well, you can start by changing the void loop() to void draw()...

Here's a quick working version. Your problems are syntactical. When you got an error it was because of a failure to encapsulate methods properly using {} curly braces.

Often happens when you comment out a conditional statement but forget the closing } further down the code.

Always worth checking before anything else! They must all add up.

Removed CENTER_DIAMETER from ellipseMode() as this is now CENTER, plus I ditched a spurious semi-colon ;

Code:

Ball[] ball; 
int objectnumber = 30; 
 
void setup (){ 
 
  size (500,500); 
  framerate(30); 
  ball = new Ball[objectnumber]; 
 
  for (int i= 0; i < objectnumber; i++){ 
    ball[i] = new Ball(#940B35,random(50,500),random(10,400),random(1,7),random(1,5), 10, 30); 
  } 
 
} 
 
void draw (){ 
  background(#280915); 
  //ball[].collide(){ 
  for (int i = 0; i < objectnumber; i++){ 
    
    ball[i].draw (); 
    ball[i].move (); 
     
 }   
     

} 
 
class Ball { 
 
 
  float size= 10; 
  float xpos; 
  float ypos; 
  float xspeed; 
  float yspeed; 
  float diameter; 
  float diameter2; 
  color c; 
  int xdirection = 1; 
  int ydirection = 1; 
 
  Ball (color c_,float xp, float yp,float xs, float ys, float d, float d2){ 
    c = c_; 
    xpos = xp; 
    ypos = yp; 
    xspeed = xs; 
    yspeed = ys; 
    diameter = d; 
    diameter2 = d2; 
 
  } 
 
  void draw(){ 
    ellipseMode(CENTER); 
    noStroke(); 
    fill (c); 
    smooth(); 
    ellipse(xpos,ypos,diameter,diameter); 
    if (diameter > 10){ 
    diameter--;} 
} 
 
  void move(){ 
    xpos = xpos +(xspeed * xdirection); 
    ypos = ypos +(yspeed * ydirection); 
    if (xpos > width-diameter2*0.5 || xpos < diameter2*0.5){ 
 xdirection *=-1; 
 diameter = diameter2;     
    } 
    if (ypos > height-diameter2*0.5 || ypos < diameter2*0.5){ 
 ydirection *=-1; 
 diameter = diameter2; 
    } 
/*} 
 
 void collide(){ 
  if (xpos == xpos - diameter){ 
  xdirection *=-1; 
  } 
  if (ypos == ypos - diameter){ 
  ydirection *=-1; 
  } 
 
 
}*/ 
} 
}