Assignment #2 Solution



In this assignment you will get practice working with Java arrays, more practice implementing your own classes, and practice creating a procedural design in Java (in the BulgarianSolitaireSimulatorclass). Like you did in assignment 1 and lab 4, you will be implementing a class whose specification we have given you, in this case a class called SolitaireBoard, to represent the board configuration for a specific type of solitaire game described further below. You will also be using tools to help develop correct code, such as assert statements along with code to verify that your class implementation is consistent.

Note: this program is due after your midterm exam, but it’s a fair amount bigger than the first assignment. We recommend getting started on it before the midterm. It only uses topics from before the midterm, so working on it now will also help you prepare for the exam (there will be paper and pencil array programming problems as part of the exam).

Note: Lab 5 has some exercises to help you get started on the assignment. You will need to read over the assignment before completing those exercises.


  • Horstmann, Special topic 11.6, assert statements

  • Horstmann, Section 11.3 Command-line arguments

  • Horstmann, Section 8.4 static methods

  • Horstmann, Section 11.2 Text Input and Output

  • Horstmann, Section 7.1.4, 7.3 Partially-filled arrays

  • CS 455 lecture, 2/5, Representation invariants

  • CS 455 lectures, 1/29, 1/31, 2/5, Partially-filled array example (i.e., Names class)

  • Horstmann, Special topic 8.1 Parameter passing

The assignment files

The starter files we are providing for you on Vocareum are listed here. The files in bold below are ones you create and/or modify and submit. The files are:

  • The interface for the SolitaireBoard class; it contains stub versions of the functions so it will compile. It also contains some named constants. You will be completing the implementation of this class. You may not change the interface for this class, but you may add private instance variables and/or private methods to it

  • A main program that does a Bulgarian Solitaire Simulation. This simulation is described further in the section on the assignment

  • README See section on Submitting your program for what to put in it. Before you start the assignment please read the following statement which you will be “signing” in the README:

“I certify that the work submitted for this assignment does not violate USC’s student conduct code. In particular, the work is my own, not a collaboration, and does not involve code created by other people, with the exception of the resources explicitly mentioned in the CS 455 Course Syllabus. And I did not share my solution or parts of it with other students in the course.”

If you choose to work on your assignment outside of the Vocareum environment, please see the detailed directions about this from assignment 1.

Note on running your program

You will be using assert statements in this program. To be able to use them, you need to run the program with asserts enabled (-ea flag). (You do not need to compile it any special way.) Here is an example:

java -ea BulgarianSolitaireSimulator

You should run with this flag set every time.

Assert statements are another tool to help us write correct code. More about how you are using them here in the section on representation invariants.

NOTE: In Eclipse you use the Run Configurations settings (in the Run menu) to change what arguments are used when running a program. You will be using both program arguments (more details in the next section) and VM arguments when running this program; -ea is a VM argument. (NOTE: VM stands for virtual machine, as in the Java Virtual Machine, which is what you are running when you do the “java” command.)

The assignment

You will be implementing a program to model the game Bulgarian Solitaire. This program will run in the console window and will not have a GUI. This is problem P7.4 from our textbook Big Java: Early Objects, 6th Edition, by Cay Horstmann. Here is his description of the problem (second paragraph is a paraphrase):

The game starts with 45 cards. (They need not be playing cards. Unmarked index cards work just as well.) Randomly divide them into some number of piles of random size. For example, you might start with piles of size 20, 5, 1, 9, and 10. In each round you take one card from each pile, forming a new pile with these cards. For example, the starting configuration would be transformed into piles of size 19, 4, 8, 9, and 5. The solitaire is over when the piles have size 1, 2, 3, 4, 5, 6, 7, 8, and 9, in some order. (It can be shown that you always end up with such a configuration.)

In the normal mode of operation your program will produce a random starting configuration and print it. It then keeps applying the solitaire step and printing the results, and stops when the final configuration is reached.

We recommend you finish playing Horstmann’s example game, started above, to see how it comes out (you can do it with just pencil and paper). (Save your work because it’s part of the associated lab assignment.)

To make it easier to test your code, your program will be able to be run in a few different modes, each of these controlled by a command-line argument. The user may supply one or both of the arguments, or neither.

Prompts for the initial configuration from the user, instead of generating a random configuration.
Stops between every round of the game. The game only continues when the user hits enter (a.k.a., return).

