How can we instantiate a collection that contains any object.

[Pavan Kumar]

Hello Friends,

I have written a simple utility method to split a collection of Strings into reasonable batches of collections. This was quite useful. And I see that the same can be used in other places of the application, where a similar logic is necessary but the collection may contain different object than String. Prior to generics, this was easy.

Could you please suggest some ways to generalize my program to accept a collection of any object type, and return a List of Collection of the type specified?

here's my code

  
public static List<Collection<String>> splitCollection(final Collection<String> source, final int batchSize){ 
        List<Collection<String>> result = new ArrayList<Collection<String>>(); 
        if( source == null  ){ 
                return result; 
        } 
        if( batchSize > source.size() || batchSize < 1 ){ 
                result.add(source); 
                return result; 
        } 
        List<String> list = new ArrayList<String>(batchSize); 
        for (String entry : source) { 
                list.add(entry); 
                if( list.size() == batchSize ){ 
                        result.add(list); 
                        list = new ArrayList<String>(batchSize); 
                } 
        } 
        if( ! list.isEmpty() ){ 
                result.add(list); 
        } 
        return result; 
} 

I've tried modifying the method parameter and the List that holds the current batch of objects to the following

   
splitGenericCollection(final Collection<? extends Object> collection, final int batchSize)
.
.
List<? extends Object> list = new ArrayList<Object>(batchSize);
for (Object object : collection) {
list.add(object); <<-- this line throws a compiler error.
}
 

So I think my question is, how can I instantiate a Collection that can hold any object?
Thanks,
Cnu.

[Jim Yingst]

I think what you want is simply

public static <E> List<Collection<E>> splitCollection(final Collection<E> source, final int batchSize){

[Pavan Kumar]

Thanks Very much Jim, for your help.

I've modified my method to the following, and it works like a charm.

public static <E>List<Collection<E>> splitGenericCollection(final Collection<E> collection, final int batchSize){
       List<Collection<E>> result = new ArrayList<Collection<E>>(); 

List<E> list = new ArrayList<E>(batchSize);
for (E object : collection) {
list.add(object);
if( list.size() == batchSize ){
result.add(list);
list = new ArrayList<E>();
}
}
if( ! list.isEmpty() ){
result.add(list);
}
return result;
}

could you perhaps throw some light on this expression, please?

  <E>List<Collection<E>>  

Believe me, I have the generics tutorial open in another tab, but I definitely want to get a heads up on that.

Thanks,
Cnu
[ December 09, 2007: Message edited by: cnu sri ]

[Jim Yingst]

[B][Cnu]: could you perhaps throw some light on this expression, please?

<E>List<Collection<E>> 

[/B]

This is part of the declaration of a generic method (see section 5 of the Generics Tutorial). The original<E> tell us two things: (a) this is a generic method, and (b) there is a single type parameter in this generic method, and its name is E. E could be any type - it's like having a ? wildcard, except that if E appears more than once, it refers to the same type everywhere.

The List<Collection<E>> is just a return type. It's like saying the method returns a List<Collection<?>>, where ? is whatever the type was of the Collection<E> input parameter. If you pass in a Collection<String>, you get back a List<Collection<String>>. If you pass in a Collection<Integer>, you get back a ist<Collection<Integer>>.

[Pavan Kumar]

Jim, Thank you for the explanation and pointer to the appropriate section in the tutorial.