2003 prentice hall, inc. all rights reserved. 1 lecture 3 variables primitive data types...

2003 Prentice Hall, Inc. All rights reserved.

1

Lecture 3

VariablesPrimitive Data Types

calculations&

arithmetic operatorsReadings: Chapter 2


2

Review

• What’s wrong with this line of code?system.out.println ("He said "Hello"");

• What’s wrong with this program?public class Welcome1

{

// main method begins execution of Java application

public static void main( String args[] )

System.out.println( "Welcome to Java" )

} // end method main

} // end class Welcome1


3

VariablesPrimitive Data Types and mathematical Operations

• Introduce Programming with an Example• Identifiers, Variables, and Constants• Primitive Data Types

– byte, short, int, long, float, double, char, Boolean

• Expressions• Mathematical Operators• Syntax Errors, Runtime Errors, and Logic Errors


4

Lets review

• Key words• Identifiers• Style• Errors


5

Reserved words:

Certain words have special meaning in Java and cannot be used as identifiers. These words are called reserved words.

So far we have seen the following reserved words:

int

public

import

static

void

class

We will see a complete list of reserved words soon.

Use of the words null, true and false is also prohibited.


6

Lets review Identifiers

• An identifier is a sequence of characters that consist of letters, digits, underscores (_), and dollar signs ($).

• An identifier must start with:– a letter– an underscore (_)– or a dollar sign ($).

• It cannot start with a digit.

• An identifier cannot be a reserved word. – (See Appendix A, “Java Keywords,” for a list of reserved words).

• An identifier cannot be true, false, or null.

• An identifier can be of any length.


7

Programming Style and Documentation

• Appropriate Comments

• Naming Conventions

• Proper Indentation and Spacing Lines

• Block Styles


8

Appropriate Comments

Include a comment at the beginning of the program:– to explain what the program does– its key features– its supporting data structures– and any unique techniques it uses.

• Include your name• class section• instruction• date• and a brief description at the beginning of the program.


9

Proper Indentation and Spacing

• Indentation– Indent 3 spaces.

• Spacing – Use blank line to separate segments of the code.


10

Block Styles

Use end-of-line style for braces.

public class Test { public static void main(String[] args) { System.out.println("Block Styles"); } }

public class Test { public static void main(String[] args) {

System.out.println("Block Styles"); } }

End-of-line style

Next-line style


11

Programming Errors

• Syntax Errors– Detected by the compiler

• Runtime Errors– Causes the program to abort (when you divide a number by

0)

• Logic Errors– Produces incorrect result


12

Syntax Errors

• Caused when the compiler cannot recognize a statement.

• These are violations of the language

• The compiler normally issues an error message to help the programmer locate and fix it

• Also called compile errors or compile-time errors.

• For example, if you forget a semi-colon, you will get a syntax error.

public class SyntaxErrors { public static void main(String[]

args) { i = 30; System.out.println(i + 4) }}


13

Run-time Errors

• Other major kind of error you’ll see

• Happens when a program is running

• The compiler cannot identify these errors at compile time.

• Will talk more about these later

public class RuntimeErrors {Public static void

main(String[] args) { int i = 1 / 0; }}


14

Logic Errors• Logic Errors

– Produces incorrect result

public class LogicalErrors {Public static void main(String[] args) { double average = 80.0 + 90.0 / 2; }}This program produces in correct answer of

90.0/2 + 80.0 = 45 + 80.0 = 125.0

Answer should be (80.0 + 90.0)/2 170.0/2 = 85.0


15Naming Conventionschoosing identifiers for variables and

classes names• Choose meaningful and descriptive names.• For Variables, class names and methods:

– Use lowercase.

– If the name consists of several words, concatenate all in one, use lowercase for the first word, and capitalize the first letter of each subsequent word in the name.

– For example, the variables radius and area, and the method computeArea.


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Naming Conventions, cont.

• Class names: – Capitalize the first letter of each word in the

name.

– For example, the class name ComputeArea.

• Constants: – Capitalize all letters in constants.

– For example, the constant PI.


17

Variables


18

Memory Concepts

• Variable names correspond to locations in the computer’s primary memory.

• Every variable has:– a name

– a type

– and a value.

• When a value is placed in a memory location the value replaces the previous value in that location (called destructive read-in)

• A variable’s value can just be used and not destroyed (called non-destructive read-out)


19

Bucket Analogy

• It is useful to think of a variable as a bucket of data.

• The bucket has a unique name, and can only hold certain kinds of data.

balance

200

balance is a variablecontaining the value200, and can containonly integers.

Note: variables are not persistent. When you exit your program, the data is deleted. To create persistent data, you must store it to a file system.