Command-line argument processing is discussed in section 11.3 of the Horstmann text. But to make things a little easier, we wrote the code for processing the command-line arguments for you. It appears in starter code you get in the main method in Here are a few examples of ways to run the program in the Linux shell:

java -ea BulgarianSolitaireSimulator -u
java -ea BulgarianSolitaireSimulator -u -s
java -ea BulgarianSolitaireSimulator

[Note: recall you are using the -ea argument for assertion-checking. The arguments after the program name are the ones that get sent to your program.]

There are more details about exactly what your output should look like in each of these operation modes in the section on the BulgarianSolitaireSimulator program.

Some of the requirements for the program relate to efficiency, testing, and style/design, as well as functionality. They are described in detail in the following sections of the document, and then summarized near the end of the document.

SolitaireBoard: interface

What follows is the specification for the SolitaireBoard class. You must implement the following methods so they work as described:

Creates a solitaire board with a random initial configuration.
SolitaireBoard(ArrayList<Integer> piles)
Creates a solitaire board with the given configuration. Example values: [20, 5, 1, 9, 10].
piles contains a sequence of positive numbers that sum to SolitaireBoard.CARD_TOTAL
void playRound()
Plays one round of Bulgarian solitaire. Updates the configuration according to the rules of Bulgarian solitaire: Takes one card from each pile, and puts them all together in a new pile. The old piles that are left will be in the same relative order as before, and the new pile will be at the end.
boolean isDone()
Returns true iff the current board is at the end of the game. That is, there are NUM_FINAL_PILES piles that are of sizes 1, 2, 3, …, NUM_FINAL_PILES in any order.
String configString()
Returns current board configuration as a string with the format of a space-separated list of numbers with no leading or trailing spaces. The numbers represent the number of cards in each non-empty pile. Example return string: “20 5 1 9 10”

In addition, we have defined two public named constants for you:

static final int CARD_TOTAL
static final int NUM_FINAL_PILES
We have defined two public named constants for you, CARD_TOTAL (45) and NUM_FINAL_PILES (9). Please write all of your code in terms of these constants, so that your program will still work if their values are changed. You should also test your code with different values for NUM_FINAL_PILES. For games that will terminate, the sum of the numbers from 1 to NUM_FINAL_PILES must equal CARD_TOTAL, so we have set CARD_TOTAL to be a value computed from NUM_FINAL_PILES. So, for example, if you change NUM_FINAL_PILES to 4, CARD_TOTAL will automatically have the value 10. See comments in for more details.

Note: No SolitaireBoard methods do any I/O.

You may have noticed that there is another method, isValidSolitaireBoard, that’s private, i.e., not part of the public interface. We will describe that later after we discuss the representation.

You may not change the public interface for this class, with the following exception: you may add a public SolitaireBoard toString method that you may want to use for debugging purposes. It would be very short and mostly consist of call to the toString method(s) of its constituent part(s). That way you can see if you are building your SolitaireBord object correctly, even before you implementconfigString. Section 9.5.1 of the textbook has more about writing a toString method. Hint: to get a String representation of an array, use Arrays.toString

SolitaireBoard: representation/implementation

For the purposes of this assignment you are required to use an array to represent the piles of cards in your solitaire board (each array element is one pile). You may have additional fields, but you may not use an ArrayList. This is going to be a partially-filled array. However, different from other situations where the number of elements in an array can change size, in this application there is an upper bound on the number of elements, so if you allocate your array in the constructor using that upper bound, you’ll never have to resize it later. You’ll figure out what that upper bound is once you think about the problem a little.

The reasons for this requirement are (1) to give you more practice with using arrays, (2) because the array capacity won’t have to change (as described in the previous paragraph), and (3) if you want to write the most efficient solution (see extra credit section below) using the ArrayList doesn’t really make it any easier.

Representation invariants

Many of the development techniques we discuss in this class, for example, incremental development, the use of good variable names, and unit-testing, are to help enable you to write correct code (and make it easier to enhance that code later). Another way to ensure correct code within a class is to make explicit any restrictions on what values are allowed to be in a private instance variable, and any restrictions on relationships between values in different instances variables in our object. Or put another way, making sure you know what must be true about our object representation when it is in a valid state. These are called representation invariants.

