I can see the immediate problem.  The code was 'improperly nested'.
Have a look over this variant..

<sscce>
public class DrawableRect extends Rect {

  /** The DrawableRect constructor just invokes the
  Rect(args) constructor */
  public DrawableRect(int x1, int y1, int x2, int y2) {
    super(x1,y1,x2,y2);
  }

  /** This is the new method defined by DrawableRect */
  public void draw(java.awt.Graphics g) {
    g.drawRect(x1, y1, (x2 - x1), (y2-y1));
  }

  public static void main(String[] args) {
    // the first version had the definition
    // of the constructor defined inside the
    // 'main' method.
    DrawableRect drawableRect =
      new DrawableRect(0,0,20,20);
  }
}

class Rect {
  int x1, y1, x2, y2;
  Rect(int x1, int y1, int x2, int y2){
  }
}
</sscce>

Note also that this is compilable code, as copy pasted.

When asking questions realted to code (even code that
will not compile) it is often useful to supply an
SSCCE, which is the next best thing.

Check here for more details..
<http://www.physci.org/codes/sscce.jsp>

HTH

Hi cjeff,

---> You have placed a method in a method! This is not allowed!...

... and the compiler tells you that your expression (it means your
method declaration) starts at a point in the source where it is not
allowed.

No. You think that what what you want can´t be done, but in fact, it
is not even understood.
What is "starting a subclass"? You can instantiate a subclass (provided
it is not abstract, but lets ignore that for a moment), but of course
only if the source for the class is syntactically/semantically correct
(which is not the case for your example). Change your "DrawableRect"
class as outlined below:
public class DrawableRect extends Rect{
   public DrawableRect(int x1,int y1,int x2,int y2){
   super(x1,y1,x2,y2)
   public static void main(String[] args){
       DrawableRect drawableRect = new DrawableRect(10,10,20,20);
   }
}

How would you do that? What commands would be necessary to "start from
Rect.java to run DrawableRect.java" ?
When a java program is running, there will be objects generated, doing
some stuff, and when they are no longer needed, they will disappear.
But you do you start this "chain of commands" or better "net of
commands"? What are the first commands that are executed by a java
program? How do you tell the java runtime which class should be used to
generate all the objects that are necessary (and that in turn may
generate new objects)? You have to give the JRE the name of the class.
The JRE will then look for that class. If it is not found, you will get
a "ClassNotFoundException". When the class that you have specified as
an entry point IS found, the JRE will look for a "public static void
main(String[] args)" method of that class. If it is no there, you will
get a "NoSuchMethodError". If the class is there and if it contains a
method with the right signature, then the commands defined in this
method will be executed. In most cases, an instance of the class will
be gerenated, and within the constructor, more objects will generated,
and so on...
HTH
Piet