findViewById giving NullPointer exception in onCreate method

Question

I took care of placing setContentView before using findViewbyId method but still it is giving me error. can any one please explain why this is happeneing. Here is my code.

public class DispActivity extends ActionBarActivity {

public String message;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_disp);

    if (savedInstanceState == null) {
        getSupportFragmentManager().beginTransaction()
                .add(R.id.container, new              PlaceholderFragment()).commit();
    }

    Intent intent = getIntent();
    message = intent.getStringExtra(MainActivity.Extra);
    EditText txt = (EditText)findViewById(R.id.edit_text23);
    System.out.println("check point 1");
    txt.setText("Having problem here");
    System.out.println("check point 2");


}

fragment_disp.xml lookss like this.

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.example.wifi.DispActivity$PlaceholderFragment" >

<EditText 
    android:id="@+id/edit_text23"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
/>
<TextView
    android:id="@+id/text"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/hello_world" />
<Button android:id="@+id/but"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:onClick="click"/>
</LinearLayout>

Error that is occuring is

05-13 23:50:46.716: I/System.out(5713): check point 1
05-13 23:50:46.716: D/AndroidRuntime(5713): Shutting down VM
05-13 23:50:46.716: W/dalvikvm(5713): threadid=1: thread exiting with uncaught     exception (group=0x419c9ba8)
05-13 23:50:46.726: E/AndroidRuntime(5713): FATAL EXCEPTION: main
05-13 23:50:46.726: E/AndroidRuntime(5713): Process: com.example.wifi, PID: 5713
05-13 23:50:46.726: E/AndroidRuntime(5713): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.wifi/com.example.wifi.DispActivity}: java.lang.ClassCastException: android.widget.TextView cannot be cast to android.widget.EditText
05-13 23:50:46.726: E/AndroidRuntime(5713):     at  android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
05-13 23:50:46.726: E/AndroidRuntime(5713):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
05-13 23:50:46.726: E/AndroidRuntime(5713):     at android.app.ActivityThread.access$800(ActivityThread.java:135)
05-13 23:50:46.726: E/AndroidRuntime(5713):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
05-13 23:50:46.726: E/AndroidRuntime(5713):     at android.os.Handler.dispatchMessage(Handler.java:102)
05-13 23:50:46.726: E/AndroidRuntime(5713):     at android.os.Looper.loop(Looper.java:136)
05-13 23:50:46.726: E/AndroidRuntime(5713):     at android.app.ActivityThread.main(ActivityThread.java:5017)
05-13 23:50:46.726: E/AndroidRuntime(5713):     at java.lang.reflect.Method.invokeNative(Native Method)
05-13 23:50:46.726: E/AndroidRuntime(5713):     at java.lang.reflect.Method.invoke(Method.java:515)
05-13 23:50:46.726: E/AndroidRuntime(5713):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
05-13 23:50:46.726: E/AndroidRuntime(5713):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
05-13 23:50:46.726: E/AndroidRuntime(5713):     at dalvik.system.NativeStart.main(Native Method)
05-13 23:50:46.726: E/AndroidRuntime(5713): Caused by: java.lang.ClassCastException: android.widget.TextView cannot be cast to android.widget.EditText
05-13 23:50:46.726: E/AndroidRuntime(5713):     at com.example.wifi.DispActivity$PlaceholderFragment.onCreateView(DispActivity.java:74)
05-13 23:50:46.726: E/AndroidRuntime(5713):     at android.support.v4.app.Fragment.performCreateView(Fragment.java:1500)
05-13 23:50:46.726: E/AndroidRuntime(5713):     at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:938)
05-13 23:50:46.726: E/AndroidRuntime(5713):     at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1115)
05-13 23:50:46.726: E/AndroidRuntime(5713):     at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:682)
05-13 23:50:46.726: E/AndroidRuntime(5713):     at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1478)
05-13 23:50:46.726: E/AndroidRuntime(5713):     at android.support.v4.app.FragmentActivity.onStart(FragmentActivity.java:570)
05-13 23:50:46.726: E/AndroidRuntime(5713):     at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1171)
05-13 23:50:46.726: E/AndroidRuntime(5713):     at android.app.Activity.performStart(Activity.java:5241)
05-13 23:50:46.726: E/AndroidRuntime(5713):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2168)
05-13 23:50:46.726: E/AndroidRuntime(5713):     ... 11 more

Answer 1

This code should be put in the fragment PlaceholderFragment.

EditText txt = (EditText)findViewById(R.id.edit_text23);
System.out.println("check point 1");
txt.setText("Having problem here");
System.out.println("check point 2");

The EditText is located in fragment_disp.xml and you are referencing that in your layout R.layout.activity_disp which is why you are getting a NPE. The activity_disp.xml doesn't have that EditText.

EDITED

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    View view = inflater.inflate(R.layout.fragment_disp, container, false);

    EditText txt = (EditText) view.findViewById(R.id.edit_text23);
    System.out.println("check point 1");
    txt.setText("Having problem here");
    System.out.println("check point 2");
}

Answer 2

You are trying to cast a TextView to an EditText and that's causing the error. Are you sure you have given the entire relevant code? The stack trace is pretty clear with line numbers.