jdk中巧妙处理 集合用元素用“，”分割

public String toString() {
        Iterator<E> i = iterator();
    if (! i.hasNext())
     return "[]";

    StringBuilder sb = new StringBuilder();
    sb.append('[');
    for (;;) {
        E e = i.next();
        sb.append(e == this ? "(this Collection)" : e);
     if (! i.hasNext())
        return sb.append(']').toString();
        sb.append(", ");
    }
    }