ArrayLists

The ArrayList Class

The ArrayList class is the most straight-forward of several classes that Java provides to represent groups or collections of objects.

• You can create an ArrayList without knowing in advance how many items the list will eventually hold, and you can add or remove items as you need.
• When you put items in an ArrayList, you do not need to give them individual variable names. Instead, you refer to them by their location, or index, in the list.
• You can go through the list in order, from first to last, or you can jump around and access the items in any order. You can use all of them, or just a few.
• All of the items in the list need to be of the same type, specified when you construct or declare and ArrayList.

For example, the following code fragment creates an empty deck of cards, adds 3 cards to the deck, and then prints the first and third card in the deck.

    ArrayList<Card> deck = new ArrayList<Card>();
    deck.add(new Card("Ace", "Clubs"));
    deck.add(new Card("Queen", "Hearts"));
    deck.add(new Card("5", "Spades"));
    System.out.println("First card: " + deck.get(0));
    System.out.println("Third card: " + deck.get(2));

A few more details:

• When you construct an ArrayList, it always starts out empty. You can get the current size using the size() method.
• Use the add(itemToAdd) method to add items.
• Use the get(i) method to get the item at index i, or set(i, newItem) to replace it. Valid indices range from 0 to theList.size() - 1.
• Use the remove(i) method to remove the item at index i.
• If there are N items in the list, valid indices range from 0 to N - 1. To get the last item in the list, use theList.get(theList.size() - 1).