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CS 303E
Lecture 3
A First Java Program
L & O Chapter 2
If you can't write it down in English,
you can't code it.
-Peter Halpern, Brooklyn, New York
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The First Java Program
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Syntax and Semantics
Syntax:
Form or grammar.
Will give syntax rules for the language.
THIS PART IS FRUSTRATING!!
Semantics:
Meaning.
What a particular element of the language causes to
happen.
THIS PART IS CHALLENGING!!
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English is Easier than Java
Git over here and tale mee where libary is at.
• The meaning can still be determined if the syntax is
not perfect. Not so in programming languages.
Kitty get over here and fast.
• The meaning of this statement?
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Variables
Variable
A place in memory that holds a value that may be
altered
Declared to have a name and type:
e.g., int fahrenheit;
Declare variable for age and another for number of
credit hours
Name variables to minimize confusion about the
semantics of the statement.
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Declaring a Variable
int numDaysLeftInSemester;
What is the variable’s type?
What is the variable’s name?
In memory 4 bytes used for an int.
?
numDaysLeftInSemester
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Literals and Expressions
Literal
A constant: e.g., 5, 9, 3.14159
Like arithmetic expressions in algebra, except use
* for multiplication, / for division, and
Java variable names may be long.
PMDAS: Precedence is:
parentheses, inner most first
multiplication and division,
addition and subtraction.
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Assignment
Assignment Statement:
Syntax: <variable> = <expression>;
<variable> and <expression> are to be replaced by the
name of a variable and an expression.
Examples:
X = X + 5; // Mathematicians leaping to their feet in protest
centigrade = (fahrenheit - 32) * 5 / 9
A = A * B;
A = AB;
Semantics: Find the value of the expression and place the
value in the variable.
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More on Assignment
Statements
First source of problems
Can’t put square peg in round hole
Can’t get in the club without the right ID
int myHappinessLevel;
myHappinessLevel = 106;
//Large assignment given
?
myHappinessLevel = 0.05;
106
myHappinessLevel
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More on Variable Names
Variable names should be long enough to aid in
the understanding of the meaning or semantics
of the program, but no longer.
x = y - (y * z);
netPay = grossPay - (grossPay * taxRate);
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Window Objects
<type> <name> = <addType> (<initial value>,
<row#>, <column#>, <width>, <height>);
Label degreesFahrenheitLabel =
addLabel ("Degrees Fahrenheit",1,1,1,1);
IntegerField degreesFahrenheitField =
addIntegerField (0,1,2,1,1);
Label degreesCentigradeLabel =
addLabel ("Degrees Centigrade",2,1,1,1);
IntegerField degreesCentigradeField =
addIntegerField (0,2,2,1,1);
Button convertButton =
addButton ("Convert",3,1,2,1);
WINDOW OBJECTS ARE SIMILAR TO VARIABLES.
THEY TAKE UP SPACE IN MEMORY.
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Window Object Dissected
IntegerField ageField =
addIntegerField (21,3,2,1,1);
Initial value =
Row =
Column =
Width =
Height =
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Where in the Window?
IntegerField ageField = addIntegerField (21, 3, 2, 1, 1);
IntegerField x = addIntegerField(0, 1, 1, 1, 1);
IntegerField y = addIntegerField(12, 2, 3, 1, 1);
1
2
3
column
1
2
3
row
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Methods
Method calls: (What is the Window Object?)
fahrenheit = degreesFahrenheitField.getNumber();
degreesCentigradeField.setNumber (centigrade);
Method definition:
public void buttonClicked (Button buttonObj)
{ fahrenheit = degreesFahrenheitField.getNumber();
centigrade = (fahrenheit - 32) * 5 / 9;
degreesCentigradeField.setNumber (centigrade);
}
Messages: another terminology. When a method is called
or invoked it sends a message to the object
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Methods in Action
•public class FahrenheitToCentigrade
•The main program is called
FahrenheitToCentigrade
• When the program is run a
FahrenheitToCentigrade object is created. This
object contains numerous Windows objects.
• How do the they communicate?
•Methods and messages!!
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Messages and Methods in
Action
FahrenheitToCentigrade
Object
degreesFahrenheitField.getNumber(); // sends message
degreesFahrenheitField
Object
Code for the method resides
here. (how does an
IntegerField getNumber?)
Object Type = IntegerField
Object Name = degreesFahrenheitField
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Edit, Compile, Run
Steps in creating and running a program:
Edit -- write the program with an editor.
Compile -- translate into byte code.
Run -- JVM executes the byte code.
If there are errors, repeat. You will never have an
error. (Gales of laughter now follow.)
