Question

What I’m doing exactly is this:
I have a class “Welcome” that calls another class “textWriter” which contains a method “write”. Welcome passes two strings to “write”, “name” is the name of the text file to write to and “something” is the thing to print.

i keep getting null pointer error on prt = new PrintStream(output);

here is the logcat

07-22 16:40:22.561: ERROR/AndroidRuntime(2245): FATAL EXCEPTION: main
07-22 16:40:22.561: ERROR/AndroidRuntime(2245): java.lang.NullPointerException
07-22 16:40:22.561: ERROR/AndroidRuntime(2245):     at android.content.ContextWrapper.openFileOutput(ContextWrapper.java:158)
07-22 16:40:22.561: ERROR/AndroidRuntime(2245):     at com.rhobot.budget.textWriter.write(textWriter.java:24)
07-22 16:40:22.561: ERROR/AndroidRuntime(2245):     at com.rhobot.budget.Welcome.setSubChoices(Welcome.java:37)
07-22 16:40:22.561: ERROR/AndroidRuntime(2245):     at com.rhobot.budget.Welcome$1.onClick(Welcome.java:76)
07-22 16:40:22.561: ERROR/AndroidRuntime(2245):     at android.view.View.performClick(View.java:2408)
07-22 16:40:22.561: ERROR/AndroidRuntime(2245):     at android.view.View$PerformClick.run(View.java:8816)
07-22 16:40:22.561: ERROR/AndroidRuntime(2245):     at android.os.Handler.handleCallback(Handler.java:587)
07-22 16:40:22.561: ERROR/AndroidRuntime(2245):     at android.os.Handler.dispatchMessage(Handler.java:92)
07-22 16:40:22.561: ERROR/AndroidRuntime(2245):     at android.os.Looper.loop(Looper.java:123)
07-22 16:40:22.561: ERROR/AndroidRuntime(2245):     at android.app.ActivityThread.main(ActivityThread.java:4627)
07-22 16:40:22.561: ERROR/AndroidRuntime(2245):     at java.lang.reflect.Method.invokeNative(Native Method)
07-22 16:40:22.561: ERROR/AndroidRuntime(2245):     at java.lang.reflect.Method.invoke(Method.java:521)
07-22 16:40:22.561: ERROR/AndroidRuntime(2245):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
07-22 16:40:22.561: ERROR/AndroidRuntime(2245):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
07-22 16:40:22.561: ERROR/AndroidRuntime(2245):     at dalvik.system.NativeStart.main(Native Method)

here is the Welcome class:

public class Welcome extends Activity
{
P p = new P(); //shared prefs helper
loadArray la = new loadArray(); //array loader
textWriter tw = new textWriter();
private EditText txtBalance;
private Button btnEnter;
int subItemCount, addItemCount;
public void onCreate(Bundle savedInstanceState)
{
    super.onCreate(savedInstanceState);
    setContentView(R.layout.welcomescreen);
    capture();
}
public static String sub_choice_key = "sub_choice_key.txt";
public static String sub_compare = "sub_compare.txt";
public static String add_choice_key = "add_choice_key.txt";
public static String add_compare = "add_compare.txt";

public void setSubChoices() throws FileNotFoundException {
    // TODO create a master list of choices
    tw.write(sub_choice_key,"-1");
    tw.write(sub_choice_key,"-2");
    tw.write(sub_choice_key,"-3");
    tw.write(sub_compare,"-1");
    tw.write(sub_compare,"-2");
    tw.write(sub_compare,"-3");
    p.pisI("subItemCount", 3, this);
}...

Here is the textWriter class

public class textWriter extends Activity{

@Override
public void onCreate(Bundle savedInstanceState)
{
    super.onCreate(savedInstanceState);
}
public void write(String name, String something) {

    try {
        FileOutputStream output;
        output = openFileOutput(name,MODE_APPEND);
        PrintStream prt;
        prt = new PrintStream(output);
        prt.println(something);
        prt.close();
    } catch (FileNotFoundException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
}

id like to emphasize that these are two different files as well, and i am importing all the right stuff i believe.. also i have the line
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
in my android manifest.

what am i doing wrong? is it context? or lack there of? if so how should i implement it?

i am doing it this way because i want to use this textWrite in other programs as an easy (less messy) way of writing data. in my main it could save me about 50 lines of code if i do it this way.
Thanks in advance-
Tricknology

Answer 1

As probablykevin said, the issue is a lack of a valid Context.

One simple fix you could do if you want your class structure to stay the same is to remove the extends Activity from textWriter and instead pass in a Context in the constructor, i.e.:

public class textWriter{
    private Context m_context;

    public textWriter(Context ctx) {
        m_context = ctx;
    }    
    public void write(String name, String something) {

        try {
            FileOutputStream output;
            output = m_context.openFileOutput(name,MODE_APPEND);
            PrintStream prt;
            prt = new PrintStream(output);
            prt.println(something);
            prt.close();
        } catch (FileNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}

You’d then need to construct it like so: textWriter tw = new textWriter(this);