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Length of ArrayList in Java

How do I find the size of an ArrayList in Java? I do not mean the number of elements, but the number of indexes.
public static void main(String[] args) {
ArrayList hash = new ArrayList(5);
System.out.println(hash.size());
}
Prints out "0." Using:
System.out.println(hash.toArray().length);
Also prints out a "0."
I have looked in http://docs.oracle.com/javase/7/docs/api/java/util/ArrayList.html but I do not see a method that will help me. Is my ArrayList reverting to a size of 0 if I do not add anything to it?
EDIT The assignment is to create a hash table using ArrayList. I am supposed to create a hash function using the formula
double hashkey = Math.floor(hash.size()*(Math.E*key-Math.floor(Math.E*key)));
Where key is an integer. hashkey then becomes the index where the value will be stored. I am using hash.size() as a placeholder at the moment, but that value should be the capacity of my ArrayList.

ArrayList.size() will give the current size.That's why hash.size() giving you the current size of your ArrayList hash. It will not give you the capacity.
You just initialized the list. Have not add any elements to your arraylist, that's why its giving 0.

There is no such method in the ArrayList API. The capacity of an ArrayList is hidden by design.
However, I think that your question is based on a misunderstanding.
How do I find the size of an ArrayList in Java? I do not mean the number of elements, but the number of indexes.
In fact, the size of a List, the number of elements in a List, and the number of indexes (i.e. indexable positions) for a List ... are all the same thing.
The capacity of an ArrayList is something different. It is the number of elements that the object could contain, without reallocating the list's backing array. However, the fact that the list has a capacity N does NOT mean that you can index up to N - 1. In fact, you can only index up to size() - 1, irrespective of the capacity.
Now to deal with your examples:
ArrayList list = new ArrayList(5);
System.out.println(list.size());
This prints out zero because the list has zero elements. The ArrayList() and ArrayList(int) constructors both create and return lists that are empty. The list currently has space for 5 elements (because you gave it an initial capacity of 5) but you can't index those slots.
System.out.println(list.toArray().length);
This prints zero because when you copy the list's contents to an array (using toArray()), the array is the same size as the list. By definition.
This does not mean that the list's backing array has changed. On the contrary, it is still big enough to hold 5 elements without reallocation ... just like before.
But ... I hear you say ... the array's length is zero!
Yes, but that is not the backing array! The toArray() method allocates a new array and copies the List contents into that array. It does NOT return the actual backing array.

Maybe you should encapsulate your ArrayList in a class and add another attribute private int capacity in that class as well.
public class AdvancedArrayList<T>
{
private int capacity;
private ArrayList<T> list;
public AdvancedArrayList<T>(int capacity)
{
this.capacity = capacity;
list = new ArrayList<>();
}
public ArrayList<T> getList()
{
return list;
}
public int getCapacity()
{
return capacity;
}
public void addElement(T element)
{
if(list.size() < capacity)
list.add(element);
else
System.out.println("Capacity is full");
}
}
Notice that size is different than capacity.

Java: 2D array of arraylists?

I am working on a sudoku solving program and I need an arraylist that holds the numbers 1 thru 9 for each of the squares on the 9x9 board. Each of these arraylists correspond to the possible numbers that could go in that square, if a number can not go in that square, it is removed from the list.
I want to be able to pull up the arraylist of the current square it is working on, like for example if I wanted to remove the number 7 from the arraylist corresponding to square (3,5)
arrayOfLists[3][5].remove(Integer.valueOf(7));
However I can't figure out how to do this. When I try to create the array I am getting this error on the line where I declare my array of arraylists
Cannot create a generic array of ArrayList
Here is my code:
//create arraylist
ArrayList<Integer> nums = new ArrayList<Integer>();
//fill arraylist with numbers 1-9
for (int i = 1; i < 10; i++) {
nums.add(i);
}
//create 9x9 array of arraylists
ArrayList<Integer>[][] array = new ArrayList<Integer>[9][9];
//fill each element of array with arraylist of numbers 1-9
for(int i = 0; i<9; i++){
for(int j = 0; j<9; j++){
array[i][j] = nums;
}
}
}
Am I doing this incorrectly or is it not possible to create an array of arraylists? If it is not possible, how should I do this then?