Representation invariants are statements that are true about the object as viewed by the implementor. Since for many classes, once a constructor has been called the other methods can be called in any order, you need to ensure that none of the constructors or mutators can leave the object in an invalid state. It will be easier to do that if you know what those assumptions are.

There are two assignment requirements for your SolitaireBoard class related to this issue listed here, and described in more detail right after that.

  1. in a comment just above or below your private instance variable definitions for SolitaireBoard, list the representation invariants for the object.

  2. write the private boolean method isValidSolitaireBoard() and call it from other places in your program as described below.

1. The representation invariant comment for SolitaireBoard
Write a list of all the conditions that the internals of a SolitaireBoard object must satisfy. That is, conditions that are always true about the data in a valid SolitaireBoard object. For example, one or more invariants would describe where the data is in a partially filled array (we did a similar example in lecture on 9/18). Another one (or more) would be related to the restriction that there are always 
CARD_TOTALcards on the board.

2. isValidSolitaireBoard() method
This private method will test the representation invariant for the internals of a solitaire board. It will return true iff it is valid, i.e., the invariants are satisfied.

Call this function at the end of every public SolitaireBoard method, including the constructors, to make sure the method leaves the board in the correct state. This is one kind of sanity check: one part of a program double-checking that another part is doing the right thing (similar to printing expected results and actual results).

Rather than putting this test in an if statement, we’re going to put it in an assert statement. For example:

assert isValidSolitaireBoard();

Assert statements are described in Special topic 11.6 of the text.

Please make sure you are running your program with assertions enabled for every run of this program, since it’s in a development stage. See earlier section for how to do this. You won’t really know if they are getting checked unless you force one to fail.

The point of these assert statements is to notify you in no uncertain terms of possible bugs in your code. The program crashing will force you to fix those bugs. For example, if a board doesn’t have CARD_TOTAL (45) cards on it, then the simulation may never terminate, or if the array has “hole” in the middle, then other methods, such as configString may not work as advertised.

BulgarianSolitaireSimulator program

Please take a look at this example for what your output must look like. This shows (part of) a run of the program with the -u option turned on (-u stands for user input mode.) It also illustrates the error-checking. User input is shown in bold and the “. . .” below represents some steps not shown here.

Number of total cards is 45
You will be entering the initial configuration of the cards (i.e., how many in each pile).
Please enter a space-separated list of positive integers followed by newline:
40   1  1 1 1
ERROR: Each pile must have at least one card and the total number of cards must be 45
Please enter a space-separated list of positive integers followed by newline:
  44 b 1 x
ERROR: Each pile must have at least one card and the total number of cards must be 45
Please enter a space-separated list of positive integers followed by newline:
100 -55
ERROR: Each pile must have at least one card and the total number of cards must be 45
Please enter a space-separated list of positive integers followed by newline:
0 45
ERROR: Each pile must have at least one card and the total number of cards must be 45
Please enter a space-separated list of positive integers followed by newline:
40 1 1 1 1 1
Initial configuration: 40 1 1 1 1 1
[1] Current configuration: 39 6
[2] Current configuration: 38 5 2
[3] Current configuration: 37 4 1 3
. . .
[30] Current configuration: 10 2 3 4 5 6 7 8
[31] Current configuration: 9 1 2 3 4 5 6 7 8

Note that we’re not forcing the user to enter exactly one space between the numbers entered for the initial configuration. E.g., if the call to in.nextLine() resulted in the following String, it should be accepted as valid input by your program:

"    40     1     1 1    1 1    "

Tabs are ok as whitespace too but we can’t show them easily here.

Here is an example of what your output should look like with the -s option turned on (-s stands for single step mode). The -u option is not set in this example, so it uses a random initial configuration. Again, only part of the run is shown here. After each “<Type return...>” the program blocks until the user hits the return key.

Initial configuration: 9 4 6 26
[1] Current configuration: 8 3 5 25 4
<Type return to continue>
[2] Current configuration: 7 2 4 24 3 5
<Type return to continue>
[3] Current configuration: 6 1 3 23 2 4 6
<Type return to continue>
. . .
[26] Current configuration: 2 3 4 5 6 7 8 10
<Type return to continue>
[27] Current configuration: 1 2 3 4 5 6 7 9 8
<Type return to continue>

If neither argument is set, then the program will take no user input, and just show the initial configuration followed by the numbered result of each round until it finishes (i.e., output like the second example above, but without the lines that say “<Type return…>”).

