1.  2003 Prentice Hall, Inc. All rights reserved. 1 Lecture 3 Variables Primitive Data Types calculations & arithmetic operators Readings: Chapter 2

2.  2003 Prentice Hall, Inc. All rights reserved. 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.  2003 Prentice Hall, Inc. All rights reserved. 3 Variables Primitive 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.  2003 Prentice Hall, Inc. All rights reserved. 4 Lets review Key words Identifiers Style Errors

5.  2003 Prentice Hall, Inc. All rights reserved. 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.  2003 Prentice Hall, Inc. All rights reserved. 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.  2003 Prentice Hall, Inc. All rights reserved. 7 Programming Style and Documentation Appropriate Comments Naming Conventions Proper Indentation and Spacing Lines Block Styles

8.  2003 Prentice Hall, Inc. All rights reserved. 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.  2003 Prentice Hall, Inc. All rights reserved. 9 Proper Indentation and Spacing Indentation –Indent 3 spaces. Spacing –Use blank line to separate segments of the code.

10.  2003 Prentice Hall, Inc. All rights reserved. 10 Block Styles Use end-of-line style for braces.

11.  2003 Prentice Hall, Inc. All rights reserved. 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.  2003 Prentice Hall, Inc. All rights reserved. 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.  2003 Prentice Hall, Inc. All rights reserved. 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.  2003 Prentice Hall, Inc. All rights reserved. 14 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

15.  2003 Prentice Hall, Inc. All rights reserved. 15 Naming Conventions choosing 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.

16.  2003 Prentice Hall, Inc. All rights reserved. 16 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.  2003 Prentice Hall, Inc. All rights reserved. 17 Variables

18.  2003 Prentice Hall, Inc. All rights reserved. 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.  2003 Prentice Hall, Inc. All rights reserved. 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 variable containing the value 200, and can contain only 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.  2003 Prentice Hall, Inc. All rights reserved. 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? sum0 Number1 3 sum? number2 2

21.  2003 Prentice Hall, Inc. All rights reserved. 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.  2003 Prentice Hall, Inc. All rights reserved. 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”

23.  2003 Prentice Hall, Inc. All rights reserved. 23 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

24.  2003 Prentice Hall, Inc. All rights reserved. 24 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

25.  2003 Prentice Hall, Inc. All rights reserved. 25 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.  2003 Prentice Hall, Inc. All rights reserved. 26 Declaring and Initializing in One Step int x = 1; double d = 1.4; float f = 1.4; Is this statement correct?

27.  2003 Prentice Hall, Inc. All rights reserved. 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.  2003 Prentice Hall, Inc. All rights reserved. 28 Assignment Statements x = 1; // Assign 1 to x; radius = 1.0; // Assign 1.0 to radius; a = 'A'; // Assign 'A' to a;

29.  2003 Prentice Hall, Inc. All rights reserved. 29 Arithmetic

30.  2003 Prentice Hall, Inc. All rights reserved. 30 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 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.  2003 Prentice Hall, Inc. All rights reserved. 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.  2003 Prentice Hall, Inc. All rights reserved. 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.  2003 Prentice Hall, Inc. All rights reserved. 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 variable z = 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 Type Variable Names Assignment Statements

34.  2003 Prentice Hall, Inc. All rights reserved. 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.  2003 Prentice Hall, Inc. All rights reserved. 35 Assignment Statements 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.  2003 Prentice Hall, Inc. All rights reserved. 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.  2003 Prentice Hall, Inc. All rights reserved. 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.  2003 Prentice Hall, Inc. All rights reserved. 38 Displaying Text in a Dialog Box Upcoming program –Application that uses dialog boxes –Explanation will come afterwards –Demonstrate another way to display output –Packages, methods and GUI

39.  2003 Prentice Hall, Inc. All rights reserved. Outline 39 Welcome4.java 1. import declaration 2. Class Welcome4 2.1 main 2.2 showMessageDial og 2.3 System.exit Program Output 1// Fig. 2.6: Welcome4.java 2// Printing multiple lines in a dialog box 3import javax.swing.JOptionPane; // import class JOptionPane 4 5public 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.java 2 // Printing multiple lines in a dialog box. 3 4 // Java packages 5 import javax.swing.JOptionPane; // program uses JOptionPane 6 7 public class Welcome4 { 8 9 // main method begins execution of Java application 10 public static void main( String args[] ) 11 { 12 JOptionPane.showMessageDialog( 13 null, "Welcome\nto\nJava\nProgramming!" ); 14 15 System.exit( 0 ); // terminate application with window 16 17 } // end method main 18 19 } // end class Welcome4

40.  2003 Prentice Hall, Inc. All rights reserved. 40 Displaying Text in a Dialog Box –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

41.  2003 Prentice Hall, Inc. All rights reserved. 41 2.4Displaying 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.  2003 Prentice Hall, Inc. All rights reserved. 42 2.4Displaying 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.  2003 Prentice Hall, Inc. All rights reserved. 43 Displaying Text in a Dialog Box –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.  2003 Prentice Hall, Inc. All rights reserved. 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

45.  2003 Prentice Hall, Inc. All rights reserved. Outline 45 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.java 2 // Addition program that displays the sum of two numbers. 3 4 // Java packages 5 import javax.swing.JOptionPane; // program uses JOptionPane 6 7 public class Addition { 8 9 // main method begins execution of Java application 10 public static void main( String args[] ) 11 { 12 String firstNumber; // first string entered by user 13 String secondNumber; // second string entered by user 14 15 int number1; // first number to add 16 int number2; // second number to add 17 int sum; // sum of number1 and number2 18 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 int 27 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.

46.  2003 Prentice Hall, Inc. All rights reserved. Outline 46 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 window 38 39 } // end method main 40 41 } // end class Addition

47.  2003 Prentice Hall, Inc. All rights reserved. 47 2.5Another 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.  2003 Prentice Hall, Inc. All rights reserved. 48 2.5Another Java Application: Adding Integers –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.  2003 Prentice Hall, Inc. All rights reserved. 49 2.5Another Java Application: Adding Integers –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.  2003 Prentice Hall, Inc. All rights reserved. 50 2.5Another Java Application: Adding Integers –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.  2003 Prentice Hall, Inc. All rights reserved. 51 Another Java Application: Adding Integers –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.  2003 Prentice Hall, Inc. All rights reserved. 52 Another Java Application: Adding Integers –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.  2003 Prentice Hall, Inc. All rights reserved. 53 Another Java Application: Adding Integers –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.  2003 Prentice Hall, Inc. All rights reserved. 54 Another Java Application: Adding Integers –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.  2003 Prentice Hall, Inc. All rights reserved. 55 Another Java Application: Adding Integers –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.  2003 Prentice Hall, Inc. All rights reserved. 56 Another Java Application: Adding Integers