Android : Calling multiple layouts in single java class

Issue

I have a java class in which i am calling multiple layouts on basis on button clicked. The button Defined on the primary page works fine. But I am Unable to do the same with the buttons on other layout pages.

My Code

public class Aptitude extends Activity implements OnClickListener {

    DBHelper helper;
    SQLiteDatabase db;
    Button btnBegin,btnnext;

    //Sub category Buttons
    Button btnaptitude5,btnaptitude4,btnaptitude3,btnaptitude2,btnaptitude1;

    String Question_ID,Title,TitleDescription,QuestionText,QuestionTemplate,QuestionImage;

    TextView tvTitle,tvInstructions,tvQuestionText;

    RadioGroup rgtemplate4images;
    ImageView img;



    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.aptitude);


        // these buttons are on a different layout xml file...

        btnaptitude1 = (Button)findViewById(R.id.btnaptitude5);
        btnaptitude2 = (Button)findViewById(R.id.btnaptitude3);
        btnaptitude3 = (Button)findViewById(R.id.btnaptitude1);
        btnaptitude4 = (Button)findViewById(R.id.btnaptitude2);
        btnaptitude5 = (Button)findViewById(R.id.btnaptitude4);

        btnaptitude1.setOnClickListener(this);
        btnaptitude2.setOnClickListener(this);
        btnaptitude3.setOnClickListener(this);
        btnaptitude4.setOnClickListener(this);
        btnaptitude5.setOnClickListener(this);

        helper=new DBHelper(this);


        btnBegin = (Button)findViewById(R.id.btnBeginAptitude);

        btnBegin.setOnClickListener(this);

    }


    @Override
    public void onClick(View v) {
        // TODO Auto-generated method stub

        if (v==btnBegin)
        {
                setContentView(R.layout.aptitudesubcategory);





            if (v== btnaptitude1)
            {               

                getAllQuestionsAptitude("1","1");

            }








//            setContentView(R.layout.template4optionsimage);
//          tvTitle = (TextView)findViewById(R.id.tvTitleTemplate4optoinsImage);
//          tvInstructions = (TextView)findViewById(R.id.tvInstructionstemplate4);
//          tvQuestionText = (TextView)findViewById(R.id.tvQuestionTextTemplate4Image);
//          
//          img = (ImageView)findViewById(R.id.imageViewtemplate4options);
//          
//          tvTitle.setText(Title);
//          tvInstructions.setText(TitleDescription);
//          tvQuestionText.setText(QuestionText);
//          
//          String imgName = QuestionImage; // specify here your image name fetched from db
//          String uri = "drawable/" + imgName;
//          int icon = getResources().getIdentifier(uri, "drawable", getPackageName());
//          img.setImageResource(icon);








        }



    }




}

Stack Trace

07-16 06:47:18.931: E/AndroidRuntime(13440): FATAL EXCEPTION: main
07-16 06:47:18.931: E/AndroidRuntime(13440): Process: com.cldonline.assesmenttool, PID: 13440
07-16 06:47:18.931: E/AndroidRuntime(13440): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.cldonline.assesmenttool/com.cldonline.assesmenttool.Aptitude}: java.lang.NullPointerException
07-16 06:47:18.931: E/AndroidRuntime(13440):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
07-16 06:47:18.931: E/AndroidRuntime(13440):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
07-16 06:47:18.931: E/AndroidRuntime(13440):    at android.app.ActivityThread.access$800(ActivityThread.java:135)
07-16 06:47:18.931: E/AndroidRuntime(13440):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
07-16 06:47:18.931: E/AndroidRuntime(13440):    at android.os.Handler.dispatchMessage(Handler.java:102)
07-16 06:47:18.931: E/AndroidRuntime(13440):    at android.os.Looper.loop(Looper.java:136)
07-16 06:47:18.931: E/AndroidRuntime(13440):    at android.app.ActivityThread.main(ActivityThread.java:5017)
07-16 06:47:18.931: E/AndroidRuntime(13440):    at java.lang.reflect.Method.invokeNative(Native Method)
07-16 06:47:18.931: E/AndroidRuntime(13440):    at java.lang.reflect.Method.invoke(Method.java:515)
07-16 06:47:18.931: E/AndroidRuntime(13440):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
07-16 06:47:18.931: E/AndroidRuntime(13440):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
07-16 06:47:18.931: E/AndroidRuntime(13440):    at dalvik.system.NativeStart.main(Native Method)
07-16 06:47:18.931: E/AndroidRuntime(13440): Caused by: java.lang.NullPointerException
07-16 06:47:18.931: E/AndroidRuntime(13440):    at com.cldonline.assesmenttool.Aptitude.onCreate(Aptitude.java:56)
07-16 06:47:18.931: E/AndroidRuntime(13440):    at android.app.Activity.performCreate(Activity.java:5231)
07-16 06:47:18.931: E/AndroidRuntime(13440):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
07-16 06:47:18.931: E/AndroidRuntime(13440):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
07-16 06:47:18.931: E/AndroidRuntime(13440):    ... 11 more