Android Finding View Components (R.id...)

I know the error is in this piece of following code.

TextView my=(TextView)findViewById(R.id.textId);

I have searched everywhere on google to resolve this issue, but none of those remedies seems to fixing my problem at all. I keep getting a nullpointer exception which I don't know why. 5 weeks ago, I had no such problem like this; however, I first encountered this problem yesterday. I have even reinstalled android studio but the problem still won't go away.

Here is my mainactivty. java code.

package com.example.fahman_khan75.testing;

import android.support.v7.app.AppCompatActivity; import android.os.Bundle; import android.view.Menu; import android.view.MenuItem; import android.widget.TextView; import android.app.Activity;

public class MainActivity extends Activity { TextView my=(TextView)findViewById(R.id.textId); @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); }

}

Here is my android manifest file

<?xml version="1.0" encoding="utf-8"?> <manifest xmlns:android="http://schemas.android.com/apk/res/android" package="com.example.fahman_khan75.testing" >

<application
    android:allowBackup="true"
    android:icon="@mipmap/ic_launcher"
    android:label="@string/app_name"
    android:theme="@style/AppTheme" >
    <activity
        android:name=".MainActivity"
        android:label="@string/app_name" >
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
</application>

</manifest>

Finally, here is my XML file

<RelativeLayout 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:paddingLeft="@dimen/activity_horizontal_margin" android:paddingRight="@dimen/activity_horizontal_margin" android:paddingTop="@dimen/activity_vertical_margin" android:paddingBottom="@dimen/activity_vertical_margin" tools:context=".MainActivity">

<TextView android:text="@string/hello_world" android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/textId" />

</RelativeLayout>