20

Memory Concepts

• Visual Representation

sum = 0;

number1 = 3;

number2 = 2;

Sum = number1 + number2; What is the value of Sum after execution of the above statement?

sum 0

Number1 3

sum ?

number2 2


21

Variable Declaration

• Before you use a variable, you must declare it. (Not all languages require this, but Java certainly does.)

• Examples:

/* Creates an integer variable */

int number;

/* Creates a double variable */

double price;

/* Creates a string variable */

Sting name;

/* Creates a character variable */

char letter;

Data type identifier

Semi-colon


22

Declaring Variables

int x; // Declare x to be an // integer variable;

double radius; // Declare radius to // be a double variable;

char a; // Declare a to be a // character variable;String name; // String variable “Bano”


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Important Point about Declarations

• In Java you can declare variables at many different places in the program. They have different meaning and scope depending on where they are declared. For now, all our variables shall be declared at the beginning of a block of code within main().

public class Sample {public static void main(String args[])

{ declare variables here

{or inside a nested block

} } // end method main

} // end class Sample


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Choose meaningful variable names to make your program more readable. For example, use income, instead of num.

Stick to lower-case variable names. For example, use income, instead of INCOME. Variables that are all capitals usually indicate a constant (more on this soon.)

Use proper case for all words after the first in a variable name. For example, instead of totalcommissions, use totalCommissions.

Avoid redefining identifiers previously defined in the Java API.

Good Programming Practices


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• DATA TYPE• integers:

– the simplest data type in Java. – Used to hold positive and negative whole numbers, e.g. 5, 25, -

777, 1.

• floats: – Used to hold fractional or decimal values, e.g. 3.14, 10.25.

• chars: Used to hold individual characters, e.g. ‘c’, ‘e’, ‘1’, ‘\n’

• We will explore each one in detail later this semester.• String: holds one or more character, e.g. “Dream

Girls”, “NYU ROCKS!”, “C”


26

Declaring and Initializingin One Step

• int x = 1;

• double d = 1.4;

• float f = 1.4;

Is this statement correct?


27

Numerical Data Types

Whole numbers:• byte 8 bits

• short 16 bits

• int 32 bits

• long 64 bits

Floating point or Decimal numbers:

• float 32 bits

• double 64 bits


28

Assignment Statements

x = 1; // Assign 1 to x;

radius = 1.0; // Assign 1.0 to radius;

a = 'A'; // Assign 'A' to a;


29

Arithmetic

Operator(s) Operation(s) Order of evaluation (precedence) * /

%

Multiplication Division Remainder

Evaluated first. If there are several of this type of operator, they are evaluated from left to right.

+ -

Addition Subtraction

Evaluated next. If there are several of this type of operator, they are evaluated from left to right.

Fig. 2.17 Precedence of arithmetic operators.


30

Arithmetic

• Arithmetic calculations used in most programs– Usage

• * for multiplication • / for division• +, -• No operator for exponentiation

• Integer division truncates remainder7 / 5 evaluates to 1

5 / 2 yields an integer 2.

5.0 / 2 yields a double value 2.5

5 % 2 yields 1 (the remainder of the division)– Remainder operator % returns the remainder

7 % 5 evaluates to 2


31

Arithmetic

• Operator precedence – Some arithmetic operators act before others (i.e.,

multiplication before addition)• Use parenthesis when needed

– Example: Find the average of three variables a, b and c• Do not use: a + b + c / 3

• Use: ( a + b + c ) / 3

– Follows PEMDAS • Parentheses, Exponents, Multiplication, Division, Addition,

Subtraction


32

Important Point about Declarations

• You must make a declaration immediately following the left brace following the main method.– at the beginning of a function and before any executable

statements or else you get a syntax error.{declaration section…statement section…

}


33

Example 1: Basic Arithmetic

/* Illustrates Integer Variables */public class addition {

public static void main(String[] args) { int x, y, z;

// Specify values of x and y x = 2; y = 3; // add x and y and place the result in z variablez = x + y;System.out.println("x has a value of " + x); System.out.println("y has a value of " + y); System.out.println("The sum of x + y is " + z); System.exit(0); }

}

Variable Declaration

Data TypeVariable Names



34

Printing Variables

• To print a variable, use the System.out.print or System.out.println statement as you would for a string.

•

• System.out.print (x);• System.out.println (x);• System.out.println ("x: " + x);

• Here the “addition” that is performed is string concatenation.


35


• Assignments statements enable one to initialize variables or perform basic arithmetic.

x = 2;y = 3;z = x + y;

• Here, we simply initialize x and y and store their sum within the variable z.


