Files From Today's Class

• Wallet.java

Today's Example

In class today, we developed a simple example designed to reinforce what we already know about developing class specification in Java. It was the first time we overloaded a constructor and also the first time we made some use of some minor language features.

Fair Warning

Check the schedule. Notice an interesting event next week!

Notable Items

• Notice the addMoney() and removeMoney() methods. Inside, you'll notice the "+=" and "-=" operator. There is also a "/=" and "*=" operator. This is just shorthand that is known to the compiler. "x += y" means "x = x + y"
  
  
• Notice that any time we displayed money, we made use of the %-operator. The is the modulus operator. It is valid upon ints and longs. Whereas "x / y" is "integer division" and drops the remainer, "x % y" gives exactly the remainder.
  
  
• Within the simpler of the constructors, when we inialize the amount, we write "amount = 0l;" rather than "amount = 0;". This isn't necessary, but it was worth showing you. Remember that Java's default integer type is the "int", so "0" is an "int". "0l" is a "long". Similarly, a "0.0f" would make it a float rather than a double.
  
  
• In the removeMoney() method we returned true or false to indicate whether or not we were able to remove the money as compared to encountering a pesky problem, such as, I don't know, not enough money. Later on, we'll learn how to actually use Java's "Exception handling facility", so that we can return "void" and still detect and manage problems, actually better.
  
  
• This code will leave off the last place of the cents digit if it is zero, "$1.0". Don't worry about this. We'll learn how to fix that later. If it bothers you, consider using and if statement to fix it.
  
  
• We've certaily used the notation before, but I noticed that it wasn't in the notes: "/* blah, blah */" is one notation for comments. "// blah blah" is another that is used when the comment doesn't cross lines. Comments are notes left within a program, to be ignored by the compiler, that are there simply to aid the human reader.

Today's Example

class Wallet {
  private String owner;
  private long amountInCents;

  public Wallet (String owner) {
    this.owner = owner;
    this.amountInCents = 0l;
  }

  public Wallet (String owner, long amountInCents) {
    this.owner = owner;
    this.amountInCents = amountInCents;
  }

  public String getOwner() {
    return owner;
  }

  public long getAmount() {
    return amountInCents;
  }

  public void addMoney(long cents) {
    amountInCents += cents;
  }

  public boolean removeMoney (long cents) {
    if (amountInCents < cents)
      return false;
    
    amountInCents -= cents;
 
    return true;
  }


  public boolean isFatter (Wallet w) {
    if (amountInCents > w.amountInCents)
      return true;

    return false;
  }

  public String toString() { 
    return owner + ": $" + (amountInCents / 100) + "." + (amountInCents % 100);    
  }


  public boolean equals(Object o) { 
    Wallet w = (Wallet) o;
    
    if (!owner.equals(w.owner))
      return false;

    if (amountInCents != w.amountInCents)
      return false;

    return true;
  }


  public static void main(String[] args) {
    Wallet gregsWallet = new Wallet ("Gregory Kesden");
    System.out.println (gregsWallet);

    Wallet forbesWallet = new Wallet ("Steve Forbes", 100); // One dollar
    System.out.println (forbesWallet);

    System.out.println (forbesWallet.getOwner() + ": $" +
                        (forbesWallet.getAmount() / 100) + "." +
                        (forbesWallet.getAmount() % 100));

    forbesWallet.addMoney (150);
    System.out.println (forbesWallet);
    System.out.println (gregsWallet);

    if (forbesWallet.removeMoney(250))
      System.out.println ("Expected success removing from Forbes Wallet.");
    else
      System.out.println ("Unxpected failure removing from Forbes Wallet.");

    if (!forbesWallet.removeMoney(1)) // Notice the !, remember, think NOT
      System.out.println ("Expected failure removing from Forbes Wallet.");
    else
      System.out.println ("Unxpected success removing from Forbes Wallet.");


    // Test one way
    if (!forbesWallet.isFatter(gregsWallet)) 
      System.out.println ("Expected: forbeswallet NOT fatter than gregswallet.");
    else
      System.out.println ("Unxpected: forbeswallet fatter than gregswallet");

    // Test symmetric way
    if (!gregsWallet.isFatter(forbesWallet)) 
      System.out.println ("Expected: forbeswallet fatter than gregswallet.");
    else
      System.out.println ("Unxpected: forbeswallet NOT fatter than gregswallet");

    // Test equal (not one greater tha the other) case
    forbesWallet.removeMoney(250);
    if (!gregsWallet.isFatter(forbesWallet)) 
      System.out.println ("Expected: gregswallet NOT fatter than forbeswallet.");
    else
      System.out.println ("Unxpected: gregswallet fatter than forbeswallet");

    if (!forbesWallet.isFatter(forbesWallet)) 
      System.out.println ("Expected: forbeswallet fatter than gregswallet.");
    else
      System.out.println ("Unxpected: forbeswallet NOT fatter than gregswallet");


  }
}