CS 206 Introduction to Computer Science II 09 / 10 / 2009 Instructor: Michael Eckmann

CS 206Introduction to Computer Science II

09 / 10 / 2009

Instructor: Michael Eckmann

Michael Eckmann - Skidmore College - CS 206 - Fall 2009

Today’s Topics• What this course will cover• More Java Review (References, FileIO, etc.)• Short lab assignment


Course Topics• This course has an emphasis on common data

structures and algorithms that work on them. After a short review of Java we learn a bit about how to analyze algorithms for efficiency. We then cover data structures such as linked lists, stacks, queues, priority queues, heaps, graphs, trees (binary and other), search trees, and hash tables. We also cover algorithms that work on those data structures such as breadth first search, depth first search, Dijkstra's algorithm, various sorting algorithms and tree balancing algorithms.


Homework Assignment• If you're new to eclipse, I have posted startup

instructions on how to run the IDE, create new programs and compile/run them within the IDE.

• The first programming assignment is now available on our assignments page.


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 - comparing• Comparing variables/data of primitive types vs.

comparing references of class types.

• ==, <, >, <=, >=, !=

• vs.

• .equals( ) and .compareTo( )

• What do these methods return? How do we use them?


Java review – method calls• some methods return nothing (are of type void)

• and some methods return something (of some particular type).

• for example:

• System.out.println("Hi");

• doesn't return anything, it just prints the text on the screen

• int i;

• i = Integer.parseInt("12");

• does return something, an integer. Therefore it makes sense to

"capture" what is being returned in a variable of type int.


Java review – method calls• It wouldn't make any sense to call that method on its own

line like so:

• Integer.parseInt("12");

• why? b/c all it does is convert that "12" to 12 and return it but nothing is "capturing" the return value (i.e. you are not using it in any way or assigning it to anything.)


Java review – writing our own classes

• instance variables vs. static class variables

• public vs. private

• constructor(s)

• set and get methods

• toString()


Writing our own classes• Create a class named TextBook with data:

– Title– Author (last name, first name)– Edition (should be numeric)

• Make the class implement the Comparable interface

- implement compareTo method with correct signature

• Implement equals method with the same signature as in Object

• Provide set and get methods for each instance variable.

- there should be no way for someone to set the edition to be < 1.

• Two textbooks should be equal if all instance variables are the

same.


Writing our own classes• The ordering of the textbooks is handled in compareTo --- that is,

compareTo holds the logic about what makes one textbook less

than another or what makes one greater than another and what

makes textbooks equal. The rules you are to follow are:

- Order the textbooks by author.

- If same author, then order by title.

- If same author and title, then order by edition.

• We will NOT make assumptions about how the TextBook class

will be used --- maybe it'll be used in an interactive program,

maybe it'll be used by a program that takes no user input or output.


Java review – ArrayList• The ArrayList class in Java API is used to store object

references in a dynamic (changing size) array type structure. You do not need to know how big your ArrayList will get, you can add references to it and it will change size.

• add(Object o) adds an object to the end of the list

• add(int idx, Object o) adds an object to the specific place (idx) in the list

• Etc. let's look at the Java API online for ArrayList and write some code using it and place TextBooks into it.


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 / 10 / 2009 Instructor: Michael Eckmann