36

Assignment Operator

• =• Read the assignment operator as “GETS” not

“EQUALS!”• This is an assignment of what’s on the right side

of = to a variable on the left• eg sum = integer1 + integer2;

– Read this as, “sum gets integer1 + integer2”

– integer1 and integer2 are added together and stored in sum


37

Displaying Text in a Dialog Box

• Display– Most Java applications use windows or a dialog box

• We have used command window

– Class JOptionPane allows us to use dialog boxes

• Packages– Set of predefined classes for us to use

– Groups of related classes called packages• Group of all packages known as Java class library or Java

applications programming interface (Java API)

– JOptionPane is in the javax.swing package• Package has classes for using Graphical User Interfaces

(GUIs)


38


• Upcoming program– Application that uses dialog boxes

– Explanation will come afterwards

– Demonstrate another way to display output

– Packages, methods and GUI

2003 Prentice Hall, Inc.All rights reserved.

Outline39

Welcome4.java

1. import declaration

2. Class Welcome4

2.1 main

2.2 showMessageDialog

2.3 System.exit

Program Output

1 // Fig. 2.6: Welcome4.java

2 // Printing multiple lines in a dialog box

3 import javax.swing.JOptionPane; // import class JOptionPane

4

5 public class Welcome4 {

6 public static void main( String args] )

7 {

8 JOptionPane.showMessageDialog(

9 null, "Welcome\nto\nJava\nProgramming!" );

10

11 System.exit( 0 ); // terminate the program

12 }

1 // Welcome4.java2 // Printing multiple lines in a dialog box.3 4 // Java packages 5 import javax.swing.JOptionPane; // program uses JOptionPane6 7 public class Welcome4 {8 9 // main method begins execution of Java application10 public static void main( String args[] )11 {12 JOptionPane.showMessageDialog( 13 null, "Welcome\nto\nJava\nProgramming!" );14 15 System.exit( 0 ); // terminate application with window16 17 } // end method main18 19 } // end class Welcome4


40


– Lines 1-2: comments as before

– Two groups of packages in Java API– Core packages

• Begin with java• Included with Java 2 Software Development Kit

– Extension packages• Begin with javax• New Java packages

– import declarations • Used by compiler to identify and locate classes used in Java

programs• Tells compiler to load class JOptionPane from javax.swing package

4 // Java packages

5 import javax.swing.JOptionPane; // program uses OptionPane


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2.4 Displaying Text in a Dialog Box

– Lines 6-11: Blank line, begin class Welcome4 and main

– Call method showMessageDialog of class JOptionPane

• Requires two arguments

• Multiple arguments separated by commas (,)

• For now, first argument always null• Second argument is string to display

– showMessageDialog is a static method of class JOptionPane• static methods called using class name, dot (.) then

method name

12 JOptionPane.showMessageDialog( 13 null, "Welcome\nto\nJava\nProgramming!" );


42

2.4 Displaying Text in a Dialog Box

– All statements end with ; • A single statement can span multiple lines

• Cannot split statement in middle of identifier or string

– Executing lines 12 and 13 displays the dialog box

• Automatically includes an OK button

– Hides or dismisses dialog box

• Title bar has string Message


43


– Calls static method exit of class System• Terminates application

– Use with any application displaying a GUI

• Because method is static, needs class name and dot (.)

• Identifiers starting with capital letters usually class names

– Argument of 0 means application ended successfully• Non-zero usually means an error occurred

– Class System part of package java.lang• No import declaration needed

• java.lang automatically imported in every Java program

– Lines 17-19: Braces to end Welcome4 and main

15 System.exit( 0 ); // terminate application with window


44

Another Java Application: Adding Integers

• Upcoming program– Use input dialogs to input two values from user

– Use message dialog to display sum of the two values


Outline45

Addition.java

1. import

2. class Addition

2.1 Declare variables (name and type)

3. showInputDialog

4. parseInt

5. Add numbers, put result in sum

1 // Addition.java2 // Addition program that displays the sum of two numbers.3 4 // Java packages 5 import javax.swing.JOptionPane; // program uses JOptionPane6 7 public class Addition {8 9 // main method begins execution of Java application10 public static void main( String args[] )11 {12 String firstNumber; // first string entered by user 13 String secondNumber; // second string entered by user14 15 int number1; // first number to add 16 int number2; // second number to add 17 int sum; // sum of number1 and number218 19 // read in first number from user as a String 20 firstNumber = JOptionPane.showInputDialog( "Enter first integer" );21 22 // read in second number from user as a String 23 secondNumber = 24 JOptionPane.showInputDialog( "Enter second integer" );25 26 // convert numbers from type String to type int27 number1 = Integer.parseInt( firstNumber ); 28 number2 = Integer.parseInt( secondNumber ); 29 30 // add numbers 31 sum = number1 + number2;32

Declare variables: name and type.

