XML : Android Changing xml

Hello I want to change text in my textView , but dont know how. I tryed to change it like this

1.   TextView textView3 = (TextView) findViewById(R.id.textView3);  textView3.append("hi");    

   But this textView is in another xml-file.

   the compiler outputs

2.   FATAL EXCEPTION: main  java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.nividimka.shopapplication/com.example.nividimka.shopapplication.ShopActivity}: java.lang.NullPointerException  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2351)  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2403)  at android.app.ActivityThread.access$600(ActivityThread.java:165)  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1373)  at android.os.Handler.dispatchMessage(Handler.java:107)  at android.os.Looper.loop(Looper.java:194)  at android.app.ActivityThread.main(ActivityThread.java:5391)  at java.lang.reflect.Method.invokeNative(Native Method)  at java.lang.reflect.Method.invoke(Method.java:525)  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:833)  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:600)  at dalvik.system.NativeStart.main(Native Method)  Caused by: java.lang.NullPointerException  at com.example.nividimka.shopapplication.ShopActivity.onCreate(ShopActivity.java:62)  at android.app.Activity.performCreate(Activity.java:5122)  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1150)  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2315)  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2403)   at android.app.ActivityThread.access$600(ActivityThread.java:165)   at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1373)   at android.os.Handler.dispatchMessage(Handler.java:107)   at android.os.Looper.loop(Looper.java:194)   at android.app.ActivityThread.main(ActivityThread.java:5391)   at java.lang.reflect.Method.invokeNative(Native Method)   at java.lang.reflect.Method.invoke(Method.java:525)   at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:833)   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:600)   at dalvik.system.NativeStart.main(Native Method)