CS 206 Introduction to Computer Science II 09 / 04 / 2008 Instructor: Michael Eckmann

Michael Eckmann - Skidmore College - CS Fall 2008 Today’s Topics More Java Review (References,...)‏ Exception Handling StringTokenizer File I/O

Michael Eckmann - Skidmore College - CS Fall 2008 Java review - References Assignment operator = e.g. Card c1 = new Card(5,0); // Five of Diamonds Card c2 = new Card(6,1); // Six of Clubs Card c3 = new Card(6,1); // Six of Clubs c1 = c3; // assign reference c3 to c1 When a reference is assigned to another reference what happens? Both references refer to the SAME object. That is, the memory address is copied. c1 and c3 are aliases for the same object.

Michael Eckmann - Skidmore College - CS Fall 2008 Java review - References Equality comparison operator == e.g. Card c1 = new Card(5,0); // Five of Diamonds Card c2 = new Card(6,1); // Six of Clubs Card c3 = new Card(6,1); // Six of Clubs if (c2 == c3) // if c2 and c3 are aliases System.out.println(“c2 and c3 are the same card”); When reference variables are compared for equality with ==, what happens? If both references refer to the SAME object, then it evaluates to true. That is, if the memory addresses are the same, (if c2 and c3 are aliases for the same object) then true is evaluated. So will the above if condition be true?

Michael Eckmann - Skidmore College - CS Fall 2008 Java review - References What if we do the following: e.g. Card c1 = new Card(5,0); // Five of Diamonds Card c2; System.out.println(“The card is “ + c1.toString()); System.out.println(“Another card is “ + c2.toString()); Problem with c2.toString() because c2 DOES NOT refer to an object. Its value is null.

Michael Eckmann - Skidmore College - CS Fall 2008 Java review - References Card c1 = new Card(5,0); // Five of Diamonds Card c2; System.out.println(“The card is “ + c1.toString()); System.out.println(“Another card is “ + c2.toString()); Either c2 needs to be assigned a non-null value from another reference c2 = c1; Or it should be assigned to a newly instantiated object. c2 = new Card(10, 3); // Ten of Spades

Michael Eckmann - Skidmore College - CS Fall 2008 Java “review” Exception handling, try/catch blocks File I/O StringTokenizer

Michael Eckmann - Skidmore College - CS Fall 2008 Exception handling Exception handling allows for a program to detect unwanted behaviour and then instead of crashing the program, “catch” the exception while the program is running and handle it by doing something to allow the program to keep running. Let's see an example with catching an exception from Integer.parseInt

Michael Eckmann - Skidmore College - CS Fall 2008 Exception handling Some methods “throw” exceptions that are required to be caught, while others like Integer.parseInt don't require catching the exception. When we write our own methods, we can throw exceptions back to the caller. For example, if we have a method that takes in one int parameter, if the value passed in is required to be >=0, then the first code we write inside the method could be to check if negative and if so, throw an exception.

Michael Eckmann - Skidmore College - CS Fall 2008 File I/O File Input and Output Reading, Writing, Appending There are many classes in Java to handle reading and writing to files. We're going to focus on a few that allow reading and writing to “text files” (human readable) as opposed to “binary files.” Also, we're only going to focus on files that are “sequential” as opposed to “random access.”

Michael Eckmann - Skidmore College - CS Fall 2008 File I/O For file Input I recommend using a BufferedReader wrapped around a FileReader For file Output I recommend using a PrintWriter wrapped around a FileOutputStream Let's look at example code.

Michael Eckmann - Skidmore College - CS Fall 2008 StringTokenizer StringTokenizer is a class that allows us to easily “divide” up a String into tokens which are separated by a delimiter character. Let's look at some example code.