Input first integer as a String, assign to firstNumber.

Add, place result in sum.

Convert strings to integers.


Outline46

Program output

33 // display result 34 JOptionPane.showMessageDialog( null, "The sum is " + sum,35 "Results", JOptionPane.PLAIN_MESSAGE ); 36 37 System.exit( 0 ); // terminate application with window38 39 } // end method main40 41 } // end class Addition


47

2.5 Another Java Application: Adding Integers

– Location of JOptionPane for use in the program

– Begins public class Addition• Recall that file name must be Addition.java

– Lines 10-11: main

– Declaration• firstNumber and secondNumber are variables

5 import javax.swing.JOptionPane; // program uses JOptionPane

7 public class Addition {

12 String firstNumber; // first string entered by user 13 String secondNumber; // second string entered by user


48


– Variables• Location in memory that stores a value

– Declare with name and type before use

• firstNumber and secondNumber are of type String (package java.lang)

– Hold strings

• Variable name: any valid identifier

• Declarations end with semicolons ;

– Can declare multiple variables of the same type at a time

– Use comma separated list

– Can add comments to describe purpose of variables

String firstNumber, secondNumber;

12 String firstNumber; // first string entered by user 13 String secondNumber; // second string entered by user


49


– Declares variables number1, number2, and sum of type int• int holds integer values (whole numbers): i.e., 0, -4, 97• Types float and double can hold decimal numbers

• Type char can hold a single character: i.e., x, $, \n, 7

• Primitive types - more in Chapter 4

15 int number1; // first number to add 16 int number2; // second number to add 17 int sum; // sum of number1 and number2


50


– Reads String from the user, representing the first number to be added

• Method JOptionPane.showInputDialog displays the following:

• Message called a prompt - directs user to perform an action

• Argument appears as prompt text

• If wrong type of data entered (non-integer) or click Cancel, error occurs

20 firstNumber = JOptionPane.showInputDialog( "Enter first integer" );


51


– Result of call to showInputDialog given to firstNumber using assignment operator =

• Assignment statement

• = binary operator - takes two operands

– Expression on right evaluated and assigned to variable on left

• Read as: firstNumber gets value of JOptionPane.showInputDialog( "Enter first integer" )

20 firstNumber = JOptionPane.showInputDialog( "Enter first integer" );


52


– Similar to previous statement• Assigns variable secondNumber to second integer input

– Method Integer.parseInt• Converts String argument into an integer (type int)

– Class Integer in java.lang• Integer returned by Integer.parseInt is assigned to

variable number1 (line 27)

– Remember that number1 was declared as type int• Line 28 similar

23 secondNumber = 24 JOptionPane.showInputDialog( "Enter second integer" );

27 number1 = Integer.parseInt( firstNumber ); 28 number2 = Integer.parseInt( secondNumber );


53


– Assignment statement• Calculates sum of number1 and number2 (right hand side)

• Uses assignment operator = to assign result to variable sum• Read as: sum gets the value of number1 + number2• number1 and number2 are operands

31 sum = number1 + number2;


54


– Use showMessageDialog to display results

– "The sum is " + sum• Uses the operator + to "add" the string literal "The sum is"

and sum• Concatenation of a String and another type

– Results in a new string

• If sum contains 117, then "The sum is " + sum results in the new string "The sum is 117"

• Note the space in "The sum is "• More on strings in Chapter 11

34 JOptionPane.showMessageDialog( null, "The sum is " + sum,35 "Results", JOptionPane.PLAIN_MESSAGE );


55


– Different version of showMessageDialog• Requires four arguments (instead of two as before)

• First argument: null for now

• Second: string to display

• Third: string in title bar

• Fourth: type of message dialog with icon

– Line 35 no icon: JOptionPane.PLAIN_MESSAGE

34 JOptionPane.showMessageDialog( null, "The sum is " + sum,35 "Results", JOptionPane.PLAIN_MESSAGE );


56


Message dialog type Icon Description

JOptionPane.ERROR_MESSAGE

Displays a dialog that indicates an error to the user.

JOptionPane.INFORMATION_MESSAGE

Displays a dialog with an informational message to the user. The user can simply dismiss the dialog.

JOptionPane.WARNING_MESSAGE

Displays a dialog that warns the user of a potential problem.

JOptionPane.QUESTION_MESSAGE

Displays a dialog that poses a question to the user. This dialog normally requires a response, such as clicking on a Yes or a No button.

JOptionPane.PLAIN_MESSAGE no icon Displays a dialog that simply contains a

message, with no icon. Fig. 2.12 JOptionPane constants for message dialogs.

2003 prentice hall, inc. all rights reserved. 1 lecture 3 variables primitive data types...

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