2003 Prentice Hall, Inc. Modified for use with this class. All rights reserved. 1 Introduction to...

2003 Prentice Hall, Inc. Modified for use with this class. All rights reserved.

1

Introduction to Computers and Programming in JAVA: V22.0002

Primitive Data Types and Operations


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Primitive Data Types and Operations

• 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


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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:

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.


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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.


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Programming Style and Documentation

• Appropriate Comments

• Naming Conventions

• Proper Indentation and Spacing Lines

• Block Styles


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Appropriate Comments

Include a comment at the beginning of each 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.


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Proper Indentation and Spacing

• Indentation– Indent two spaces.

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


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Block Styles

Use a consistent 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


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Programming Errors

• Syntax Errors– Detected by the compiler

• Runtime Errors– Causes the program to abort

• Logic Errors– Produces incorrect result


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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.


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Run-time Errors

• These occur once a program is running; typically it will “crash” or simply abort.

• The compiler cannot identify these errors at compile time.

• For example, a statement which calls for division by zero causes a run-time error: the compiler would not detect it but the program cannot run in this case.


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Naming Conventions

• 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.


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Variables


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Memory Concepts

• Variables – Every variable has a name, a type, a size and a value

• Name corresponds to location in memory

– When new value is placed into a variable, it replaces (and destroys) the previous value

– Reading variables from memory does not change them


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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.


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Memory Concepts

• Visual Representation– sum = 0;

– sum = 3;

– sum = 2;

– Sum = number1 + number2; after execution of statement

sum 0

sum 3

sum 2


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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.


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Arithmetic

• Arithmetic calculations used in most programs– Usage

• * for multiplication

• / for division

• +, -• No operator for exponentiation

• Integer division truncates remainder

7 / 5 evaluates to 1

– The Remainder operator: % returns the remainder 7 % 5 evaluates to 2


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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 (aka “Please excuse my dear Aunt Sally”)


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Variable Declaration

• Java is a “strongly typed” language.

• Before you use a variable, you must declare it.

• Examples:

/* Creates an integer variable */

int number;

/* Creates a float variable */

float price;

/* Creates a character variable */

char letter;


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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;


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Data Types:• integers: the simplest data type in Java. They are

used to hold positive and negative whole numbers, e.g. 5, 25, -777, 1.

• doubles: 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 as well as some additional primitive data types.


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Declaring and Initializingin One Step

• int x = 1;

• double d = 1.4;

• float f = 1.4;


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Numerical Data Types

byte 8 bits

short 16 bits

int 32 bits

long 64 bits

float 32 bits

double 64 bits


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Assignment Statements

x = 1; // Assign 1 to x;

radius = 1.0; // Assign 1.0 to radius;

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


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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…

}


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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



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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 “+” symbol is for string concatenation.


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• 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.


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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


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Assignment Operators

• Given the following:

x = 2;

x = x + 1;

System.out.println ("x: " + x);• There are actually several ways to rewrite this

more concisely.

Short Cut Operator

• One option is to use the += operator

x = 2;

x += 1; // same as x = x + 1;

System.out.println ("x: " + x);• There are similar operators for *, -, /.%

– x = x * 5 is equivalent to x *= 5;– x = x – 5; is equivalent to x -= 5;– x = x / 5; is equivalent to x /= 5;– x = x % 5; is equivalent to x %= 5;

• Good Practice: place a space before and after your short cut operators.

Increment Operator

• A second option is to use an increment operator:

x++ Post-Increment Operator

++x Pre-Increment Operator• Both operators will increment x by 1, but they do

have subtle differences.

Pre v. Post Increment

• PostIncrement Operator (x++):– use the current value of x in the expression.

Then, increment by 1.• PreIncrement Operator (++x):

– Increment x by 1. Then, use the new value of x in the expression.

How about a real example?

// Preincrementing v. PostIncrementing public class PrePost{

public static void main (String[] args){

int c = 5;System.out.println (c);System.out.println (c++);System.out.println (c);System.out.println();c = 5;System.out.println (c);System.out.println (++c);System.out.println (c);

}}

Output:

5

5

6

5

6

6

Post Increment

Pre Increment 2000 Prentice Hall, Inc. All rights reserved. Modified by Evan Korth

Pre v. Post Decrement

• PostDecrement Operator (x--):– use the current value of x in the expression.

Then, decrease by 1.• PreDecrement Operator (--x):

– Decrease x by 1. Then, use the new value of x in the expression.

• Good practice: Place unary operators directly next to their operands, with no intervening spaces.


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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)


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• 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.

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welcome3.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 // welcome3.java2 // Printing multiple lines in a dialog box.3 4 // Java packages 5 import javax.swing.JOptionPane; // program uses JOptionPane6 7 public class welcome3 {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 welcome3


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– 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!" );


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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


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– 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 welcome3 and main

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


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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


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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.


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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


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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


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– 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


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– 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


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– 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" );


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– 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" );


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– 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 );


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– 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;


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– 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 );


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– 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 );


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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.


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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)

2003 Prentice Hall, Inc. Modified for use with this class. All rights reserved. 1 Introduction to...

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