public void paint( Graphics g )
{
g.drawOval(150, 50, 200, 200); //Draw a circle
}
Explanation
drawOval( int X, int Y, int width, int height ) this instruction draws an oval in the specified 'x' and 'y' coordinates, been this coordinates the left upper corner of a rectangle who perfectly fits the oval. The last two numbers are the width and height.
X coordinate from the upper left corner. From the center location, move half of the width of the circle. (minus 100)
Y coordinate from the upper left corner. From the center location, move half of the height of the circle. (minus 100)