unable to start an android app logact showing null pointer and classcast exception, the java file and xml file is as shown below

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http://stackoverflow.com – this is the code used in tha java class.it doesnt show any errors in eclipse.the logcat shows null pointer exception and class caste exception package com.myfirstapp; import android.app.Activity; import android.os.Bundle; import android.view.View; import android.widget.*; public class MyfirstappActivity extends Activity { /** Called when the activity is first created. */ int count ; Button add,sub; TextView txt; @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); count = 0; add = (Button) findViewById(R.id.add_one); sub = ( (HowTos)