new OutputStreamWriter(fOut);
 osw.write(whatToLog);
 osw.flush();
 osw.close(); }
     catch (IOException ioe) { ioe.printStackTrace(); }  
     return true; }

and call it from other activity like that

Logger.INSTANCE.doLog("foobar");  

app chrashes with NullPointerException (at line with openFileOutput). I suppose it`s because of improper use of singleton/activity here and now rewriting the code to run as a service. But maybe there are some better ideas to solve an issue? Or some workarounds?

Thanks for your contributions in advance!

Answer 1

You based your singleton on an Activity that you didn’t start as an Activity. Therefore, it doesn’t have a valid Context, which is necessary for the IO calls. See Blundell’s answer for better singleton, with one change: As per android.app.Application javadoc, your singleton should get the application context from the given context via Context.getApplicationContext(). See why does AndroidTestCase.getContext().getApplicationContext() return null for another example.