Android: listview crashes

I'm using android listview and its working perfectly fine. my implementation as below

ListView listview = (ListView)findViewById(R.id.list);
setListAdapter(new ArrayAdapter<String>(MyIncidentActivity.this,
    R.layout.row_incident, R.id.label_incident, db_results));

Now I introduced onItemClickListener as below and application crashes for no reason.

listview.setOnItemClickListener(new OnItemClickListener() {

            @Override
            public void onItemClick(AdapterView<?> arg0, View arg1, int arg2,
                    long arg3) {
                // TODO Auto-generated method stub

            }
        });

logcat

08-12 14:44:40.105: ERROR/AndroidRuntime(14525): FATAL EXCEPTION: main
08-12 14:44:40.105: ERROR/AndroidRuntime(14525): java.lang.RuntimeException: Unable to start activity ComponentInfo{a.b/a.b.MyIncidentActivity}: java.lang.NullPointerException
08-12 14:44:40.105: ERROR/AndroidRuntime(14525):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1647)
08-12 14:44:40.105: ERROR/AndroidRuntime(14525):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1663)
08-12 14:44:40.105: ERROR/AndroidRuntime(14525):     at android.app.ActivityThread.access$1500(ActivityThread.java:117)
08-12 14:44:40.105: ERROR/AndroidRuntime(14525):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:931)
08-12 14:44:40.105: ERROR/AndroidRuntime(14525):     at android.os.Handler.dispatchMessage(Handler.java:99)
08-12 14:44:40.105: ERROR/AndroidRuntime(14525):     at android.os.Looper.loop(Looper.java:123)
08-12 14:44:40.105: ERROR/AndroidRuntime(14525):     at android.app.ActivityThread.main(ActivityThread.java:3683)
08-12 14:44:40.105: ERROR/AndroidRuntime(14525):     at java.lang.reflect.Method.invokeNative(Native Method)
08-12 14:44:40.105: ERROR/AndroidRuntime(14525):     at java.lang.reflect.Method.invoke(Method.java:507)
08-12 14:44:40.105: ERROR/AndroidRuntime(14525):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
08-12 14:44:40.105: ERROR/AndroidRuntime(14525):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
08-12 14:44:40.105: ERROR/AndroidRuntime(14525):     at dalvik.system.NativeStart.main(Native Method)
08-12 14:44:40.105: ERROR/AndroidRuntime(14525): Caused by: java.lang.NullPointerException
08-12 14:44:40.105: ERROR/AndroidRuntime(14525):     at a.b.MyIncidentActivity.onCreate(MyIncidentActivity.java:50)
08-12 14:44:40.105: ERROR/AndroidRuntime(14525):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
08-12 14:44:40.105: ERROR/AndroidRuntime(14525):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1611)
08-12 14:44:40.105: ERROR/AndroidRuntime(14525):     ... 11 more

Can someone help me find out the reason. I'm lost with this error. within same application same implementation works fine but don't know why its not working.

回答1:

Well from you code where you are using

setListAdapter(adapter);

indicates that you are using ListActivity, and while using ListActivity you get your list as follow:

listView = this.getListView();

or you can use

listView = this.findViewById(android.R.id.list);

and you are using

listView = this.findViewById(R.id.list);

which is wrong in case of ListActivity, and ListView cannot be found and results in NullPointerException.

回答2:

Try like this.

Public class ABCD implements OnItemClickListener{
          ListView listview;//class varible

           listview = (ListView)findViewById(R.id.list);
           setListAdapter(new ArrayAdapter<String>(MyIncidentActivity.this,
           R.layout.row_incident, R.id.label_incident, db_results));
           listview.setOnItemClickListener(this);

        @Override
    public void onItemClick(AdapterView<?> arg0, View arg1, int arg2, long arg3) {
        // TODO Auto-generated method stub

    }

}