XML : How To Set Width/Height Of An Android Relative Layout Via Programming?

I am working on a Android apps using Android Studio and just a new to it. I want to make outer main layout on currentscreenwidth + 300dp and height should be 100% means match_parent. In that main outer layout, I am creating two layouts. Inner first layout will have a fix 300dp width and the other layout should have screenwidth+300dp. For that I have to main layout width currentscreenwidth + 300dp becasue one of my inner layout have fixed 300dp and other one have match_parent and placed in a row. For this I am using below codes but its not working. When I manually write width about 1000dp to outer layout then my inner layout work fine as I want but I want the fix calculation of currentscreenwidth + 300dp. How Can I Do This?

activity_main.xml:

  <?xml version="1.0" encoding="utf-8"?>  <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"      xmlns:tools="http://schemas.android.com/tools"      android:id="@+id/mainbody"      android:layout_width="match_parent"   //currentscreenwidth + 300dp      android:layout_height="match_parent"      tools:context="com.domain.project.MainActivity">        <RelativeLayout          android:id="@+id/menubar"          android:layout_width="300dp"   //300dp          android:layout_height="fill_parent">      </RelativeLayout>      <RelativeLayout          android:layout_toRightOf="@+id/menubar"          android:layout_width="match_parent"    //currentscreenwidth          android:layout_height="fill_parent">      </RelativeLayout>    </RelativeLayout>    

MainActivity.java:

  package com.domain.project;    import android.support.v7.app.AppCompatActivity;  import android.os.Bundle;  import android.util.DisplayMetrics;  import android.view.Display;  import android.widget.Button;  import android.widget.RelativeLayout;    public class MainActivity extends AppCompatActivity {        @Override      protected void onCreate(Bundle savedInstanceState) {          super.onCreate(savedInstanceState);          setContentView(R.layout.activity_main);            // Get Width And Height Of Screen On Page Load          Display display = getWindowManager().getDefaultDisplay();          DisplayMetrics outMetrics = new DisplayMetrics();          display.getMetrics(outMetrics);          float density  = getResources().getDisplayMetrics().density;          float dpHeight = outMetrics.heightPixels / density;          float dpWidth  = outMetrics.widthPixels / density;            int width = Math.round(dpWidth);          RelativeLayout rl = (RelativeLayout) findViewById(R.id.mainbody);          //rl.getLayoutParams().height = 100;          rl.getLayoutParams().width = width + 300;        }      }