1207

CS110 Dec 7 and Dec 9
Final Review Sessions

From the first day of class

Goals of course

understanding of what computers can / can't do.
programmer mindset -- helpful in understanding and using programs
programming
JAVA -- final -- will not ask fir Java specific knowledge. If needed, it will be given.

The Web

as you know it "invented" in @1991 by Tim Berners-Lee
URLs

parts of a URL
given a URL, what do you know?
https -- go to www.genunet.com

compare to allwhois.com for genunet.com

HTML

hypertext markup language
"markup" refers to having specific "markups" to control how text looks. E.g., <b>, <i>, <ol><li>, <table><tr><td><th>, ...
"hypertext" refers to providing links to other texts -- so linear reading of documents is not assumed.

Java

Applets allow easy distribution of code -- we have always worked with applets
Relatively simple -- still need to take my word for this
Object oriented -- not really used this in this class

idea is that find something related to what you want then change it to make that thing your own

we have always done this with "applet"
"extends" is the related thing that we are going to change.

Safe -- exception handling -- language does not let you do things like read off the end of an array.
Large libraries -- get awt and all the drawing stuff.

consider the difference between drawing a circle and drawing a cardioid

Declarations in Java:

class

public class Xxx extends Yyy implements Z1z1, Z2z2, ...
java convention, the name has initial cap

Methods -- must exist with a class

public void methodName(int param1, float param2, ...
if not void then need a return statement
java convention: name has initial lower case.

Class Variables

public int xx = 5;
live as long as the class instance
int, float, boolean, Color, Image, ...
java convention: name has initial lower case.

Method variables

like class variable but not "public" and only live as long as the method execution.
rewrite the class to do the method variable vs class variable counter thing.

Extending Classes

Several ways

Overriding methods:

most classes come with a set of methods (applet has about 270)

when you "extend" you get to use all of them.

You can also change any of them by "overwriting"

when overwriting you copy -- exactly -- the declaration line of the method (actually parameter names may change)

Overwriting methods is the principle way in which you change a method from its default behavior to the behavior you want.

Methods of Applet commonly overwritten

paint

init

Introducing new methods

do something else. Recall in the applet for drawing a cardioid the polar2cart method. Also, Many of you introduced methods for drawing your Dr. Who characters.

Introducing new class variables

Consider this program (that makes an applet):

public class T extends java.applet.Applet {
    int cv = 0;
    public void init() {
        cv = cv + 10000;
    }
    public void paint(java.awt.Graphics gg) {
        int mv = 0;
        java.awt.Font f = getFont();
        java.awt.Font ff = new java.awt.Font(f.getFontName(), java.awt.Font.BOLD, 40);
        gg.setFont(ff);
        gg.drawString("cv = " + cv, 10, 50);
        gg.drawString("mv = " + mv, 10, 100);
        cv = cv + 1;
        mv = mv + 1;
    }
}

what does it do?: All that this does is to count the number of times that paint has been called. The count is accumulated in cv. On each call of paint, mv is incremented from 0 to 1 but then this info is forgotten when the paint method ends.
What could be improved?: cv wuld be better named classVariable or perhaps even better, paintcounter. mv would be better named methodVariable.
Why does it not start with "import ...."?: The are not needed. By referring to java.applet.Applet in the class definition, you tell java exactly where to find Applet. "inports" do the same work.

Programming Arithmetic:

generally may combine any numeric types with an operator
resulting type will be the one that uses the most storage space
numeric types

int, float (also short, long, double, byte) although diff langs have different types
int+float ==> float
int/int ==> int!!! (does truncation of decimals) so 3/2==>1, 7/10==>0

Type coercion: (float)3/2==>1.5, (float)(3/2)==>1.

Allows programmer to have more control over results

Widgets and Events:

java has a large number of "widgets" (see java.awt package, also see javax.swing package)

common in many other PL (or have been added on to other PL)

Java supports a concept known as "event driven programming"

idea is that things cause events and you write programs to respond to those events
events are cuased by

most widgets (via user interactions)
the windowing system (e.g., window getting de-iconified causes an event which results in execution of paint

button, textfield -- action event
scrollbar -- adjustment event
window, textfield, ... -- keyboard event, mouse event,
window -- paint event

To handle events programs must be set up to listen to those events
applets handle paint events by default.
widgets must be told what things are set up to listen to the events they create

Booleans ...

booleans have two possible values: true of false
mostly useful in that they allow programs to do different things depending on boolean is true or false

in all of these the boolean expression is always encolsed in ().

constructs using booleans: if, while, for
The following are functionally identical:

if (5>4)
{ .....

int a=5;
int b=4;
if (a>b)
{ ....

int a=5;
int b=4;
boolean c=a>b
if (c==true)
{....

int a=5;
int b=4;
boolean c=a>b
if (c)
{....

Combining:

logical AND --- &&: true && true && false ==> false

stops evaluating at first false
if (a && b && c) { // do this .... is the same as if (a) { if (b) { if (c) { //do this

logical OR --- ||: true || true || false ==> true

stops evaluating at first true
if (a || b || c} { // do this is the same as: if (a) { // do this } if (b) { //do this } if (c) { // do this }
Note that this is not quite exactly the same as the "// do this" will be done three times in the expanded version

Interation

2 basic forms

while (condition) { }

for (int i=0; i<n; i++)

use for when you have an explicit number of times you want to do something. E.g., I want to catch 15 fish
use while when you have a good termination condition, but that condition is not obviously numbered e.g., while (alive) { wakeUp(); getOutOfBed(); ...

Arrays

Used to keep together a number of pieces of information of the same data type

e.g. the names of the beatles, the letters of the alphabet, the names of students in this class
not -- the names and g.p.a. of students

Initialization:

String[] aa = new String[4];

creates a thing that will hold 4 Strings

Refering

use numbers to refer to array elements and those number can be variables
int i=3; then aa[i] is the same as aa[3]

2-D arrays

String[][] aa = new String[4][3];