public abstract class List {
  public abstract Object getFirst();
}

public class Empty extends List {
  public Object getFirst() {
    throw new RuntimeException( "Empty has no first");
  }
}

public class Larger extends List {
  private Object first;
  private List rest;
  public Larger( Object first, List rest ) {
    this.first = first;
    this.rest = rest;
  }
  public Object getFirst() {
    return this.first;
  }
}


















class Window {
  int width, height;
  boolean displayed = false;
  
  Window( int width, int height ) {
    if ( width < 0 ) 
      throw new RuntimeException("Window requires a positive width");
    if (height < 0)
      throw new RuntimeException("Window requires a positive height");
    this.width = width;
    this.height = height;
  }

  void display() {
    displayed = true;
  }

  void drawPixels(Object pixels) throws WindowHiddenException {
    if ( displayed )
   	pixels.equals(pixels); //Do something to draw the pixels
    else
        throw new WindowHiddenException("Window must be visible to draw");
  }

}

class Graphics {

  Window myView;
  

  Graphics( Window myView ) {
    this.myView = myView;
  }

  void alwaysDraw() {
    try {
      myView.drawPixels( "Draw a square");
    } catch ( WindowHiddenException e ) {
       myView.display();
       this.alwaysDraw();
    }
  }
}

class WindowHiddenException extends Exception {
  WindowHiddenException( String s ) {
    super( s );
  }
}