A correct program will always terminate. For some values of SolitaireBoard.CARD_TOTAL a game won’t terminate; so do not change the initialization expression that sets this value based on the current value of SolitaireBoard.NUM_FINAL_PILES (the code to do that is already present in the starter version); this way it should still terminate even if you change NUM_FINAL_PILES to some other positive value.

Your output for a particular input must match what’s shown above character-by-character (e.g., the messages displayed and the error handling should be the same), so we can automate our tests when we grade your program. This means the new piles must appear in the particular order shown, not only in how they are printed, but the order of the numbers in the String returned by the SolitaireBoard‘s configString method. Of course, this does not include the “. . .” for the parts we left out in our example runs: yours would show the missing rounds instead.

The submit script will do a few of the automated tests (and give you are report on the results), thus we recommend you try your first submit early, so you would have time to still fix your code and resubmit before the final deadline. See the section on submitting for more details about this.

Error checking required

You are required to check that the list of numbers entered are actually all numbers (integers) and represents a valid configuration. Section 11.2.7 of the text discusses how to use a Scanner to check whether a string is a valid integer. The previous section showed examples of invalid input, and what the program’s response should be.

Converting a String into an array of numbers

In user mode, you will not be able to read in the numbers directly into an ArrayList of integers using repeated calls to nextInt, because we’re using newline as a sentinel (i.e., a signal that that’s the end of the input data), and nextInt skips over (as in, doesn’t stop for) newlines. So you’ll want to first read the input all at once using the Scanner nextLine method, and then convert them to an ArrayList of integers.

How do we do such a conversion? Section 11.2.5 of the textbook (called Scanning a String) shows one way of solving the problem of processing an indeterminate number of values all on one line. It takes advantage of the fact that the Scanner class can also be used to read from a String instead of the keyboard. Once we have our String of ints from the call to nextLine(), you create a second Scanner object initialized with this string to then break up the line into the parts you want, using the Scanner methods you are already familiar with.

Structure of BulgarianSolitaireSimulator

The code for BulgarianSolitaireSimulator is too long to be readable if you put it all into the main method. One could design and add another class, to deal with the simulation, but instead here you’ll use a procedural design to organize the code; we’ll review procedural design here.

A good design principle (for procedural as well as object-oriented programming) is to keep each of your methods small, for easier program readability. In object-oriented programming, the class design sometimes naturally results in small methods, but sometimes you still need auxiliary private methods. The same principles apply for a procedural design. Since we haven’t given you a predefined method decomposition for the BulgarianSolitaireSimulator, you will have to create this decomposition yourself.

A procedural design in Java is just implemented as static methods in Java that pass data around via explicit parameters. Static methods are discussed in Section 8.4 of the text, and this use of them was also discussed in a sidebar in Lab 4. You have seen a few examples of this in other test programs we have written, for example of lab 4, and we developed in lecture. You have also seen some utility classes in Java that have static methods: Math and Arrays.

If you have learned about procedural design in other programming classes, you know that global variables are a no-no. This is another example of the principle of locality. Thus, in designing such a “main program” class, we don’t create any class-level variables, because they become effectively global variables (see also Style Guideline #9). The “main class” does not represent any overall object. Instead you will create variables local to main that will get passed to (or returned from) its helper methods, as necessary.

Note: the next section discusses a limit on method length as one of our style guidelines for this course.

Another design issue that comes up often in both object-oriented and procedural designs is code reuse. You’ve seen a couple examples of one kind of code reuse in lecture where we’ve created a parameterized method that generalizes some action to avoid repeating code, calling the method every time we need to do that action instead. For example, doOneLookup in PartialNamesTester from the lecture on 9/13. In particular, for this assignment, if there are four possible modes in which the program can be run (i.e., two flags that can be turned on and off independently), a good solution would not repeat the same code four times! Imagine, if we added two more options, we’d have to have it repeated sixteen times!

As we’ve discussed in lecture: one clue is if you find yourself writing the same or similar code sequence multiple times, it may be an indication that there’s a better (and shorter) way to do it.

