Carpet Calculator Program Final Edit
Your code is near perfection, if you want to "get OCD" about it you can skip all the empty lines, comments on self-explanatory methods and fields and open code blocks on the same line, for example:

 public Room_Dimension(double length, double width)
{
    super(); 
    this.length = length;
    this.width = width;
}


and all the rest, could be written as:

 public Room_Dimension(double length, double width){
    super(); 
    this.length = length;
    this.width = width;
}


to save space, so that the class is visible within the screen with no need of scrolling. Additionally, search for program layout and class interface best practices, to get insight on the subject.