CSCI 151 - Prelab 2

Due at the start of class on Monday September 24

In this prelab, you will tackle some of the non-programming issues related to the second full lab assignment.  Please write or type up your solutions, and hand in a paper copy by 10 am on Monday.  Sign the Honor Pledge somewhere on the paper.  Late prelabs are not accepted.

Note that there are 5 numbered questions here.

MyArrayList

In this week's lab, you will construct your own implementation of the ArrayList data structure, which we will call MyArrayList.  You will likely be using this data structure frequently.  It has a fairly simple interface and behaves similarly to the Python list structure.

Soon you will be very familiar with using this data structure and its methods -- two different add methods, get, set, remove, isEmpty, size, and so forth.

MyArrayList implements the Java List interface. That is, you have to provide implementation for all of the (abstract) methods contained in the interface. There are more than 20 such methods, however, which is more than we want to do in a single lab.  Fortunately, java.util contains an abstract class AbstractList that implements many of the required methods.  Some of the methods still aren't implemented (that is, they are abstract) and some of them may have an inefficient implementation (that is, you'll want to override them), but it's useful nonetheless.  Thus your MyArrayList class should extend AbstractList in order to take advantage of this.

    Question 1:

    You'll be using generics for your implementation, and taking advantage of the fact that List and AbstractList are similarly parameterized.  Given the information above, give the declaration (i.e. "public class...") for MyArrayList.  Remember to properly indicate the parent and that we'll be using Generics to handle the storage type.  (Refer to Weiss 4.7 for information on generics.)


    Question 2:

    Skim through the documentation for AbstractList (specifically the description at the top of the web page).  What methods must you implement to create a concrete child class?  What methods do you need to override so that the list is modifiable?


Backing Storage

As you might expect from the name, the underlying storage for an ArrayList is an array.  That is, an ArrayList is an object with an array as a data member.  Thus, the number of slots in the array (its capacity; i.e., its length) might be different than the number of items currently in the MyArrayList (its size).  The size must always be less than or equal to the capacity -- otherwise we're not storing everything.  (Note: you don't need to make things smaller after removing items.)

    Question 3:

    Which of the methods that you identified in question 2 might require that you resize the array in order to be sure all the data can fit?


Resizing Options

When you add an item to a MyArrayList, you may need to resize the underlying array.  Otherwise, you might not being able to fit all of your data into your structure.  Let's consider how much larger we should make the array each time we resize.

You will write a private resize() method that increases the length of the data array as part of your assignment. You will need to make a new array of larger size, copy the data from the old array into the new array, and change the reference to point to your new array. Let's assume that our storage array is declared AnyType data[].

    Expanding the array by 1 each time an item is added:  Suppose you start with an ArrayList of size 0 and capacity 1, (i.e., data.length==1), and you add n elements one-by-one to the ArrayList.  After the first item is added, you'll have to call resize for each additional add.  When you resize, you'll copy all the previous values over to the new array.

    Consider the following questions.




    Question 4:


    Doubling the size of the array each time an item is added:  Now suppose you change your resize method so that it doubles the length of the data array, instead of only incrementing by one.  Rebuild the table below using this strategy.

     Number of items (size) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 ... n-1 n=2p n+1
     data.length 1 2 4 4 8 8 8 8 16 16 16 16 16 16 16 16 32 ... n n=2p 2*n
     calls resize? (Y/N) N Y Y N Y











    ...
    N Y
     # assignment statements in resize( ) 0 1 2 0 4













    0 n

    You are to determine the total number of assignments; the chart will help you determine this value by summing the values in the bottom row.  You may assume that n is in the form 2p+1 -- that's the worst case scenario.  It may be useful to remember that 1+2+...+2p= 2p+1-1 = 2*2p-1.

    *** Total number of assignments performed (in terms of n, so get rid of all p's, and simplify):



Testing

An important part of developing a reusable Abstract Data Type is being able to determine if you've correctly implemented all of its methods.  A portion of the lab will be to write some test programs to test your MyArrayList implementation, to see if you can spot any problems with it.

A good programming technique is to "write a little, test a little."  By being able to come up with short tests that you can use right after you create a method, you will have greater confidence and insight as to the location of any bug that appears later.


    Question 5:

    Describe three short tests that will test an aspect of one of the methods you are planning to implement.  Describe (in pseudocode) what actions you will take in the test and what the expected outcome should be.  For example, I may want to test the constructor.  I would call the constructor and then print out the value given by size() as well as looking at the array's length to see if it is what I expect.  Put some thought into where your programming mistakes are likely to be, and write tests that will check them.  Frequent sources of errors come from adding/removing items, repeated operations, and dealing with edge cases (is that supposed to be less-than or less-than-or-equal?)

     Test #1  




     Test #2  




     Test #3  





handing in your prelab

If you adhered to the honor code in this assignment, add the following statement at the top of your prelab.

I have adhered to the Honor Code in this assignment.

Write or type your solutions and hand in a paper copy by the deadline listed at the top of this prelab.  Late prelabs will not be accepted.