A note about the Scanner

This (and all Java programs that read from the console) should only have one Scanner object to read from If you make multiple such Scanner objects your program will not work with our test scripts. You will also have problems if you try to open and close multiple Scanners from in your code. Once you create that one Scanner, you can pass it as a parameter to other methods to be able to use it in different places. Here is a little program with an example of this:

public class MyClass {
   public static void main(String[] args) {
      Scanner in = new Scanner(;  // create the Scanner
      . . .
      int dataVal = in.nextInt();  // using "in" directly in main
      . . .
      // readAndValidateString will also read some more input
      String moreData = readAndValidateString(in); // pass "in" as a parameter here
      . . .

   // prompts for a String from "in", reads it, and validates it.
   private static String readAndValidateString (Scanner in) { 
      // don't create another Scanner for here; use the parameter instead
      . . .
      String theString =;
      . . .
      return theString;
   . . .

As you may know by now, you can create Scanner objects that have different data sources. We are not saying you can only have one Scanner, but rather that you should only have one that was created using new Scanner(

Summary of requirements

As on the first assignment, there are several requirements for this assignment related to design, testing, and development process strewn throughout this document. We’ll summarize those and the functional requirements here:

  • implement SolitaireBoard class according its public interface (see SolitaireBoard: interface)

  • use the representation for SolitaireBoard described in the section about that.

  • write representation invariant comments for SolitaireBoard class.

  • implement and use private SolitaireBoard method isValidSolitaireBoard as described here.

  • implement BulgarianSolitaireSimulator with the user interface described in the section about the BulgarianSolitaireSimulator program.

  • only use the expression new Scanner( at most once in your program (see previous section)

  • do the error checking described in the Error Checking section.

  • your code will also be evaluated on style, documentation, and design. We will deduct points for programs that do not follow the published style guidelines for this course (they are also linked from the Assignments page). (Note: For pa1 we only deducted points for problems related to some of the style guidelines.) One guideline we want you to be especially aware of is the limit of 30 lines of code at most allowable in a method. This is exclusive of whitespace, comment lines, and lines that just have a curly bracket by itself (i.e., you should not sacrifice code-readability to make your code fit into this limit). Hopefully, it’s obvious that putting multiple statements or declarations on a single line decreases code readability and is not desirable; that would also result a loss in style points.

For these “style” points we will also take into account the general quality of your design (e.g., see hints in the previous section).

Extra credit

We won’t be discussing big-O until nearer the due date, so we’re making this one part extra credit: You will receive a small amount of extra credit if your playRound and isDone methods execute in time linear in the number of piles (i.e., O(n)) and if your playRound uses a constant amount of extra space (e.g., it does not allocate a new array). However, your isDone method may use more space to meet the O(n) time requirement.

If you are a student who is struggling in the class or starting the program on the late side, I recommend not attempting the extra credit, or only doing so if you have extra time after you have a complete working program (and save a backup of that working program).

README file / Submitting your program

You will be submitting, and README. Make sure your name and loginid appear at the start of each file. Reminder: if you developed your code locally on your laptop, before submitting you will need to upload your code to Vocareum, and compile and re-test it there before submitting it

Here’s a review of what goes in the README: This is the place to document known bugs in your program. That means you should describe thoroughly any test cases that fail for the the program you are submitting. (You do not need to include a history of the bugs you already fixed.) You also use the README to give the grader any other special information, such as if there is some special way to compile or run your program. You will also be signing the certification shown near the top of this document.

When you are ready to submit the assignment press the big “Submit” button in your PA2 Vocareum work area. The submit script will check that you have the correct files in your work area and whether they compile. In addition, it will run your program on a few test cases, in particular to check if your output matches the correct output character-by-character, and to check if your program has the correct output in the case of incorrect input from the user. If you fail any of these tests, you are unlikely to pass many of the other automated tests we will be doing when grading your code, so please go back and fix your code so it produces the correct output.

Passing these submit checks is not necessary or sufficient to submit your code (the graders will get a copy of what you submitted either way). (It would be necessary but not sufficient for getting full credit.) However, if your code does not pass all the tests we would expect that you would include some explanation of that in your README. One situation where it might fail would be if you only completed a subset of the assignment (and your README would document what subset you completed.)

You are allowed to submit as many times as you like, but we will only grade the last one submitted. If you are unsure of whether you submitted the right version, there’s a way to view the contents of your last submit in Vocareum after the fact: see the item in the file list on the left called “Latest Submission”.