1.   if (bola.x<bola.boladiam/2) {//rebote horizontal
2.       bola.velx= abs (bola.velx);
3.     }

The problem is the stutter in the bounce between balls. Here´s another try:

ArrayList <Bola> bolas;
  int size=700;
  int i;
  int j;
  float acum;
  float vlimy=15;//velocidad limite
  float vlimx=5;
 
  class Bola{
  float x;
  float y;
  float boladiam;
  float velx;
  float vely;
  int r;
  int g;
  int b;
  //auxiliar para cambiar valores
  Bola(int xini, int yini,float diamini,int rini,int gini, int bini){
    x=xini;
    y=yini;
    boladiam=40;
    velx=1;
    vely=0;
    boladiam=diamini;
    r=rini;
    g=gini;
    b=bini;
  }
}

void setup(){
  noStroke();
  size(size,size);
  bolas=new ArrayList();
}

void draw(){
 frameRate(42);
 background(255);
 for(i=0;i < bolas.size();i++){
   Bola bola = (Bola) bolas.get(i);
   fill(bola.r,bola.g,bola.b,200);
   ellipse(bola.x,bola.y,bola.boladiam,bola.boladiam);//dibuja la bola
   bola.vely=bola.vely + 0.1;//acelera
   bola.x=bola.x + bola.velx;//mueve en x
   bola.y=bola.y + bola.vely;//mueve en y
   if (bola.y > size-bola.boladiam/2){//rebote vertical
     bola.vely=-bola.vely;
   }
   if (bola.y+bola.boladiam/2-size>0.15){
     bola.y=size-bola.boladiam/2;
     if (abs(bola.vely)<0.1){
       bola.vely*=1.25;
     }
   }
    
   if (bola.x>size-bola.boladiam/2){//rebote horizontal
     bola.velx=-abs(bola.velx);
   }
   if(bola.x<bola.boladiam/2){
     bola.velx=abs(bola.velx);
   }
   for(j=i+1;j < bolas.size();j++){//rebote entre ellas
   if(bola.velx<-vlimx){bola.velx=-vlimx;}
   if(bola.velx>vlimx){bola.velx=vlimx;}
   if(bola.vely<-vlimy){bola.vely=-vlimy;}
   if(bola.velx>vlimy){bola.velx=vlimy;}
     Bola otrabola = (Bola) bolas.get(j);
     float dist=sqrt ((bola.x-otrabola.x)*(bola.x-otrabola.x)+(bola.y-otrabola.y)*(bola.y-otrabola.y));
     if(dist<(bola.boladiam+otrabola.boladiam)/2){
      
       if(sqrt((bola.velx-otrabola.velx)*(bola.velx-otrabola.velx)+(bola.vely-otrabola.vely)*(bola.vely-otrabola.vely))<10){
       while (dist<(bola.boladiam+otrabola.boladiam)/2){
        dist=sqrt ((bola.x-otrabola.x)*(bola.x-otrabola.x)+(bola.y-otrabola.y)*(bola.y-otrabola.y));
         if (bola.velx<otrabola.velx){
           bola.x-=1;
           otrabola.x+=1;
            bola.velx-=0.1;
           otrabola.velx+=0.1;
         }
         else{
          bola.x+=1;
          otrabola.x-=1;
          bola.velx+=0.1;
          otrabola.velx-=0.1;
         }
         if (bola.vely<otrabola.vely){
           bola.y-=1;
           otrabola.y+=1;
           bola.vely+=0.1;
          otrabola.vely-=0.1;
         }
         else{
          bola.y+=3;
          otrabola.y-=3;
          bola.vely+=0.1;
          otrabola.vely-=0.1;
         } 
       }
       }
      
         acum=bola.velx;
         bola.velx=otrabola.velx;
         otrabola.velx=acum;
         acum=bola.vely;
         bola.vely=otrabola.vely;
         otrabola.vely=acum;
        
      
     }
    
   }
 }
}

void mousePressed() {
  bolas.add ( new Bola(mouseX,mouseY,10+mouseX/10,int(random(100,255)),int(random(0,255)),int(random(0,255))));
}

No it isn't.

 

I`ve tried to make a loop that separate balls that go in the same direction, and another kind of stutter appears.

 

I've also limited the ball's speed.