// sometimes generics is enough

class cell1<T>
{
    T info;
    cell1<T> next;

    public cell1(T x, cell1<T> y) {info=x; next=y;}
}

// but want to do more

class cell2<T extends Comparable<T>>
{
    T info;
    cell2<T> next;

    public cell2(T x, cell2<T> y) {info=x; next=y;}

    // check if list is sorted
    public boolean sorted()
    {
	boolean ax = true;
	for(cell2<T> i=this; i.next!=null && ax;i=i.next)
	    if (i.info.compareTo(i.next.info)>0) ax = false;
	return ax;
    }//sorted
}//cell2  -- this is what we did for heaps.


// but this is also sometimes not enough. sometimes we won't know how
// to define a procedure until we know exactly what T is.

abstract class abcell<T>
{
    T info;
    abcell<T> next;
    // no constructor in abstract class!

    public abstract T merge(); // merge all elements of list into one value

    // other methods in abstract class can "call" merge
    public void printmerged() // can't call right now, why?
    {  T x = merge();
       System.out.println(x);
    }// printmerged is "polymorphic"
}//abcell

class cell3a extends abcell<Double>
{
    // inheriting Double info  and  abcell<Double> next
    public cell3a(Double x, cell3a y) {info=x; next=y;}
    
    // now implement merge
    public Double merge()
    {
	Double ax = 0.0;
	for (abcell<Double> i=this;i!=null;i=i.next) ax = ax + i.info;
	return ax;
    }
}// cell3a


class cell3b extends abcell<Double>
{
    // inheriting Double info  and  abcell<Double> next
    public cell3b(Double x, cell3b y) {info=x; next=y;}
    
    // now implement merge
    public Double merge()
    {
	Double ax = this.info; 
	for (abcell<Double> i=this;i!=null;i=i.next) ax = ax * i.info;
	return ax;
    }
}// cell3b

class cell3c extends abcell<String>
{
   public cell3c(String x, cell3c y) {info=x; next=y;}

   public String merge()
   {
       String ax = "";
       for(abcell<String> i=this;i!=null;i=i.next) ax = i.info + ax;
       return ax;
   }
}

public class abstracts
{
    public static void main(String[] aaa)
    {
	cell3a A = new cell3a(1.2,new cell3a(2.4,null));
	cell3c B = new cell3c("world",new cell3c("hello ",null));
	System.out.println(A.merge());
	B.printmerged(); // can now call printmerged.
    }
}