CPSC 120 Lab 12
Dependencies Among Objects of the Same Class

Lab Objectives

  • Gain a better understanding of the equals method and references.
  • Experience using static variables in a class.
  • Gain experience (and a better understanding of ) passing an object of the same class as a parameter.

As usual, create a lab12 subdirectory for today's lab, open up Mozilla Firefox and the Web version of this handout, and open Emacs.

Comparing Objects of the Same Class

All Java classes are derived from the Object class in the Java Class Library. That class provides default versions of several methods. Two of those methods are toString and equals. We have used toString often. We know that if we want to print an object we can use the object in a print statement and, if we have written a toString method in our class, that method will be invoked automatically and the string it returns will be printed. If we do not have a toString method, the reference (a hexadecimal number) to the object will be printed. Similarly, we can use equals to compare objects from a class we have written but if we haven't written our own equals method, the method will compare references. In this exercise you will define equals for two different classes, Die and PairOfDice. Save the files Die.java, PairOfDice.java, and TestEquals.java to your directory and do the following.

  1. The Die class is from the text book (we've used it before). The constructor initializes the face on a die to 1. The PairOfDice class, written in Post Lab 10, models a pair of dice (its instance variables are two Die objects). Study the TestEquals program to see what it does, then compile and run it. Study the output to see what is happening. You should notice the following:

    1. Even though all three pairs of dice are initially snake eyes (two 1s) they aren't all "equal" because the references are not the same. Be sure you understand why pair1 and pair2 are not equal, pair2 and pair3 are not equal, but pair1 and pair3 are equal!

    2. When pair1 and pair2 are rolled 10 times note the values of pair3. Be sure you understand why pair3 has the values it has!

  2. Open up Die.java and add an equals method. Two Die objects will be equal if they have the same face value. The equals method signature is as follows. You add the body.
         public boolean equals (Die otherDie)

  3. Open up PairOfDice.java and add an equals method. When writing an equals method for a class you need to decide what you mean for two objects to be equal. For a pair of dice there are two possibilites - you could mean the sum on the dice is the same (that's often what is needed in games) or you could mean that the dice have the same faces (so, for example if the faces are 3 and 4 on one pair, then another pair would be equal with a 3 and 4 (OR 4 and 3) but not 2 and 5). Write your method using the latter definition; that is, two pair are equal if the dice have the same face values. Note the following:

  4. Compile and run TestEquals again. Now all pairs should be equal.

  5. Add a for loop to the program that rolls pair1 and pair2 20 times and counts the times they are equal. Print out each roll in the loop along with a message if the two pair are equal. After the loop print the number of times the rolls were the same. So your output should look something like this:
         Roll 1: [1 : 6] and [3 : 2]
         Roll 2: [3 : 4] and [4 : 3] - equal
         Roll 3: [6 : 2] and [4 : 4]
         Roll 20: [1 : 1] and [5 : 2]
         The pair were equal 3 times.
    Run the program enough to be sure that your equals is correct. Note above that [6 : 2] and [4 : 4] are not equal even though the sum is the same but [3 : 4] and [4 : 3] are equal.

  6. Print PairOfDice.java and TestEquals.java to hand in.

Static Methods and Variables; Objects of the Same Class as Parameters

File Account.java contains a class representing a bank account similar to the one we used in earlier labs. (Note that a second constructor has been added that takes only two parameters and generates the account number randomly.) Save this file to your directory and use it in the exercises below.
  1. Suppose the bank wants to keep track of how many accounts exist. We'll add a static variable to do this as follows:
    1. Declare a private static integer variable numAccounts to hold this value. Like all instance and static variables, it will be initialized (to 0, since it's an int) automatically.
    2. Add code to both constructors to increment this variable every time an account is created.
    3. Add a static method getNumAccounts that returns the total number of accounts. (Think about why this method should be static -- its information is not related to any particular account.)

  2. Now open TestAccounts1.java, study it to see what it does, and save it to your directory. Run it to test your Account class.

  3. Next we will add the capability of transferring funds from one account to another. We'll do it two different ways as follows:

    1. Add a method to the Account class that allows money to be transferred between accounts. This could be done by using two separate methods transferIn(Account otherAcct, double amt) and transferOut(Account otherAcct, double amt), or by using just one method transfer(Account otherAcct, double amt) and defining (and clearly documenting - points off if you don't!) it so that it transfers money FROM the current account TO the account that is passed as a parameter. Use this second strategy (just one method). Inside the transfer method you can manipulate the balance directly or use the deposit and withdraw methods you have already written. Either way, first you'll still have to check that the balance in the from account is sufficient; if not, print a message and don't carry out the transfer.

    2. Now save TestAccounts2.java to your directory. Add code as indicated by the comments to transfer $50 from Madison's account to Spud's account and then to transfer $25 from Spud's account to Madison's account. Print both accounts after each transfer. Don't fill in the rest of the missing code yet. Run it to be sure it works.

    3. Add a static method to the Account class that lets the user transfer money between two accounts without going through either account. You can call the method transfer just like the other one -- you are overloading it. Your new method should take two Account objects and an amount and transfer the amount from the first account to the second account. The signature will look like this:
      public static void transfer(Account fromAcct, Account toAcct, double amt)

    4. Following the next comment in TestAccounts2.java, fill in code to use the static transfer method to transfer another $10 from Spud's account to Madison's account. Test your program.

    5. Following the next comment in TestAccounts2.java, fill in code to transfer $250 from Madison's account to Spud's account. (Use the transfer method of your choice.) Run the program to see what happens. If your code is correct this will not be allowed (insufficient funds).

  4. Finally we will add the capability of closing accounts and consolidating accounts to the class. Do the following:

    1. Add a method void close() to your Account class that closes an account. It should set the name to null (note: this is the null reference which is not the same as an empty string), the balance to 0, and the account number to 0. It should also decrement the total number of accounts.

    2. Also add a static method Account consolidate(Account acct1, Account acct2) to your Account class that creates a new account whose balance is the sum of the balances in acct1 and acct2 and closes acct1 and acct2. The new account should be returned. Two important rules of consolidation:
      • Only accounts with the same name can be consolidated. The new account gets the name on the old accounts.
      • Two accounts with the same number cannot be consolidated. (This would be an easy way to double your money!)

      Check these conditions before creating the new account. If either condition fails, do not create the new account or close the old accounts; print a useful message and return null.

    3. Following the comments in TestAccounts2.java, add code to create a second account for Madison with a balance of 500. Print this account and the total number of accounts. Then consolidate Madison's two accounts into account madConsolidated; print all three accounts and the total number of accounts.

    4. Add code to consolidate Spud's account with itself, putting the new account into spudConsolidated. Of course, this consolidation shouldn't work, but it's useful to see what happens when you try it. Print both spudAcct and spudConsolidated after the call.

    5. Finally, add code to consolidate Madison's consolidated account with Spud's account, putting the new account into jointAcct. Once again, the consolidation shouldn't work. Print all three accounts after the call to consolidate. Run the program and make sure it does what it should.

  5. Print Account.java and TestAccounts2.java to turn in.

What to turn in

Turn in hardcopy of PairOfDice.java, TestEquals.java, Account.java, and TestAccounts2.java. Tar your lab12 directory and email it to your instructor.