If you want to reference any UI component, defined in the Layout XML file, then you need to use the findViewById method.
In your Activity class add the following code snippet
TextView helloText = (TextView) findViewById(R.id.helloId);
if(helloText != null){
Log.v(TAG, “Lets check the value in hello id “+helloText.getText());
}
Once you get a reference you can perform operations on the element.
Code: https://github.com/ankur-srivastava/HelloAndroid/blob/master/app/src/main/java/com/edocent/helloandroid/MainActivity.java