Java: How to initialize a List of Typed Objects?

Question

So I've been searching a while for the answer to this and I'm just not sure how it works.

I'm trying to make a list of BloomFilter<String> objects.

The class definition for the BloomFilter is:

public class BloomFilter<E> implements Serializable { ...

The <E> allows the user to pick what type of elements are going into the filter. In my case, I need strings.

Somewhere else in the program, I need 4 BloomFilter<String> objects.

My question is: How do I initialize the following line?

private static BloomFilter<String> threadedEncrpytionFilters[] = null;
threadedEncryptionFilters = ???

This seems similar to creating a list of ArrayLists? Is that also possible?

Your question is a bit unclear... Do you want a List of these Objects or an array? — Jon7, Aug 26 '11 at 20:45

score 4 · Accepted Answer · answered Aug 26 '11 at 21:03

After seeing that someone alread answered this question I wanted to remove this answer but I see by the comments that people are still confused so here it goes :)

The specification clearly states, that what you want to do is illegal. Meaning:

BloomFilter<String> threadedEncrpytionFilters[] = new BloomFilter<String>[4];

won't compile. You can't create an array of concrete generic classes. When it comes to generics you can store in arrays only:

raw types
unbounded wildcard parameteriezd types

The workaround to your problem is, as already stated, to change the array to a List<BloomFiler<String>>.

This behaviour is actually pretty logical if you take into account how Java handles generic types at different stages (compile, runtime etc). After understanding that you'll see that arrays of concrete generic types wouldn't be type-safe. Here's a mighty good read on this subject: http://www.angelikalanger.com/GenericsFAQ/FAQSections/ParameterizedTypes.html#FAQ104

score 2 · Answer 2 · edited Aug 26 '11 at 20:48

2

List<BloomFilter<String>> list = new ArrayList<BloomFilter<String>>();
list.add(new BloomFilter<String>());
list.add(new BloomFilter<String>());
list.add(new BloomFilter<String>());
// ... and so on

edited Aug 26 '11 at 20:48

Marcelo

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answered Aug 26 '11 at 20:46

Brian Kent

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Perfect! Thank you! I didn't realize you could nest type parameters like that. – theangryhornet Aug 26 '11 at 20:48
You can also add to your answer how to instance and declare arrays too: `BloomFilter[] threadedEncrpytionFilters = new BloomFilter[100];` – Marcelo Aug 26 '11 at 20:51
@Marcelo: the whole point here is that your code wont compile. – Mateusz Dymczyk Aug 26 '11 at 20:55
@Zenzen you are right. Check this answer out: http://stackoverflow.com/questions/1025837/initialize-java-generic-array-of-type-generic/1025915#1025915 – Marcelo Aug 26 '11 at 21:03

score 1 · Answer 3 · answered Aug 26 '11 at 20:46

1

Consider this:

private static List<BloomFilter<String>> threadedEncrpytionFilters = 
    new ArrayList<BloomFilter<String>>();

answered Aug 26 '11 at 20:46

MByD

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Java: How to initialize a List of Typed Objects?

3 Answers3