Hashtable with integer key in Java

Question

I'm trying to create a Hashtable as in the following:

Hashtable<int, ArrayList<byte>> block = new Hashtable<int, ArrayList<byte>>();

but I am getting an error on both int and byte saying "Dimensions expected after this token".

If I use something like:

Hashtable<String, byte[]> - all is good. Can someone explain why?

Thanks.

Answer 1

In Java's core collection classes you can only store reference types (something that extends a java.lang.Object). You cannot store primitives like int and byte. Note that an array like byte[] is no primitive but also a reference type.

As @Giuseppe mentioned, you can define it like this:

Hashtable<Integer, ArrayList<Byte>> table = new Hashtable<Integer, ArrayList<Byte>>();

and then put primitive int's in it as keys:

table.put(4, ...);

because since Java 1.5, autoboxing will automatically change the primitive int into an Integer (a wrapper) behind the scenes.

If you need more speed (and measured the wrapper collection classes are the problem!) you could use a 3rd party library that can store primitives in their collections. An example of such libraries are Trove and Colt.

Answer 2

You can use Integer instead of int and if you're using java 1.5+ the boxing/unboxing feature will make your life easy when working with it.

Hashtable<Integer,byte[]> block = new Hashtable<Integer,byte[]>();

Answer 3

Java generics can't be instantiated with primitive types. Try using the wrapper classes instead:

Hashtable<Integer, ArrayList<Byte>> block = new Hashtable<Integer, ArrayList<Byte>>();