Problem in passing ArrayList

[Gaurav Jain]

I dont want to pass an ArrayList from one class to another as i dont want the client to explicitly type cast the objects present in the ArrayList that is the client should not be bothered to know that what type of object does the ArrayList contains.
What are the other options available that is what can i pass instead of ArrayList?

[William Brogden]

I'm not sure I understand what you want, but I find it convenient to use the toArray( exampleArray ) method. For instance, this fragment reads lines to an ArrayList but returns a String[]

   ArrayList lines = new ArrayList();
   String s = read.readLine();
   while( s != null ){
     lines.add( s );
     s = read.readLine();
   }
   String[] ret = new String[ lines.size() ];
   return (String[]) lines.toArray( ret );
  

If you hand toArray an array of the right size, it fills in the elements, casting to the array type.
Bill

[Stefan Wagner]

Or you build a Wrapper and use an ArrayList inside:

class FooList
{
ArrayList al;
// ...
public void add (Foo foo)
{
al.add (foo);
}

public Foo get (int i)
{
return (Foo) al.get (i);
}
// ...

[Jim Yingst]

Or use Java 1.5 to return a List<Foo> (a List whose contents are all Foo instances and do not need to be cast to Foo before you use them).

[Tony Morris]

 
// decorator design pattern 
class FooList implements List
{
    private List l;
 
    public void add(Foo f)
    {
        l.add(f);
    }
 
    public void add(Object o)
    {
        throw new UnsupportedOperationException();
    }
}

[Warren Dew]

Jim, do you have more information or a link on generics in Java 1.5? How similar are they to C++ templates?

[Tony Morris]

There is a particular distinction to made between 1.5 generics and C++ templates.

http://java.sun.com/j2se/1.5/pdf/generics-tutorial.pdf
http://www.jcp.org/aboutJava/communityprocess/review/jsr014/

[Jim Yingst]

Well Tony's already given links to the nitty gritty details; excellent. There's also a good overview of 1.5 changes here. and note that JDK 1.5 beta is out now, so you can try out generics yourself. (Along with lots of other changes.)