Question

I sometimes get a null pointer exception with my code. Oddly enough, this does not happen every time, and I’m not sure why. Help?

package net.obviam.droidz;
public class HomeView extends Activity {
    public void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);



     setContentView(R.layout.menu);



    Button page1 = (Button) findViewById(R.id.Button01);
         page1.setOnClickListener(new View.OnClickListener() {
                 public void onClick(View view) {
//                   Intent i = getIntent();
                     Intent myIntent = new Intent(view.getContext(), DroidzActivity.class);
                     startActivityForResult(myIntent, 0);
                 }

     });


}



protected void onDestroy() {
//      Log.d(TAG, "Destroying..");
        super.onDestroy();
    }
    @Override
    protected void onStop() {
//      Log.d(TAG, "Stopping...");
        super.onStop();
    }

}

The logcat says that the problem is with line 19, the setOnClickListener. I’m not sure what’s wrong with it though. It doesn’t appear to be referencing anything.

The Logcat reads like this:

02-17 16:52:13.816: E/AndroidRuntime(23756): FATAL EXCEPTION: main
02-17 16:52:13.816: E/AndroidRuntime(23756): java.lang.RuntimeException: Unable to start activity ComponentInfo{net.obviam.droidz/net.obviam.droidz.HomeView}: java.lang.NullPointerException
02-17 16:52:13.816: E/AndroidRuntime(23756):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1651)
02-17 16:52:13.816: E/AndroidRuntime(23756):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1667)
02-17 16:52:13.816: E/AndroidRuntime(23756):    at android.app.ActivityThread.access$1500(ActivityThread.java:117)
02-17 16:52:13.816: E/AndroidRuntime(23756):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:935)
02-17 16:52:13.816: E/AndroidRuntime(23756):    at android.os.Handler.dispatchMessage(Handler.java:99)
02-17 16:52:13.816: E/AndroidRuntime(23756):    at android.os.Looper.loop(Looper.java:130)
02-17 16:52:13.816: E/AndroidRuntime(23756):    at android.app.ActivityThread.main(ActivityThread.java:3687)
02-17 16:52:13.816: E/AndroidRuntime(23756):    at java.lang.reflect.Method.invokeNative(Native Method)
02-17 16:52:13.816: E/AndroidRuntime(23756):    at java.lang.reflect.Method.invoke(Method.java:507)
02-17 16:52:13.816: E/AndroidRuntime(23756):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:842)
02-17 16:52:13.816: E/AndroidRuntime(23756):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:600)
02-17 16:52:13.816: E/AndroidRuntime(23756):    at dalvik.system.NativeStart.main(Native Method)
02-17 16:52:13.816: E/AndroidRuntime(23756): Caused by: java.lang.NullPointerException
02-17 16:52:13.816: E/AndroidRuntime(23756):    at net.obviam.droidz.HomeView.onCreate(HomeView.java:19)
02-17 16:52:13.816: E/AndroidRuntime(23756):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
02-17 16:52:13.816: E/AndroidRuntime(23756):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1615)
02-17 16:52:13.816: E/AndroidRuntime(23756):    ... 11 more

And here is the manifest file:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="net.obviam.droidz"
android:versionCode="1"
android:versionName="1.0" >

<uses-sdk android:minSdkVersion="10" />

<application
    android:icon="@drawable/images"
    android:label="@string/app_name" android:debuggable="true">
    <activity android:name=".HomeView" android:label="@string/app_name">
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />
            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
    <activity android:name=".DroidzActivity"></activity>

</application>

And here is the xml file that the button is found in:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent">

 <Button android:text="Start Game!"
        android:id="@+id/Button01"
        android:layout_width="250dp"
        android:textSize="18dp"
        android:layout_height="55dp">
    </Button>



</LinearLayout>

Answer 1

After reading through the stack trace provided, I think the problem is when you start the new intent, you to pass the Activity’s context, NOT the View’s:

Button page1 = (Button) findViewById(R.id.Button01);
page1.setOnClickListener(new View.OnClickListener() {
    public void onClick(View view) {
        Intent myIntent = new Intent(getContext(), DroidzActivity.class); // fix View.getContext() to getContext()
        startActivity(myIntent);    // change to startActivity
    }
});

As to why this is an intermitent problem, I have no idea.