Anytime I see a list of lists, alarm bells start ringing. The situations where you actually want such a thing are rare indeed, and this is not one of them.
You've got a fixed board consisting of 9 fixed squares, columns and rows, each position of which may take a number 1-9.
Use an array for all of these concepts, because they are fixed in size and you need direct access to each element - collections offer no benefit and would be a hindrance. Use logic (possibly sets) to ensure numbers are used only once in each zone.

Use a bit field instead of an array list. That is, use an integer where bits 1-9 represent the possibilities of the numbers. Testing, adding, removing a single number is O(1), and it has a fixed memory size. Encapsulate the integer in its own object that knows the operations.

A few things:
1) In your for loop, array[i][j] = nums; This is going to result in the same object in each element of the array. If you call remove() on one element of the array, it's going to affect all the others. You want to build a separate list object for each element.
2) Program to interfaces; declare nums as a List as opposed to ArrayList.
3) Use a List of Lists as opposed to any array of Lists.
List<List<List<Integer>>> list = new ArrayList<List<List<Integer>>>();
for(int i = 0; i<9; i++){
List<List<Integer>> row = new ArrayList<List<Integer>>();
for(int j = 0; j<9; j++){
List<Integer> nums = new ArrayList<Integer>();
for (int k = 1; k < 10; k++) {
nums.add(i);
}
row.add(nums);
}
list.add(row);
}
// You can still get an element by index
int x = list.get(3).get(1).remove(6);
But this is kind of unwieldy. You might want to consider writing a class that represents the board. That way you'll at least have operations that better abstract this.

You could completely remove the use 2d stuff and keep a single list by giving each square a unique number from 1...81. So if you are working with 3,5 cell that means it's the 9*2+5 = 23rd item in the list. That will greatly simplify the list manipulation. You could use a single method to give the unique cell index given the (3,5) kind of reference

OK, I'm going to post this as an answer since it seems to work for me and I haven't yet seen any pitfalls.
private static class IntegerArrayList extends ArrayList<Integer> {
IntegerArrayList () { super(); }
IntegerArrayList (Collection<? extends Integer> c) { super(c); }
IntegerArrayList (int initialCapacity) { super(initialCapacity); }
}
Now you can say something like
IntegerArrayList[][] array = new IntegerArrayList[9][9];
and elements like array[1][2] will inherit all the ArrayList methods (array[1][2].remove(something) works fine). I made the class private static thinking you could nest it in some other class if that's the only place you'll use it, but you can make it public if you like. Also, I copied all three constructors from ArrayList; you could eliminate unneeded ones but I don't see a compelling reason to.
I think the issue is that new ArrayList<Integer>[9][9] is prohibited because it would create an array that wouldn't do type checking (because of "type erasure"). But I think adding your own non-generic type that inherits from ArrayList<Integer> restores the type safety.
But I'm not a generic expert, and it wouldn't surprise me if someone more knowledgeable than I spots a problem with this solution. But it seemed to work fine for me, with no compiler warnings about unchecked type stuff or anything.
(P.S. I'm posting this as a possible general solution to a problem that gets asked a lot. But in reality, for this particular problem, I might just use a fixed-size array of boolean instead of an ArrayList, like others, or I might even do bit-diddling on integers if speed is a real issue.)

Java ArrayList IndexOutOfBoundsException despite giving an initial capacity

When I do
ArrayList<Integer> arr = new ArrayList<Integer>(10);
arr.set(0, 1);
Java gives me
Exception in thread "main" java.lang.IndexOutOfBoundsException: Index: 0, Size: 0
at java.util.ArrayList.rangeCheck(Unknown Source)
at java.util.ArrayList.set(Unknown Source)
at HelloWorld.main(HelloWorld.java:13)
Is there an easy way I can pre-reserve the size of ArrayList and then use the indices immediately, just like arrays?

How about this:
ArrayList<Integer> arr = new ArrayList<Integer>(Collections.nCopies(10, 0));
This will initialize arr with 10 zero's. Then you can feel free to use the indexes immediately.

Here's the source from ArrayList:
The constructor:
public ArrayList(int initialCapacity)
{
super();
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Capacity: "+ initialCapacity);
this.elementData = new Object[initialCapacity];
}
You called set(int, E):
public E set(int index, E element)
{
rangeCheck(index);
E oldValue = elementData(index);
elementData[index] = element;
return oldValue;
}
Set calls rangeCheck(int):
private void rangeCheck(int index)
{
if (index >= size) {
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
}
It may be subtle, but when you called the constructor, despite initializing an Object[], you did not initialize size. Hence, from rangeCheck, you get the IndexOutOfBoundsException, since size is 0. Instead of using set(int, E), you can use add(E e) (adds e of type E to the end of the list, in your case: add(1)) and this won't occur. Or, if it suits you, you could initialize all elements to 0 as suggested in another answer.

