Question

Is it possible to use Java code to write to the main.xml file?
I want to make some changes to the layout but have it based from values received from Java?

Answer 1

Yes you can. Read more about YOURLAYOUT.LayoutParams

http://developer.android.com/reference/android/widget/LinearLayout.LayoutParams.html
Use the addRule() and specify what you need to your layout object.
Change the values before you do setContentView();

Something like:

 RelativeLayout rl = new RelativeLayout(this);
       RelativeLayout.LayoutParams rlp= new RelativeLayout.LayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.FILL_PARENT);
       rlp.addRule(RelativeLayout.VISIBLE); //set the rules and parameters before you do setLayoutParams();
       rl.setLayoutParams(rlp);  
       setContentView(rl);

Or if you want to make changes on the actual layout from the xml then you do the inflating

  LayoutInflater inflater = (LayoutInflater)this.getSystemService(Context.LAYOUT_INFLATER_SERVICE);

    View v = inflater.inflate(R.layout.main, null);
    RelativeLayout rl = (RelativeLayout)v.findViewById(R.id.relativeLayout1);
    RelativeLayout.LayoutParams rlp= new RelativeLayout.LayoutParams(new LayoutParams(RelativeLayout.LayoutParams.WRAP_CONTENT,RelativeLayout.LayoutParams.WRAP_CONTENT));
    rlp.addRule(RelativeLayout.VISIBLE);
    rl.setLayoutParams(new LayoutParams(RelativeLayout.LayoutParams.WRAP_CONTENT,RelativeLayout.LayoutParams.WRAP_CONTENT));   
    setContentView(rl);