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The Edit, Compile, Run Visual
Program Design
EDIT
Java Libraries
MyProg.java
Compile
Byte Code
Produced Here
Java Libraries
MyProg.class
Program Input
Program is run by a
Java Virtual Machine
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RUN
Program Output
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Errors
Syntax errors -- caught by the compiler.
Run-time errors -- caught when the program is
run.
Semantic errors -- caught by the programmer.
Program runs but doesn’t do what you wanted.
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Syntax Error
Forgetting a semicolon on
int fahrenheit
Sometimes not so helpful
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Runtime Error
Divide by zero is classic example.
Change FahrenheitToCentigrade and add a
variable for the divisor.
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Semantic Errors
The hardest to find and fix
Right after pushing convert.
– Not so much!
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FahrenheitToCentigrade Program, text p. 15
import java.awt.*;
import BreezyGUI.*;
public class FahrenheitToCentigrade extends GBFrame
{ Label degreesFahrenheitLabel =
addLabel ("Degrees Fahrenheit",1,1,1,1);
IntegerField degreesFahrenheitField =
addIntegerField (0,1,2,1,1);
Label degreesCentigradeLabel =
addLabel ("Degrees Centigrade",2,1,1,1);
IntegerField degreesCentigradeField =
addIntegerField (0,2,2,1,1);
Button convertButton = addButton
("Convert",3,1,2,1);
(continued on next slide)
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FahrenheitToCentigrade Program (continued)
int fahrenheit;
int centigrade;
public void buttonClicked (Button buttonObj)
{ fahrenheit = degreesFahrenheitField.getNumber();
centigrade = (fahrenheit - 32) * 5 / 9;
degreesCentigradeField.setNumber (centigrade);
}
public static void main (String[] args)
{ Frame frm = new FahrenheitToCentigrade();
frm.setSize (200, 150);
frm.setVisible (true);
}
}
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Window Design
[See Fig. 2.3, text p. 20.]
Remember:
<type> <name> = <addType> (<initial value>,
<row#>, <column#>, <width>, <height>);
So far we have:
• Label
• IntegerField
• Button
Many more to come.
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CS 303E
Lecture 4
Java Basics 1
Chapter 3
Once a person has understood the way variables
are used in programming,
(s)he has understood the quintessence of
programming.
-Dijkstra
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Case 3.1 -- Area of Circle
Makes simple transformations from program
FahrenheitToCentigrade.
• If you can reuse your code in this class, do it!
Type double numbers, DoubleField boxes.
• Extraordinarily similar to what?
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Request and Analysis
Request
• write a program that computes the area of a circle.
Design
• Area of circle = r2
• get radius from user
• compute and display area
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Area of Circle Design
get radius
area = 3.14 * radius * radius
display area with 2 digits after the decimal
point
• Again, input - process - output.
• concerns?
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Elements of the GUI
Label radiusLabel = addLabel ("Radius",1,1,1,1);
DoubleField radiusField
= addDoubleField (0,1,2,1,1);
Label areaLabel = addLabel ("Area",2,1,1,1);
DoubleField areaField = addDoubleField (0,2,2,1,1);
Button computeButton
= addButton ("Compute",3,1,2,1);
Remember: initial value, row, column, width, height
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The Guts of Button Clicked
public void buttonClicked (Button buttonObj){
radius = radiusField.getNumber();
area = 3.14 * radius * radius;
areaField.setNumber (area);
areaField.setPrecision(2);
}
What is areaField.setPrecision(2); ????
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Case 3.2 -- Income Tax
More simple transformations from program
FahrenheitToCentigrade.
Analysis a little more in depth.
• flat tax rate of 20%, $10,000 standard deduction, $2,000
deduction for each dependent,
• Must know something about figuring taxes to do this or must
ask requestor to clarify.
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Income Tax Program
Type int and double variables, IntegerField and
DoubleField boxes.
An additionl box for input. (2 inputs)
Get both inputs for computation.
Yet again
• input - process - output
Look at program in JBuilder.
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Income Tax Program Variables
int grossIncome;
int numberOfDependents;
double taxableIncome;
double incomeTax;
Double is a data type. Floating point numbers:
3.14, 12.232301, 0.000001
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Income Tax Button Clicked
public void buttonClicked (Button buttonObj){
grossIncome = grossIncomeField.getNumber();
numberOfDependents = numberOfDependentsField.getNumber();
taxableIncome =
grossIncome - 10000 - numberOfDependents * 2000;
incomeTax = taxableIncome * 0.20;
incomeTaxField.setNumber (incomeTax);
incomeTaxField.setPrecision (0);
}
Is the taxable income variable necessary?
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Constants
What do the numbers 10000, 2000, and 0.20
represent in the income tax program?
These numbers should be declared as constants.
final int STANDARD_DEDUCTION = 10000;
final int DEPENDENT_DEDUCTION = 2000;
final double TAX_RATE = 0.20;
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Constants
Constants share some similarities with
variables.
