Declaring an ArrayList of Arrays

[mlad]

Hiya, I'm making a mock cache in Java for coursework. I have managed to write a direct-mapped cache using Arrays, but I want to use ArrayLists for a fully-associative cache. I am trying to declare an Arraylist of Arrays using two int variables to assign the intial length of the arraylist (numBlocks) and the length of the arrays within it (blockSize).

This is the statement:

//removed

However when I try to compile I get the error "']' expected". It must be something to do with passing in the blockSize variable because if I remove that I can compile. I'm using BlueJ if that matters, cheers.

[iluxa]

you point out the error correctly. Here's what you need to do in order to initialize all arrays to the required size:

public static ArrayList<int[]> cache = new ArrayList<int[]>(numBlocks);
for (int i = 0; i < cache.size (); i ++)
{
  cache.add (new int [blockSize]);
}

[mlad]

Thanks, this is helpful but I am now adding them as I need them instead (I am new-ish to Java so I am mostly using trial and error, and will probably try both ways in the end). Also, I have read that I can't store primitive types in ArrayLists, is this true? I am trying to create and ArrayList<int> and ArrayList<boolean> and they're giving me some hassle!

[iluxa]

True. You need to use ArrayList<Integer> and ArrayList<Boolean>

ArrayList<Integer> a = new ...

int x = 48;
a.add (new Integer (x));   // works
a.add (x);                 // also works, although in the background it's really doing new Integer ()

[Fubarable]

Thanks, this is helpful but I am now adding them as I need them instead (I am new-ish to Java so I am mostly using trial and error, and will probably try both ways in the end). Also, I have read that I can't store primitive types in ArrayLists, is this true? I am trying to create and ArrayList<int> and ArrayList<boolean> and they're giving me some hassle!

Yep, what you heard is correct, since ArrayLists (all collections) hold objects only. If you need a collection of ints, you'll have to use the Integer wrapper class instead, same for Boolean/boolean.

Edit: I'm sloooow tonight! It must be my age...

[mlad]

Ok I have changed that it is working now. Hopefully my last question, I am trying to assign the elements of the arrays (the ones in my ArrayList).

//removed

The above would be in a for loop, but doesn't work as it says array required, but int found.

//removed

And populating a temporary array and assigning the ArrayList entry to it doesn't work as it is unexpected type. Could you tell me the correct syntax for the top piece please?

[mlad]

I have figured it out now, its cache.get(g)[i] for future reference.

All problems solved, thanks everyone!!