float x =0;
float y =0;
float snelheid_omlaag = 1;
;void setup()
{
fullScreen();
}
void draw()
{
ellipse(x, y, 50, 50);
x=x+random(-90, 90);
y=y+random(-40, 40)+snelheid_omlaag;
i want the keep inside window code here
}
Hi @MoonAlien822 ,
Welcome to the forum! 
Please format your code by using the </> button when editing a message or put backticks around your code like this : ``` code``` → code
Also it’s better to put the content of your question in the body of the message, not the title! 
If you want the ellipse to stay in the window, try to think in pixels and coordinates. What does that mean if a circle is outside the visible screen?
You can easily make a drawing on paper and try to see which information you have in order to solve that.
About the coordinate system and shapes, there’s this tutorial on the Processing website :