• they have a data type
• they take up space in memory
The major differences is constants cannot be
changed in the program.
0.20
TAX_RATE = 12;
causes syntax error
TAX_RATE
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Binary Represenation
Bits , bytes.
Integers, floating point numbers, characters, Strings and
other data, and instructions
All are represented in binary using bits, short for
binary digit, 1s and 0s
Memory -- an array of bytes.
Self Test
How much memory to store a picture 20 by 30 pixels,
each pixel can be 1 of 256 possible colors.
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Variable Names
Reserved words -- can’t use these for variable names.
Java already has a meaning for them.
• Not that many of them.
abstract
boolean
break
byte else
case extends
catch
char finally
class float
const
default if
private
do
implements
double import
public
instanceof
return
int
short
final
interface
long
super
native
switch
for
new
continue
goto
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throw
protected
throws
transient
try
void
static
volatile
while
synchronized
packagethis
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Varaiable Names Continued
Identifier -- a name. Composed of letters, digits, ‘_’ and
‘$’ and beginning with a letter or ‘_’.
Don’t use ‘$’.
Upper- and lower-case letters are considered different.
“Case sensitive.”
Common practice to capitalize the first letter of each
word in a variable name except the first
Constant names all caps with ‘_’ between words
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Valid or not?
just_in_time
c1212
int
class num
my_happiness_factor TANJ
x
battingAverage
classTime
X
5bestTimes
gross%
total+Income totalGold
_total_time
TotalIncome
public
tax&
variable
1time
money$
5
aVariableToHoldMoneySpentSoFar
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Variable Names Continued
 "Make things as simple as possible--but no simpler.” - A.
Einstein
 Meaningful variable names make a program easier to
understand and maintain. One of the major principles of this
course.
 incomeTax versus
it
 totalArea
versus
 studentNumber
versus
theNumberAssignedToThisStudent
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Expressions
Operator precedence -- see p. 45.
Unary ‘-’ is precedence 2 (omitted).
x = -3 + -4
% means modulo
•
•
•
•
The remainder from integer division.
Go back to second grade, 21  5 = 4 r 1
21 / 5 = 4
21 % 5 = 1
Integer vs. floating point ‘/’
• integer division truncates the result
• floating point gives a floating point answer
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Unary plus and minus
Expression
Unary minus:
–3 * –4
3 * –4
3 * – –4
Unary plus:
3 * +4
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Value
Better
12
–12
12
(–3) * (–4)
3 * (–4)
3 * (– (–4))
12
3 * (+4)
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Increment and Decrement
Operator
x ++;
++ x;
x – –;
– – x;
Equivalent
x = x + 1;
x = x + 1;
x = x – 1;
x = x – 1;
Called
postfix increment
prefix increment
postfix decrement
prefix decrement
Too confusing, DON’T DO IT!
z = 2 * x++;  z = 2 * x; x++;
z = 2 * ++x;  x++; z = 2 * x;
 Write code that is easy to understand and maintain
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Extended Assignment
Operators
Operator
sum += x;
diff –= x;
product *= x;
quot /= x;
rem %= x;
Confusing:
int a = 1, b = 2;
a += b += 3;
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Equivalent
sum = sum + x;
diff = diff – x;
product = product * x;
quot = quot / x;
rem = rem % x;
b = 5; a = 6;
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Expressions Practice
Value of variables after each statement?
int x;
int y;
int z = 12; //yes, this is allowed and encouraged!
final int MAX_UNITS = 15;
double a = 2.5;
x = z + 2 * 5;
y = 5;
y = y + MAX_UNITS;
z = y % 7 + x * 2;
MAX_UNITS = z;
x = 15 / 4 - 2;
y = 15 / (4 - 2);
x = a;
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Data Types Revisted
Numeric types:
int, double, and others
integers
floating point
byte short int long float double
less inclusive more inclusive
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Numeric Types (p. 124)
Type
Storage
Range
byte
1 byte
–128 to 127
short
2 bytes
–32,768 to 32,767
int
4 bytes
–2,147,483,648 to 2,147,483,647
long
8 bytes
–9,223,372,036,854,775,808 to
9,223,372,036,854,775,807
float
4 bytes
3.40282347x10+38 to
 1.40239846x10-45
double
8 bytes
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 1.79769313486231570x10+308 to
 4.94065645841246544x10-324
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Mixed-Mode Operations
Java converts the value of
a less inclusive type (like int) to
a more inclusive type (like double)
before performing an operation:
int i = 5;
double d = 3.5;
System.out.println (d + i);
// Displays 8.5
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CS 303E
Lecture 5
Java Basics 2
Chapter 3
"In a way, staring into a computer screen is like
staring into an eclipse. It's brilliant and you don't
realize the damage until its too late."
