31-ArrayList.pdf

Mehran Sahami

Handout #31

CS 106A

October 31, 2007

ArrayLists Reference for Hangman

Based on a handout by Patrick Young

This handout gives you a quick reference for some of the concepts related to ArrayLists

that may be useful to you for implementing Part III of the Hangman assignment.

ArrayLists

You can think of an ArrayList as being a special object which is used to store a list of

other objects. In your case, you’ll be using an ArrayList to store the list of String s which

can be chosen as words for Hangman.

Using ArrayLists

ArrayLists are defined in the java.util package. To use ArrayLists, you’ll need to

import the package:

import java.util.*;

Declaring an ArrayList variable

ArrayLists can store a variety of different types of information—for example, you can

create an ArrayList of GOval s, an ArrayList of GRect s, or in the case of Hangman an

ArrayList of String s.

When declaring a variable of type ArrayList, you’ll need to specify what type of data is

stored in the ArrayList using a special angular bracket notation. Your declaration will

look something like this:

private ArrayList<String> wordList;

The information in the angular brackets tells Java that this is an ArrayList of String s.

ArrayLists themselves are objects and are created using constructors, just as GOval s and

GRect s are created using constructors. Here is how we would create a new ArrayList and

assign it to our variable wordList :

wordList = new ArrayList<String>();

Adding elements to your ArrayList

Call the add method to add elements to your ArrayList. Because your ArrayList has been

declared to contain String s, you can call the add method with a String as an argument:

String word = readLine("?"); // reading a String from the user

wordList.add(word);

Accessing elements of your ArrayList

To access an element of your ArrayList, you can call the get method, passing in the index

of the element you want to access. Remember, ArrayLists (as with regular arrays in Java)

are zero-indexed—in other words, the first element in the list is actually at index 0, the

second element is at index 1, and so forth. Because your list is declared to contain

String s, Java assumes that the element returned by get is a String :

int index = 0; // index of first element in ArrayList

String word = wordList.get(index);

Determine the size of your ArrayList

Your ArrayList will keep track of the number of element which you have added. To

determine the number of elements in the ArrayList, call the size method:

int totalCount = wordList.size();

Chapter 11.8 is your friend for more information about ArrayLists

There’s a lot more to learn about ArrayLists. This handout is just providing a quick

references for the Hangman assignment. We will cover ArrayLists more extensively in

class and you can read more about ArrayLists in Chapter 11.8 of the class textbook.

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