cs303ethe first java program1 cs 303e lecture 3 a first java program l & o chapter 2 if you...

CS303E The First Java Program 1

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


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!!


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?


VariablesVariable

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.


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


Literals and Expressions Literal

A constant: e.g., 5, 9, 3.14159

Like arithmetic expressions in algebra, except use* for multiplication, / for division, andJava variable names may be long.

PMDAS: Precedence is: parentheses, inner most first multiplication and division, addition and subtraction.


AssignmentAssignment 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 / 9A = A * B;

A = AB;Semantics: Find the value of the expression and place the value

in the variable.


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;

More on Assignment Statements

myHappinessLevel

? 106


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);


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.


Window Object Dissected

IntegerField ageField = addIntegerField (21,3,2,1,1);

Initial value =Row = Column =Width =Height =


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

row

1 2 3 column


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


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!!


Messages and Methods in Action

FahrenheitToCentigradeObject

degreesFahrenheitFieldObject

degreesFahrenheitField.getNumber(); // sends message

Object Type = IntegerFieldObject Name = degreesFahrenheitField

Code for the method resides here. (how does an IntegerField getNumber?)


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.)


The Edit, Compile, Run Visual

EDIT

Compile

RUN

Program Design

MyProg.java

Java Libraries

MyProg.class

Program Input

Java Libraries

Program Output

Byte CodeProduced Here

Program is run by aJava Virtual Machine


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.


Syntax Error Forgetting a semicolon on int

fahrenheit

Sometimes not so helpful


Runtime Error Divide by zero is classic example. Change FahrenheitToCentigrade and add a

variable for the divisor.


Semantic Errors The hardest to find and fix

Right after pushing convert.– Not so much!


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)


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); } }


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.


CS 303ELecture 4

Java Basics 1Chapter 3

Once a person has understood the way variables are used in programming,(s)he has understood the quintessence of programming.

-Dijkstra


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?


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


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?


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


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); ????


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.


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.


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


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?


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;


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.

TAX_RATE

0.20 TAX_RATE = 12; causes syntax error


Binary RepresenationBits , 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.


Variable Names Reserved words -- can’t use these for variable names.

Java already has a meaning for them.• Not that many of them.

abstract default if private throwboolean do implements protected throwsbreak double import public transientbyte else instanceof return trycase extends int short voidcatch final interface static volatilechar finally long super whileclass float native switchconst for new synchronized

continue goto packagethis


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


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


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 ta

studentNumber versus theNumberAssignedToThisStudent


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


Unary plus and minus

Expression Value Better

Unary minus:–3 * –4 12 (–3) * (–4)3 * –4 –12 3 * (–4)

3 * – –4 12 3 * (– (–4))Unary plus:

3 * +4 12 3 * (+4)


Increment and DecrementOperator Equivalent Called

x ++; x = x + 1; postfix increment++ x; x = x + 1; prefix incrementx – –; x = x – 1; postfix decrement– – x; x = x – 1; 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


Extended Assignment Operators

Operator Equivalentsum += x; sum = sum + x; diff –= x; diff = diff – x;

product *= x; product = product * x;quot /= x; quot = quot / x;

rem %= x; rem = rem % x;

Confusing:int a = 1, b = 2;a += b += 3; b = 5; a = 6;


Expressions PracticeValue 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;


Data Types Revisted

Numeric types:

int, double, and others

less inclusive more inclusive

byte short int long float double

integers floating point


Numeric Types (p. 124)Type Storage Rangebyte 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 1.79769313486231570x10+308 to 4.94065645841246544x10-324


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


CS 303E

Lecture 5

Java Basics 2

Chapter 3

"In a way, staring into a computer screen is likestaring into an eclipse. It's brilliant and you don't realize the damage until its too late."- Bruce Sterling


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>);


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);


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.


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.


Strings

String variables:String status;

String customer = “Bill”;

String firstName, lastName;Several objects in one declaration,like any other type.

String given aninitial value.

String with noinitial value.


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”;


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?


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.


Message Box


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.


Text FieldsTextField -- 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 Stringname.setText (aString) -- sets field content.


Case 3.3 - Vital Statisticsimport 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?


Window Layout


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); } }


System.out.printlnSystem.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.


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


Assignment 2

Handed out today


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); // Displays 5.0i = d; // Syntax errorSystem.out.println (i);

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