Of course.  The class definition for HelloObject
works like a cookie cutter.
We can cut out as many cookies as we want.
Here is an expanded program that creates several HelloObjects
(and several String objects.)
class HelloObject 
{
  String greeting;
  HelloObject( String st )
  {
    greeting = st;
  }
  void speak() 
  { 
    System.out.println( greeting );
  }
}
class HelloTester
{
  public static void main ( String[] args )  
  {
    HelloObject object1 = new HelloObject("Mercury"); 
    HelloObject object2 = new HelloObject("Venus"); 
    HelloObject object3 = new HelloObject("Earth"); 
    HelloObject object4 = new HelloObject("Mars"); 
    object1.speak();
    object2.speak();
    object3.speak();
    object4.speak();
  }
}
 
The program writes this to the monitor:
Mercury Venus Earth Mars