I believe the issue here is that although you have suggested the allocated space of entries in the Array, you have not actually created entries.
What does arr.size() return?
I think you need to use the add(T) method instead.

Programming aside, what you are trying to do here is illogical.
Imagine an empty egg carton with space for ten eggs. That is more or less what you have created. Then you tell a super-precise-and-annoying-which-does-exactly-what-you-tell-him robot to replace the 0th egg with another egg. The robot reports an error. Why? He can't replace the 0th egg, because there is no egg there! There is a space reserved for 10 eggs, but there are really no eggs inside!

You could use arr.add(1), which will add 1 in the first empty cell, i.e. the 0-indexed one.
Or you could create your own list:
public static class PresetArrayList<E> extends ArrayList<E> {
private static final long serialVersionUID = 1L;
public PresetArrayList(int initialCapacity) {
super(initialCapacity);
addAll(Collections.nCopies(initialCapacity, (E) null));
}
}
Then:
List<Integer> list = new PresetArrayList<Integer>(5);
list.set(3, 1);
System.out.println(list);
Prints:
[null, null, null, 1, null]

This is not an Java-specific answer but an data structure answer.
You are confusing the Capacity concept with the Count (or Size) one.
Capacity is when you tell the list to reserve/preallocate a number of slots in advance (in this ArrayList case, you are saying to it create an array of 10 positions) in its' internal storage. When this happens, the list still does not have any items.
Size (or Count) is the quantity of items the list really have. In your code, you really doesn't added any item - so the IndexOutOfBoundException is deserved.

While you can't do what you want with arraylist, there is another option: Arrays.asList()

Capacity is used to prepare ArrayLists for expansion. Take the loop
List<Integer> list = new ArrayList<>();
for(final int i = 0; i < 1024; ++i) {
list.add(i);
}
list starts off with a capacity of 10. Therefore it holds a new Integer[10] inside. As the loop adds to the list, the integers are added to that array. When the array is filled and another number is added, a new array is allocated twice the size of the old one, and the old values are copied to the new ones. Adding an item is O(1) at best, and O(N) at worst. But adding N items will take about 2*1024 individual assignments: amortized linear time.
Capacity isn't size. If you haven't added to the array list yet, the size will be zero, and attempting to write into the 3rd element will fail.

ArrayList proper use?

Have a question.. I have a list of mp3 filenames
to add a new file name use:
musicList.add(new Mp3(id, filename));
now I wanted to create a array containing this lists
private static final int LIST_COUNT = 8;
public static List<List<Mp3>> musicLists = new ArrayList<List<Mp3>>(LIST_COUNT);
private void parseMus(){
musicLists = new ArrayList<List<Mp3>>(LIST_COUNT);
...
//gettin mp3 list id,filename, length
...
musicLists.get(listNr).add(new Mp3(id,filename,length));
...
}
but it gives me errors:
03-15 21:01:36.030: E/AndroidRuntime(3393): java.lang.IndexOutOfBoundsException: Invalid index 4, size is 0
Second quwestion!
now I edited code and have no errors... BUT now what i get is that all list is filled with same lists.. so when i try to see:
for(int i =0; i< Settings.musicLists.size();i++){
if(D)Log.e("visio added","MP3 file muslist nr="+i+"= "+Settings.musicLists.get(i));
}
I get 8 identical rows shown... what I do wrong?

musicLists = new ArrayList<List<Mp3>>(LIST_COUNT);
You seem to think that this line creates a list of 8 lists. However, what it really does is to create an empty list of lists with initial capacity 8. That is, it doesn't actually populate any lists into musicLists.
In order for this you do what you expect it to, you need to initialize musicLists and then use musicLists.add(foo) to add 8 List<Mp3> objects to it.

