Drawing a bounding rectangle to select what area to record

Question

How do I draw that semi-transparent rectangle on the screen? That cannot be a JFrame be

Answer 1

You could use a transparent, undecorated frame in order to create a basic border.

public class ScreenRectangle extends JFrame {

    public ScreenRectangle() {
        this.setUndecorated(true);
        this.setBackground(new Color(0, 0, 0, 0.25F));
        // opacity ranges 0.0-1.0 and is the fourth paramater
        this.add(new DrawPanel());
    }

    private class DrawPanel extends JPanel {

        @Override
        public void paintComponent(Graphics g) {
            super.paintComponent(g);
            g.drawRect(0, 0, this.getWidth(), this.getHeight());
            // any other drawing
        } 
    }
}

The frame may also need to be setOpaque, or the panel size may need to be handled, but this is the general idea of it.