- Bruce Sterling
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BreezyGUI Window Objects
Pages 48 - 51 in Lambert and Osborne.
Window objects must have:
• type, name, initial value, location, extent
types so far are:
• Label, IntegerField, Button, DoubleField
<type> <name> = <addType> (<initial value>,
<row#>, <column#>, <width>, <height>);
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BreezyGUI Methods
These calls require window_object.method_name:
fahrenheit = degreeFahrenheitField.getNumber();
Some return a value (e.g., getNumber),
some don’t (e.g. setNumber).
Some require a parameter or argument (besides the
window object), which goes in the parentheses.
someField.setNumber (number);
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BreezyGUI Methods
Some methods require multiple parameters
• someObject.someMethod(param1, param2, …
paramn);
Multiple parameters must have the right number,
order, and type.
• Wrong number or type leads to syntax error
• Wrong order may lead to syntax or semantic error
List of BreezyGUI methods on page 51.
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Strings
String is a type, similar to int and double.
String is an object type like BreezyGUI window
objects, int and double are primitives.
String literals -- use double quotes:
“Hi there! My name is Raoul.” -- any
characters
“” -- the empty string.
“453” is a string, 453 is an int.
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Strings
String variables:
String status;
String with no
initial value.
String customer = “Bill”;
String firstName, lastName;
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String given an
initial value.
Several objects in
one declaration,
like any other type.
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String Concatenation
+ concatenates Strings:
String firstName = “Maria”;
String lastName = “Ruiz”;
String name = firstName + “ ” + lastName;
// name = “Maria Ruiz”
\n is the newline character (one character):
String name = “Jose Garcia\n”;
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Number + String Converts
int a = 5, b = 6;
String result = a + " plus " + b + " equals " + (a + b);
// result = “5 plus 6 equals 11”
Conversion of a number to a String occurs with
number + String or String + number.
number + number still does addition.
(a + b) above is needed to make this addition, not
String concatenation. What happens if no parenthesis?
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Message Boxes
BreezyGUI provides message boxes:
messageBox (“Hey there!”);
Pops up a separate window called a message box
containing the text provided as an argument.
String line1 = “Top line.\n”;
String line2 = “Bottom line.”;
messageBox (line1 + line2);
Message box contains 2 lines.
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Message Box
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Message Box Behavior
Message Boxes are modal.
Nothing can be done in the original window
until the message box is acknowledged by
closing it  clicking ‘x’ in upper right corner.
Too many message boxes make for a very
annoying program. Use sparingly.
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Text Fields
TextField -- BreezyGUI window component type
containing one line of String data.
Declared like other window components:
TextField <name> = addTextField (<initial value>,
<row#>, <col#>, <width>, <height>);
Methods:
name.getText() -- returns String
name.setText (aString) -- sets field content.
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Case 3.3 - Vital Statistics
import java.awt.*;
import BreezyGUI.*;
public class VitalStatistics extends GBFrame{
Label nameLabel = addLabel ("Name",1,1,1,1);
TextField nameField = addTextField ("",1,2,1,1);
Label heightLabel = addLabel ("Height",2,1,1,1);
IntegerField heightField =
addIntegerField (0,2,2,1,1);
Label weightLabel = addLabel ("Weight",3,1,1,1);
IntegerField weightField =
addIntegerField (0,3,2,1,1);
Button displayButton =
addButton ("Display",4,1,2,1);
// What will window look like?
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Window Layout
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String name, outputString;
int height, weight;
public void buttonClicked (Button buttonObj){
name = nameField.getText();
height = heightField.getNumber();
weight = weightField.getNumber();
outputString = "Name:
" + name + "\n" +
"Height: " + height + " inches\n" +
"Weight: " + weight + " pounds\n";
messageBox (outputString);
}
// how many lines of text in message box?
public static void main(String[] args){
Frame frm = new VitalStatistics();
frm.setSize (200, 150);
frm.setVisible(true);
}
}
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System.out.println
System.out.println (expression); // with newline
System.out.print (expression); // without newline
Prints the value of expression to the DOS window.
Expression is converted to a String if it is not one.
Useful for debugging.
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Debugging in JBuilder and
Codewarrior
System.out.println statements are very useful
May also step through program
• F8 Step Over
• F7 Trace Into
Setting Breakpoints can halt execution at a
statement in the program.
Contents of variables and objects may be
examined
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Assignment 2
Handed out today
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Mixed-Mode Assignment
Java converts the value of a less inclusive type
to a more inclusive type before performing an
assignment, but not vice versa:
int i = 5;
double d = 3.5;
d = i;
System.out.println (d);
i = d;
System.out.println (i);
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// Displays 5.0
// Syntax error
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