int circleX, circleY;
int r = 100;   
boolean circleOver = false;
void setup() { 
size(800, 800);
strokeWeight(3);
background(255, 255, 255);
}

boolean overCircle(int x, int y){
  float disX = x - mouseX;
  float disY = y - mouseY;
  if(sqrt(sq(disX) + sq(disY)) < r ) {
    return true;
  } else {
    return false;
  }
}

void update(int x, int y) {
  if( overCircle(circleX, circleY) ) {
    circleOver = true;
  }else{
    circleOver = false;
  }
}

void draw() {
  update(mouseX, mouseY);
  if (circleOver){
    circleX = int (random(100,700));
    circleY = int (random(100,700));
  }else{
    clear();
    background(255, 255, 255);
  }
  circle(circleX, circleY, r);
}