How to create an array of a collection? [duplicate]

Question

Possible Duplicate:
What’s the reason I can’t create generic array types in Java?

HashSet<Integer>[] rows = new HashSet<Integer>[9]; 

gives me a compilation error: generic array creation.

Answer 1

The simple answer: do not mix arrays with generics!

Use a list of hashsets:

ArrayList<HashSet<Integer>> rows = new ArrayList<HashSet<Integer>>(); 

The problem here is that Java specification doesn't allow you to declare an array of generics object.

A workaround of it is to use the wildcard, but you will lose the type-safety:

HashSet<?>[] rows = new HashSet<?>[9];  for (int i = 0; i < rows.length; ++i)     rows[i] = new HashSet<Integer>(); 

This, in your case, won't create problems when you are going to check if an item is contained: you can easily do rows[0].contains(4) but when adding new content you will be forced to cast the row to the right type and suppress the warning of unchecked cast itself:

((HashSet<Integer>)rows[0]).add(4); 

A side note: if you feel pioneer just download the Trove Collections framework that has a not-generics, highly optimized version of an integer hashset made to work with primitive type, I'm talking about the TIntHashSet class: it will solve your problem and you'll end up having faster code.