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Shakedown

Setting size of an ArrayList of ArrayList's (Java)

9 posts in this topic

So I'm trying to create an ArrayList of a certain size. This ArrayList will contain ArrayList<Integer> of an undetermined size. I've tried this: ArrayList<ArrayList<Integer>> listOfLists = new ArrayList<ArrayList<Integer>>(size); // Say size = 4 print(listOfLists.size()); // prints "0" So, how do I create an ArrayList of a determined size that contains ArrayLists of undetermined sizes? Also, the reason I'm doing this is because the Java API doesn't contain a multimap. I want to be able to store a set of integers for a particular integer. If I had a multimap to do this, then the set of integers would be all the keys, and the particular integer would be the value. Perhaps there's an easier way to do this than creating an ArrayList of ArrayLists?
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Correct me if i'm wrong but your talking about 2d arrays...

So effectively Array[x][y]...

You can define in java from memory a new array via:

int[][] Array;

You can also use:

int[][] Array = new int[5][];

which would create an array of 5 rows without initializing the columns which can then be defined by:

Array[0] = new int[10]; //row 0 has 10 columns
Array[1] = new int[4]; //row 1 has 4 columns
Array[2] = new int[2]; //row 2 has 2 columns

to find the size of the individual lists i would imagine you would have to use the size on each of the rows to find the width of the row...
So,
array[0].length should give the size of array 0, array.length should give the size of the array, so in this case should return 3, while array[0].length should return 10. though i'm not sure about the array[x].length actually working.

Its been a while since i've done anything with java, but from memory this should work in a jsdk environment or at least allude to something that might help.

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I'm having trouble declaring and initializing these containers. Using a Vector isn't going to change the declaration and initialization.
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Quote:

So I'm trying to create an ArrayList of a certain size. This ArrayList will contain ArrayList<Integer> of an undetermined size.

I've tried this:

ArrayList<ArrayList<Integer>> listOfLists = new ArrayList<ArrayList<Integer>>(size); // Say size = 4

print(listOfLists.size()); // prints "0"


The value given in the ArrayList constructor is not the size, but the capacitiy. For ArrayLists this means it is the initial size of the backing array, so you can put up to capacity amount of values into the list without thw list being grown (which involves a new memory allocation and an array copy).

Quote:

So, how do I create an ArrayList of a determined size that contains ArrayLists of undetermined sizes?



ArrayList<Integer>[size] arrayOfLists = new ArrayList<Integer>[size]();
for( int i=0; i<size; i++ ) arrayOfLists[i]= new ArrayList<Integer>();



Quote:

Also, the reason I'm doing this is because the Java API doesn't contain a multimap. I want to be able to store a set of integers for a particular integer. If I had a multimap to do this, then the set of integers would be all the keys, and the particular integer would be the value. Perhaps there's an easier way to do this than creating an ArrayList of ArrayLists?


I don't see why you need a multimap for this...


HashMap<Integer,Integer> ints= new HashMap<Integer,Integer>();
ints.put(2,3);
ints.put(4,3);
ints.put(9,3);
ints.put(12,3);



Also you might be interested in a faster collection-implementation for primitives:

trove4j
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Ah, thank you for the helpful response cylab.

Quote:

The value given in the ArrayList constructor is not the size, but the capacitiy. For ArrayLists this means it is the initial size of the backing array, so you can put up to capacity amount of values into the list without thw list being grown (which involves a new memory allocation and an array copy).


Hmm...ahh, I see. Good call. If I remember correctly, in C++ you're able to declare the size of your container when you create the container. I figured that was the same functionality in Java.

Quote:

I don't see why you need a multimap for this...


I expected I needed a multimap for the same reasons that you need a multimap in C++. I thought that a HashMap could only map 1 key to 1 value, as does a map in C++. It appears from the Java API that I was wrong though [smile].
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Quote:
Original post by Shakedown
I expected I needed a multimap for the same reasons that you need a multimap in C++. I thought that a HashMap could only map 1 key to 1 value, as does a map in C++. It appears from the Java API that I was wrong though [smile].


No, you were correct. cylab is creating the reverse of the mapping you want.

However, the natural way to map an integer to an unknown-sized list of integers would be to, well, make a map from integers to unknown-sized-lists-of-integers. That is to say, HashMap<Integer, ArrayList<Integer>>.

Although that now contradicts your assertion that you know ahead of time what size the list-of-lists would be.
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Quote:
Original post by Shakedown
Quote:

The value given in the ArrayList constructor is not the size, but the capacitiy. For ArrayLists this means it is the initial size of the backing array, so you can put up to capacity amount of values into the list without thw list being grown (which involves a new memory allocation and an array copy).


Hmm...ahh, I see. Good call. If I remember correctly, in C++ you're able to declare the size of your container when you create the container. I figured that was the same functionality in Java.


It *is* the same. You're just getting your terminology wrong. C++ containers have the same concept of capacity vs. size. 'capacity' is the amount of space allocated for elements, while 'size' is the number of elements actually in the container. When you tell a C++ container to allocate a certain space for a certain number of elements, via a constructor or through methods like vector::reserve, it's not the size but the 'capacity' that grows. It's the same in Java.
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Quote:
Original post by Zahlman
No, you were correct. cylab is creating the reverse of the mapping you want.


Oh, sorry, seems I misinterpreted this statement from the OP:
Quote:

If I had a multimap to do this, then the set of integers would be all the keys, and the particular integer would be the value.


If you really need a multimap, you could utilize apache's commons-collections:
Multimap
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