Oy ! generics...simple newbie questions

Question

I did read the faq- but have all sorts of problems. This is my first generics program, please critique and why am I getting this compile time error ?!
Also: if I wanted to use List<List<M>>, how would this change ?
public class GenericsTest
static class Library{
ArrayList> aislesofmedia = new ArrayList>();
public Library(){
for(int i=0; i< 20; i++) { aislesofmedia.add(i, new ArrayList(20)); }
public void storeMedia(M m, int rowid, int shelfid)
ArrayList row = (ArrayList) aislesofmedia.get(rowid);
row.add( shelfid, m);
aislesofmedia.add( (ArrayList) row); /*ERROR! Description Resource Path Location Type*
*The method add(ArrayList<M>) in the type ArrayList<ArrayList<M>> is not applicable for the arguments (ArrayList<M>) GenericsTest.java myproj/src line ...* /
static class MediaClass {
protected String name;
MediaClass( String s) { name=s; }
static class Book extends MediaClass
{ public Book(String s) { super(s); }
public static void main(String[] args)
Library lib = new Library();
Book b1= new Book("Oliver Twist");
lib.storeMedia(b1, 1,1); }
}Edited by: anikam on Oct 15, 2009 3:30 PM

Answer 1

Thankyou for your patience.
I am still trying to understand of generics... so I understand that I could use a different construct ...
Here is the class based on ArrayList<ArrayList<M>>
import java.util.List;
import java.util.ArrayList;
public class gtest {
    static class Library<M>{
        ArrayList<ArrayList<M>> aislesofmedia = new ArrayList<ArrayList<M>>();
        public Library(){
             for(int i=0; i< 10; i++)
                  aislesofmedia.add(i,  new ArrayList<M>(20));                  
        public <M> void storeMedia(M m, int rowid, int shelfid)
               /* The following line does not compile
              Compile error: Type mismatch: cannot convert from ArrayList<M> to ArrayList<M>
              It will compile with ArrayList<M> e = (ArrayList<M>)aislesofmedia.get(rowid); */
             ArrayList<M> e = aislesofmedia.get(rowid);             
             e.add( shelfid, m);                
        public <M> M retrieveMedia(int rowid, int shelfid)
/* The following line does not compile
              Compile error: Type mismatch: cannot convert from ArrayList<M> to ArrayList<M>
              It will compile with ArrayList<M> e = (ArrayList<M>)aislesofmedia.get(rowid); */
             ArrayList<M> row =  aislesofmedia.get(rowid);             
             return row.get(shelfid);                
    public static void main(String[] args)
        Library<String>    lib = new Library<String>();
        String book1 = "Oliver Twist";
        String book2 = "where the wild things are";
        lib.storeMedia(book1, 0,0);
        lib.storeMedia(book2,0,1);
        String m = lib.retrieveMedia(0, 0);
        String m2 = lib.retrieveMedia(0, 1);
        System.out.println(m+", "+m2);
}Edited by: anikam on Oct 16, 2009 10:54 AM