int current = 0;
MyCircle [] circles = new MyCircle [4];

int f = 0; // fill color
boolean circleOver = false;
float circleX = 250;
float circleY = 250;  // Position of circle
int circleSize = 50;   // Diameter of circle

void setup() {
  ellipseMode(CENTER);
  size(500, 500);
  fill(f,150);
  noStroke();
  smooth();
  for (int i=0; i <= 3; i++) {
    circles[i] = new MyCircle(circleX, circleY, circleSize+(i*50));
  }
}
void draw() {
  update(mouseX, mouseY);
  fill(f,150);
  background(255-f);
  for (int i=0; i <= 3; i++) {
    circles[i].display();
    circles[i].move(mouseX, mouseY, 10+(i*20));
  }
}

void update(int x, int y) {
  if( overCircle(circleX, circleY, circleSize) ) {
    circleOver = true;
    f = 0;
  } 
  else {
    circleOver = false;
    f = 255;
  }
}

class MyCircle
{
  float xpos; // xposition
  float ypos ; // yposition
  int d;

  MyCircle(float ixp, float iyp, int id) {
    xpos = ixp;
    ypos = iyp;
    d = id;
  }

  void move (float posX, float posY, float damping) {
    float dif = ypos - posY;
    if (abs(dif) > 1) {
      ypos -= dif/damping;
    }
    dif = xpos - posX;
    if (abs(dif) > 1) {
      xpos -= dif/damping;
    }
  }

  void display() {
    ellipse(xpos, ypos, d, d);
  }
}

boolean overCircle(float x, float y, int diameter) 
{
  float disX = x - mouseX;
  float disY = y - mouseY;
  if(sqrt(sq(disX) + sq(disY)) < diameter/2 ) {
    return true;
  } 
  else {
    return false;
  }
}