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Michael Eckmann - Skidmore College - CS 206 - Fall 2009
Today’s Topics• Introduction of myself• Review the syllabus• List of Topics we'll cover• Testing & Debugging• Start Java Review (References, ...)• discussion of what we'll do in lab tomorrow
Michael Eckmann - Skidmore College - CS 206 - Fall 2009
Reading
• Chapter 1 – sections 1.1 & 1.3
• Appendix H
Michael Eckmann - Skidmore College - CS 206 - Fall 2009
Who is your instructor?• I'm Mike Eckmann and this is my sixth year at Skidmore.
Before that I was at Lehigh University in PA.• I studied Mathematics and Computer Engineering and
Computer Science all at Lehigh University.• I was employed as a programmer (systems analyst) for eight
years.
Michael Eckmann - Skidmore College - CS 206 - Fall 2009
Syllabus
• Office hours
• Text book
• Class room
• Assignments – Hw's &
programs
• Collaboration policy
• Grading schema
• Attendance
• Readings
http://www.skidmore.edu/~meckmann/2009Fall/cs206/index.html
The most up-to-date syllabus will be found on the course web page.
Michael Eckmann - Skidmore College - CS 206 - Fall 2009
Suggestions on how to succeed in this (or any? CS) course
• Keep up on the reading before the class period that we cover it.
• Participate in class -- ask questions in class (best), via email or in my office.
• Start the programs as soon as they are assigned. You will run into snags. Debugging can take a long, long time.
• These are based on my observations of successful students in past classes.
Michael Eckmann - Skidmore College - CS 206 - Fall 2009
JavaDoc
• Using JavaDoc to “self” document your code.
• /**
• Class description
• Method description
• @author
• @param
• @precondition
• @postcondition and/or @returns
Michael Eckmann - Skidmore College - CS 206 - Fall 2009
JavaDoc• preconditions are those things that are supposed to be
true before a piece of code (say a method) executes
• postconditions are those things that are guaranteed to be true after a piece of code executes (assuming the preconditions were met.) -- including any changes the code makes to instance variables, output, and what is returned
• what might be the precondition for a “get” aka “accessor” method?
• Let's look at my test program ForJavaDoc.java
Michael Eckmann - Skidmore College - CS 206 - Fall 2009
Testing & Debugging
• This semester our programs can be larger and more complex than in cs106. Therefore it's a good idea to first design your code and then code it and then think about how to test your programs for accuracy.
• Determining good test data to input to our programs is the first meaningful step in determining if our code is correctly written.
Michael Eckmann - Skidmore College - CS 206 - Fall 2009
Testing & Debugging
• Any ideas on what makes good test data?
• How have you tested your programs in the past?
Michael Eckmann - Skidmore College - CS 206 - Fall 2009
Testing & Debugging
• Choose test data for which you know the correct output.
• Pick test cases that are likely to cause errors.
• What kinds of things do you think often cause errors?
Michael Eckmann - Skidmore College - CS 206 - Fall 2009
Testing & Debugging
• Choose test data for which you know the correct output.
• Pick test cases that are likely to cause errors.
• What kinds of things do you think often cause errors?
– boundary values
• e.g. testing code that requires a month number (1
through 12) so we would try what numbers?
Michael Eckmann - Skidmore College - CS 206 - Fall 2009
Testing & Debugging
• Another necessary requirement of testing your code is to “fully exercise the code.”
– what do you think that means?
Michael Eckmann - Skidmore College - CS 206 - Fall 2009
Testing & Debugging
• If you find out that one or more of your tests yield incorrect results, then what?
Michael Eckmann - Skidmore College - CS 206 - Fall 2009
Testing & Debugging
• If you find out that one or more of your tests yield incorrect results, then what?
– Debug
• Maybe first read the code by eye and find out what is wrong.
• If that doesn't help, then could use a debugger that will allow you to run your program line by line or stop at certain places in your code and display values of variables, etc.
Michael Eckmann - Skidmore College - CS 206 - Fall 2009
Testing & Debugging
• Once you find the bug, fix it, then you should run all the tests again to make sure your fix didn't cause problems for other input that did work before.
Michael Eckmann - Skidmore College - CS 206 - Fall 2009
Java review - References• A reference variable (aka reference) is a variable that
stores the memory address at which an object lives.
• References are variables of class types and arrays. Differentiate references from variable of the eight primitive types.
• null is the value a reference has when it does not refer to an object.
Michael Eckmann - Skidmore College - CS 206 - Fall 2009
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?
Michael Eckmann - Skidmore College - CS 206 - Fall 2009
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 206 - Fall 2009
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)
System.out.println(“c2 and c3 are the same card”);
• When reference variables are compared for equality with ==, what happens?
Michael Eckmann - Skidmore College - CS 206 - Fall 2009
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 206 - Fall 2009
Java review - References• Not equal 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 not aliases
System.out.println(“c2 and c3 are not the same”);• When reference variables are compared with !=, what happens? If
both references refer to the SAME object, then it evaluates to false. That is, if the memory addresses are the same, (if c2 and c3 are aliases for the same object) then false is evaluated.
• So will the above if condition be false?
Michael Eckmann - Skidmore College - CS 206 - Fall 2009
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());
Michael Eckmann - Skidmore College - CS 206 - Fall 2009
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 206 - Fall 2009
Java review - ReferencesCard 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 206 - Fall 2009
Java “review”• Exception handling, try/catch blocks (import java.util.*;)
• File I/O (import java.io.*;)
• StringTokenizer
Michael Eckmann - Skidmore College - CS 206 - Fall 2009
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 206 - Fall 2009
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 206 - Fall 2009
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 206 - Fall 2009
File I/O• For file Input I recommend using a
BufferedReader wrapped around a FileReader
• For file Output I recommend using aPrintWriter wrapped around a FileOutputStream
• Let's look at example code.