How to resize ArrayList

I come from a C++ background and I want to have a matrix of

ArrayList<arrayList<E>> javamatrix 

In C++ I would just do

std::vector<std::vector<T> > cppmatrix;
std::vector<T>vcol(cols);
cppmatrix.resize(rows,vcol);

I can't seem to find a built-in resize() function for ArrayLists for this task, so should I use another collection? Is no way to do this except using for loops with javamatrix.add()?

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P.S I want it to be initialized in the const开发者_StackOverflow中文版ructor with its size as that size might be queried before I edit elements or add or remove.

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There is no resize equivalent that automatically constructs and adds elements. You must do this yourself. However, ensureCapacity is equivalent to vector's reserve. It will ensure you have room, but not change the actual size.

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You shouldn't need to resize arraylists. The size you initially pass in is just its starting size. If you attempt to add items beyond its current size, it will automatically resize.

From the documentation:

Each ArrayList instance has a capacity. The capacity is the size of the array used to store the elements in the list. It is always at least as large as the list size. As elements are added to an ArrayList, its capacity grows automatically. The details of the growth policy are not specified beyond the fact that adding an element has constant amortized time cost.

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Mostly, a 'resize()' operation is not needed because (a) ArrayList's auto-resize as you add elements, and (b) it's unclear what values you would store in the ArrayList<>, e.g. 'null' is not very useful. E.g. in your case you'd probably need a loop anyway to create MatrixCell objects.

For those readers who want to know how to resize an ArrayList to make it smaller, it mystifies me why ArrayList was designed without a 'resize()' method. Perhaps it's because novice programmers are likely to see that method and then not realise that ArrayList<> auto-resizes.

In Java this idiom works to reduce the size of an ArrayList<>:

    list.subList(n,list.size()).clear();

It works because the 'subList' returns a List backed by the original ArrayList<>, so therefore the 'clear()' operates on the original 'ArrayList<>'.

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I know this question is very old already but this link may help java arraylist ensureCapacity not working , The code adds "Null" value in order to adjust the current size.

Instead of using purely ensureCapacity you can have ensureSize

public static void ensureSize(ArrayList<?> list, int size) {

    list.ensureCapacity(size);
    while (list.size() < size) {
        list.add(null);
    }
}