Java Code:
import org.eclipse.swt.SWT;
import org.eclipse.swt.graphics.Region;
import org.eclipse.swt.widgets.Button;
import org.eclipse.swt.widgets.Display;
import org.eclipse.swt.widgets.Event;
import org.eclipse.swt.widgets.Listener;
import org.eclipse.swt.widgets.Shell;

public class NonRectangularClass {

  static int[] createCircle(int xOffset, int yOffset, int radius) {
    int[] circlePoints = new int[10 * radius];
    for (int loopIndex = 0; loopIndex < 2 * radius + 1; loopIndex++) {
      int xCurrent = loopIndex - radius;
      int yCurrent = (int) Math.sqrt(radius * radius - xCurrent
          * xCurrent);
      int doubleLoopIndex = 2 * loopIndex;

      circlePoints[doubleLoopIndex] = xCurrent + xOffset;
      circlePoints[doubleLoopIndex + 1] = yCurrent + yOffset;
      circlePoints[10 * radius - doubleLoopIndex - 2] = xCurrent
          + xOffset;
      circlePoints[10 * radius - doubleLoopIndex - 1] = -yCurrent
          + yOffset;
    }

    return circlePoints;
  }

  public static void main(String[] args) {
    final Display display = new Display();
    final Shell shell = new Shell(display, SWT.NO_TRIM);

    Region region = new Region();
    region.add(createCircle(50, 50, 50));
    region.subtract(createCircle(50, 50, 20));
    shell.setRegion(region);
    shell.setSize(region.getBounds().width, region.getBounds().height);
    shell.setBackground(display.getSystemColor(SWT.COLOR_BLUE));

    Button button = new Button(shell, SWT.PUSH);
    button.setText("Exit");
    button.setBounds(35, 6, 35, 20);
    button.addListener(SWT.Selection, new Listener() {
      public void handleEvent(Event event) {
        shell.close();
      }
    });

    shell.open();
    while (!shell.isDisposed()) {
      if (!display.readAndDispatch())
        display.sleep();
    }
    region.dispose();
    display.dispose();
  }
}