Chapter 2 Elementary Programming 1

Introducing Programming with an Example Listing 2.1 Computing the Area of a Circle This program computes the area of the circle. 2 ComputeArea Run IMPORTANT NOTE: To enable the buttons, you must download the entire slide file slide.zip and unzip the files into a directory (e.g., c:\slide).

Trace a Program Execution public class ComputeArea { /** Main method */ public static void main(String[] args) { double radius; double area; // Assign a radius radius = 20; // Compute area area = radius * radius * ; // Display results System.out.println("The area for the circle of radius " + radius + " is " + area); } 3 no value radius allocate memory for radius animation

Trace a Program Execution public class ComputeArea { /** Main method */ public static void main(String[] args) { double radius; double area; // Assign a radius radius = 20; // Compute area area = radius * radius * ; // Display results System.out.println("The area for the circle of radius " + radius + " is " + area); } 4 no value radius memory no value area allocate memory for area animation

Trace a Program Execution public class ComputeArea { /** Main method */ public static void main(String[] args) { double radius; double area; // Assign a radius radius = 20; // Compute area area = radius * radius * ; // Display results System.out.println("The area for the circle of radius " + radius + " is " + area); } 5 20 radius no value area assign 20 to radius animation

Trace a Program Execution public class ComputeArea { /** Main method */ public static void main(String[] args) { double radius; double area; // Assign a radius radius = 20; // Compute area area = radius * radius * ; // Display results System.out.println("The area for the circle of radius " + radius + " is " + area); } 6 20 radius memory area compute area and assign it to variable area animation

Trace a Program Execution public class ComputeArea { /** Main method */ public static void main(String[] args) { double radius; double area; // Assign a radius radius = 20; // Compute area area = radius * radius * ; // Display results System.out.println("The area for the circle of radius " + radius + " is " + area); } 7 20 radius memory area print a message to the console animation

Identifier sequence of characters that consist of letters, digits, underscores (_), and dollar signs ($). 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. 8

Variables // Compute the first area radius = 1.0; area = radius * radius * ; System.out.println("The area is “ + area + " for radius "+radius); // Compute the second area radius = 2.0; area = radius * radius * ; System.out.println("The area is “ + area + " for radius "+radius); 9

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; 10

Assignment Statements x = 1; // Assign 1 to x; radius = 1.0; // Assign 1.0 to radius; a = 'A'; // Assign 'A' to a; 11

Declaring and Initializing in One Step int x = 1; double d = 1.4; 12

Constants final datatype CONSTANTNAME = VALUE; final double PI = ; final int SIZE = 3; Must be declared at initialization. 13

Numerical Data Types 14

Numeric Operators 15

Integer Division +, -, *, /, and % 5 / 2 yields an integer / 2 yields a double value will be truncated (not rounded) if assigned to an integer 5 % 2 yields 1 (the remainder of the division) 16

Problem: Displaying Time Write a program that obtains hours and minutes from seconds. 17 DisplayTime Run

Number Literals A literal is a constant value that appears directly in the program. For example, 34, 1,000,000, and 5.0 are literals in the following statements: int i = 34; long x = ; double d = 5.0; 18

Arithmetic Expressions 19 is translated to (3+4*x)/5 – 10*(y-5)*(a+b+c)/x + 9*(4/x + (9+x)/y) Remember: Order of operations!

Shortcut Assignment Operators 20 OperatorExampleEquivalent +=i += 8i = i + 8 -=f -= 8.0f = f *=i *= 8i = i * 8 /=i /= 8i = i / 8 %=i %= 8i = i % 8

Increment / Decrement 21 ++varpreincrement var++postincrement --varpredecrement var--postdecrement What is the difference between ++var and var++?

Numeric Type Conversion Consider the following statements: byte i = 100; long k = i * 3 + 4; double d = i * k / 2; 22

Conversion Rules When performing a binary operation involving two operands of different types, Java automatically converts the operand based on the following rules: 1. If one of the operands is double, the other is converted into double. 2. Otherwise, if one of the operands is float, the other is converted into float. 3. Otherwise, if one of the operands is long, the other is converted into long. 4. Otherwise, both operands are converted into int. 23

Type Conversion Widening: convert numeric type of smaller range to type of larger range ▫Done implicitly Narrowing: convert numeric type of larger range to type of smaller range ▫May result in overflow ▫Explicit casts 24

