Transcript ArrayLists
The ArrayList Class in Java Computer Science 3 Gerb Reference: Objective: Understand ArrayLists in Java and how they implement the List ADT. The ArrayList class • The ArrayList class in Java implements the List interface • Uses an array as its data structure • If you need more space – Creates a new array – Copies everything from the old array to the new • Implements operations in the List interface Creating an ArrayList • Two constructors • First constructor takes no parameters ArrayList myList = new ArrayList(); – Picks an initial size for the array • Second constructor takes an integer ArrayList myList=new ArrayList(60); – Uses the number passed to the constructor as the initial size for the array • DON’T BE CONFUSED: 60 is the size of the storage array – NOT the size of the list – All ArrayLists are empty (size = 0) when created Elements of the ArrayList class • Elements in an ArrayList are all of type Object. • Add, set, etc. expect arguments of type Object – Every Java class is a subclass of Object – Includes arrays, strings and user defined classes – Does not include primitive types (int, double, boolean). Wrapper Classes • Java provides “wrapper” classes that allow integers and doubles to be passed as Objects. – Called Integer, Boolean, and Double (note upper case) – Can store these in ArrayList Integer int1 = new Integer(5); Double d1 = new Double(4.01); myList.add(int1); myList.add(d1); • If you try to put an integer or other primitive type into an ArrayList, Java automatically converts it into a wrapper. MyList.add(4); //will be added as an Integer • This conversion is called autoboxing Adding an item to an ArrayList • Use the add operation myList.add(“The String”); • Adds the item after the last item in the list – E.g. if 3 items already in the list, it becomes the 4th • Argument can be any class instance or array • Note that add changes the size of the list Changing an item in an ArrayList • Use the set method to change an item in an ArrayList myList.set(3,”New String”); • First number is the index – Must be less than the size of the list. Throws an exception otherwise – Start numbering from zero. I.e. 2nd item is numbered 1, third item numbered 2, etc. • Second argument can be any class instance or array • Does not change the size of the list Retrieving an item from an ArrayList • Use the get method to get an item from an ArrayList System.out.print(myList.get(3)); • Parameter is the index – Like set, parameter must be less than the size of the list. – Like set, start numbering from zero. – Returns an Object Removing an item from an ArrayList • Use the remove method to take an item out of an ArrayList myList.remove(3); • Parameter is the index of the item to be removed – Start numbering from zero, – Must be less than list size. • Index of all later items in the list are moved down one – E.g. if you remove item #1, item #2, becomes the new item #1 – Item #3 becomes the new item #2, etc. • Decreases the size of the list by one • Returns the item removed Use size to get the number of items in the list • Takes no parameters, returns an integer System.out.print(myList.size()); • Integer equal to the number of add operations minus the number of remove operations • Not to be confused with the size of the underlying array Summary • ArrayList class offers operations on a list – – – – – add set get remove size • Items are numbered from zero • Items are stored in an array which is enlarged when necessary.