public class SubstanceList {

private final Substance[] substances = { new Water(), 
                                         new Carbon(),  
                                         new MgSiO3() }; 
private int index;

public SubstanceList() {
   index = 0;
   for(int i=0; i<substances.length; i++) { substances[i].setID(i); }
}

public int length() { return substances.length; }

public Substance at(int i) {
   if(i < 0) { index = 0; return substances[index]; }
   else if(i >= substances.length) { index = substances.length-1; return substances[index]; }
   else return substances[index = i];
}

public int indexOf(Substance s) {
   for(int i=0; i < substances.length; i++) {
      if(s.equals(substances[i])) return i;
   }
   System.out.println("**** Error in SubstanceList.indexOf => substance not found****");
   return -1;
}

public Substance current() {
   return substances[index];
}

public Substance next() {
   index++;
   if(index == substances.length) index = 0;
   return substances[index];
}

public Substance previous() {
   index--;
   if(index < 0) index = substances.length-1;
   return substances[index];
}

}