As VeeArr said you need to fill the list with lists before you can get the lists.
// this creates an empty list of initial capacity 8
musicLists = new ArrayList<List<Mp3>>(8);
// musicList.size() is still 0
// we can add as much items as we want, the list will dynamically grow.
for (int i = 0; i < 8; i++) {
List<Mp3> emptySubList = new ArrayList<Mp3>();
// emptySubList.size() is 0 each as well.
musicList.add(emptySubList);
}
// musicList.size() is 8 now.
// do your stuff...
musicLists.get(listNr).add(new Mp3(id,filename,length));
The initial capacity you can specify for an ArrayList is just a way to improve memory consumption.
ArrayLists can dynamically grow unlike array[]s. They do that by internally keeping an array[] where the data you put in your ArrayList is actually stored. If the size they need gets bigger than the array[] can hold they create a bigger array[] and copy the content to that new version (afaik the size is always doubled).
Thats nice because you don't need to worry about doing that yourself. But copying arrays is expensive. Therefore you can define an initial size to hint your ArrayList that it should expect a certain number of elements. It's usually fine if you don't give an initially capacity unless you really know how much items you need.

java arraylist ensureCapacity not working

Either I'm doing this wrong or i'm not understanding how this method works.
ArrayList<String> a = new ArrayList<String>();
a.ensureCapacity(200);
a.add(190,"test");
System.out.println(a.get(190).toString());
I would have thought that ensureCapacity would let me insert a record with an index up to that value. Is there a different way to do this?
I get an IndexOutOfBounds error on the third line.

No, ensureCapacity doesn't change the logical size of an ArrayList - it changes the capacity, which is the size the list can reach before it next needs to copy values.
You need to be very aware of the difference between a logical size (i.e. all the values in the range [0, size) are accessible, and adding a new element will add it at index size) and the capacity which is more of an implementation detail really - it's the size of the backing array used for storage.
Calling ensureCapacity should only ever make any difference in terms of performance (by avoiding excessive copying) - it doesn't affect the logical model of what's in the list, if you see what I mean.
EDIT: It sounds like you want a sort of ensureSize() method, which might look something like this:
public static void ensureSize(ArrayList<?> list, int size) {
// Prevent excessive copying while we're adding
list.ensureCapacity(size);
while (list.size() < size) {
list.add(null);
}
}

So as others have mentioned ensureCapacity isn't for that.
It looks like you want to start out with an ArrayList of 200 nulls? Then this would be the simplest way to do it:
ArrayList<String> a = new ArrayList<String>(Arrays.asList( new String[200] ));
Then if you want to replace element 190 with "test" do:
a.set(190, "test");
This is different from
a.add(190, "test");
which will add "test" in index 190 and shift the other 9 elements up, resulting in a list of size 201.
If you know you are always going to have 200 elements it might be better to just use an array.

Ensuring capacity isn't adding items to the list. You can only get element 190 or add at element 190 if you've added 191 elements already. "Capacity" is just the number of objects the ArrayList can hold before it needs to resize its internal data structure (an array). If ArrayList had a getCapacity(), then doing this:
ArrayList<String> a = new ArrayList<String>();
a.ensureCapacity(200);
System.out.println(a.size());
System.out.println(a.getCapacity());
would print out 0 and some number greater than or equal to 200, respectively

ArrayList maintains its capacity (the size of the internal array) separately from its size (the number of elements added), and the 'set' method depends on the index already having been assigned to an element. There isn't a way to set the size. If you need this, you can add dummy elements with a loop:
for (int i = 200; --i >= 0;) a.add(null);

Once again JavaDoc to clarify the situation:
Throws: IndexOutOfBoundsException
- if index is out of range (index < 0 || index > size()).
Note that size() returns the number of elements currently held by the List.

ensureCapacity just makes sure that the underlying array's capacity is greater than or equal to the argument. It doesn't change the size of the ArrayList. It does't make any changes visible through the API, so you won't notice a difference except that it will probably be longer before the ArrayList resizes it's internal array.

Adding 190 null entries to an ArrayList reeks of a misuse of the data structure.
Think about using a standard primitive array.
If you require a generics or want more efficient use of space then consider SparseArray or even a Map like a HashMap may be appropriate for your purposes.

public static void fillArrayList(ArrayList<String> arrayList, long size) {
for (int i = 0; i < size + 1; i++) {
arrayList.add(i,"-1");
}
}
public static void main(String[] args) throws Exception {
ArrayList<String> a = new ArrayList<String>(10);
fillArrayList(a, 190);
a.add(190,"test");
System.out.println(a.get(190).toString());
}