Method that can return a collection OR list of any type

veryangryman · 07-01-2014 5:40pm #1

Hi, ive a requirement to return the following in one method.

This should be implemented so that any list/collection can be filtered.

So the method signature needs something exotic i guess...

I know a small bit of generics but overall am kinda stuck on this.

public List<? extends Collection> doStuff(List listToBeHandled<? extends Collection>, double percentage, int offset)

Is there any handy way to do this? My above approach aint working. Thanks if you can help

Skrynesaver · 07-01-2014 5:47pm

So your method has to handle a list of any type of object, I wonder is there a class that every other class can be cast into which you could then cast your list from ...

veryangryman · 07-01-2014 5:56pm

Skrynesaver wrote: »

So your method has to handle a list of any type of object, I wonder is there a class that every other class can be cast into which you could then cast your list from ...

I wish your Sarcasm was helpful. As the question says, i need it to handle the 2 grouping mechanisms Lists or Collections, not just lists. The object type doesnt matter for the purpose of the question.

CreepingDeath · 07-01-2014 6:32pm

A list extends the collection class.
Why can't you do this ?

    public Collection<?> doStuff(Collection<?> col, double percentage, int offset)
    {
        // do stuff
        
        return(null);
    }

cregganna · 07-01-2014 11:23pm

CreepingDeath wrote: »
A list extends the collection class.
Why can't you do this ?
    public Collection<?> doStuff(Collection<?> col, double percentage, int offset)
    {
        // do stuff
        
        return(null);
    }

Better still parametrise the generic either through a type on the implemented interface or even directly on the method signature:

    public <T> Collection<T> doStuff(Collection<T> col, double percentage, int offset);

That way you can garuntee that the return collection type matches that passed in.

veryangryman · 08-01-2014 12:37pm

Bingo.

Thanks for the help folks!

Method that can return a collection OR list of any type

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