creating a file on android

I am trying to create a file with my android application. I need to write to the file in a specific class. The code I currently have for it is list开发者_如何转开发ed below. I keep getting a nullpointerexception when it is trying to write to the file. The exact error is listed below. I am new to android, so please be detailed.

Can someone please tell me what I am doing wrong and how i can fix this issue?

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//the class where i need to create and write to the file
public class DataRobot {
    Context tThis;
    FileOutputStream fOut;
    OutputStreamWriter writer;

    public DataRobot(SmartApp smartApp) extends Activity {
        tThis = (Context) smartApp;
    }
    public void onCreate(Bundle savedInstanceState) {
        try {
            //file = new File("/sdcard", "test.csv");
            fOut = openFileOutput("test.csv", MODE_WORLD_READABLE);
            writer = new OutputStreamWriter(fOut);
        } catch (NullPointerException e) {
            e.printStackTrace();
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
    public void analyzeData(SmartDataObject temp) {
        data = temp;
        try {
                //the following line is where the error is occurring.
            writer.write(Double.toString(data.getHeartRate()));
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
     }
}   

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//the error
03-26 03:47:53.924: WARN/System.err(273): java.lang.NullPointerException
03-26 03:47:53.934: WARN/System.err(273):     at cpe495.smartapp.DataRobot.analyzeData(DataRobot.java:80)
03-26 03:47:53.946: WARN/System.err(273):     at cpe495.smartapp.SmartApp$1.dataReceivedReceived(SmartApp.java:49)
03-26 03:47:53.956: WARN/System.err(273):     at cpe495.smartapp.ConnectDevice.fireDataReceivedEvent(ConnectDevice.java:79)
03-26 03:47:53.956: WARN/System.err(273):     at cpe495.smartapp.ConnectDevice.run(ConnectDevice.java:46)
03-26 03:47:53.965: WARN/System.err(273):     at java.lang.Thread.run(Thread.java:1096)

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It looks like you are doing this in your constructor. You can't do that. You need to wait until onCreate(), which is when you know the object has been initialized.

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You should acces files via content providers, I think the path is wrong... look at this file tutorial

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The following code is what i figured out and is working. Sorry it took so long to post.

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public class DataRobot {
    Context tThis; //was enabled for preferences
    private FileOutputStream fOut;
    private OutputStreamWriter writer;

    public DataRobot(SmartApp smartApp) {
        tThis = (Context) smartApp;

    }

    public void analyzeData(SmartDataObject temp) {
        try {
            fOut = tThis.openFileOutput("test.csv", tThis.MODE_APPEND);
            writer = new OutputStreamWriter(fOut);
        } catch (NullPointerException e) {
            e.printStackTrace();
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }

        try {
            writer.write("Test");//0
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}