CS 206 Introduction to Computer Science II 09 / 09 / 2009 Instructor: Michael Eckmann

CS 206Introduction to Computer Science II

09 / 09 / 2009

Instructor: Michael Eckmann

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


Reading

• Chapter 1 – sections 1.1 & 1.3

• Appendix H


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.


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.


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.


JavaDoc

• Using JavaDoc to “self” document your code.

• /**

• Class description

• Method description

• @author

• @param

• @precondition

• @postcondition and/or @returns


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


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.


Testing & Debugging

• Any ideas on what makes good test data?

• How have you tested your programs in the past?


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?


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?


Testing & Debugging

• Another necessary requirement of testing your code is to “fully exercise the code.”

– what do you think that means?


Testing & Debugging

• If you find out that one or more of your tests yield incorrect results, then what?


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.


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.


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.


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


c1 = c3; // assign reference c3 to c1

• When a reference is assigned to another reference what happens?


Java review - References• Assignment operator =

• e.g.




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.


Java review - References• Equality comparison operator ==

• e.g.




if (c2 == c3)

System.out.println(“c2 and c3 are the same card”);

• When reference variables are compared for equality with ==, what happens?


Java review - References• Equality comparison operator ==

• e.g.




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?


Java review - References• Not equal comparison operator !=

• e.g.




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?


Java review - References• What if we do the following:

• e.g.


Card c2;

System.out.println(“The card is “ + c1.toString());

System.out.println(“Another card is “ + c2.toString());


Java review - References• What if we do the following:

• e.g.


Card c2;



• Problem with c2.toString() because c2 DOES NOT refer to an object. Its value is null.


Java review - ReferencesCard c1 = new Card(5,0); // Five of Diamonds

Card c2;



• 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


Java “review”• Exception handling, try/catch blocks (import java.util.*;)

• File I/O (import java.io.*;)

• StringTokenizer


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


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.


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.”


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.


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.

Documents

CS 206 Introduction to Computer Science II 09 / 09 / 2009 Instructor: Michael Eckmann