Type Casting Implicit casting double d = 3; (type widening) Explicit casting int i = (int)3.0; (type narrowing) int i = (int)3.9; (truncate decimal) What is wrong?int x = 5 / 2.0; 25

Character Data Type char letter = 'A'; (ASCII) char numChar = '4'; (ASCII) char letter = '\u0041'; (Unicode) char numChar = '\u0034'; (Unicode) 26 Four hexadecimal digits. NOTE: Increment (decrement) operators on char variables get the next (preceding) Unicode character. What character is printed by these statements? char ch = 'a'; System.out.println(++ch);

Escape Sequences for Special Characters 27 Description Escape Sequence Unicode Backspace \b\u0008 Tab \t\u0009 Linefeed \n\u000A Carriage return \r\u000D Backslash \\\u005C Single Quote \ ' \u0027 Double Quote \ " \u0022

Appendix B: ASCII Character Set 28 ASCII Character Set is a subset of the Unicode from \u0000 to \u007f

Casting between char and Numeric Types 29 int i = ' a ' ; // Same as int i = (int) ' a ' ; char c = 97; // Same as char c = (char)97; Unicode representation of characters

The String Type String message = "Welcome to Java"; Compare char and String:  char: single character String: several characters  char literal ‘a’  String literal “A”  char is a primitive data type String is predefined class (reference type) More on classes later! 30

String Concatenation (+ operator) // Three strings are concatenated String message = "Welcome " + "to " + "Java"; // String Chapter is concatenated with number 2 String s = "Chapter" + 2; // s becomes Chapter2 // String Supplement is concatenated with character B String s1 = "Supplement" + 'B'; // s becomes SupplementB 31

Problem: Monetary Units 32 This program lets the user enter the amount in decimal representing dollars and cents and output a report listing the monetary equivalent in single dollars, quarters, dimes, nickels, and pennies. Your program should report maximum number of dollars, then the maximum number of quarters, and so on, in this order. ComputeChangeRun

Trace ComputeChange int remainingAmount = (int)(amount * 100); // Find the number of one dollars int numberOfOneDollars = remainingAmount / 100; remainingAmount = remainingAmount % 100; // Find the number of quarters in the remaining amount int numberOfQuarters = remainingAmount / 25; remainingAmount = remainingAmount % 25; // Find the number of dimes in the remaining amount int numberOfDimes = remainingAmount / 10; remainingAmount = remainingAmount % 10; // Find the number of nickels in the remaining amount int numberOfNickels = remainingAmount / 5; remainingAmount = remainingAmount % 5; // Find the number of pennies in the remaining amount int numberOfPennies = remainingAmount; remainingAmo unt remainingAmount initialized Suppose amount is 11.56

Trace ComputeChange int remainingAmount = (int)(amount * 100); // Find the number of one dollars int numberOfOneDollars = remainingAmount / 100; remainingAmount = remainingAmount % 100; // Find the number of quarters in the remaining amount int numberOfQuarters = remainingAmount / 25; remainingAmount = remainingAmount % 25; // Find the number of dimes in the remaining amount int numberOfDimes = remainingAmount / 10; remainingAmount = remainingAmount % 10; // Find the number of nickels in the remaining amount int numberOfNickels = remainingAmount / 5; remainingAmount = remainingAmount % 5; // Find the number of pennies in the remaining amount int numberOfPennies = remainingAmount; remainingAmo unt Suppose amount is numberOfOneDol lars numberOfOneDolla rs assigned animation

Trace ComputeChange int remainingAmount = (int)(amount * 100); // Find the number of one dollars int numberOfOneDollars = remainingAmount / 100; remainingAmount = remainingAmount % 100; // Find the number of quarters in the remaining amount int numberOfQuarters = remainingAmount / 25; remainingAmount = remainingAmount % 25; // Find the number of dimes in the remaining amount int numberOfDimes = remainingAmount / 10; remainingAmount = remainingAmount % 10; // Find the number of nickels in the remaining amount int numberOfNickels = remainingAmount / 5; remainingAmount = remainingAmount % 5; // Find the number of pennies in the remaining amount int numberOfPennies = remainingAmount; remainingAmo unt Suppose amount is numberOfOneDol lars remainingAmount updated animation

Trace ComputeChange int remainingAmount = (int)(amount * 100); // Find the number of one dollars int numberOfOneDollars = remainingAmount / 100; remainingAmount = remainingAmount % 100; // