Processing - Java Swing

I need to have my ellipse initially start in the middle of the canvas, and then start following the cursor once you put the cursor inside the canvas.

Here is my code

void setup() { size(600,600); }

void draw() { int ballX = mouseX; int ballY = mouseY;

int ballColour = 0;

int diameter = 25;

background(255); fill(ballColour); ellipse(ballX,ballY,diameter,diameter); }

Thank you!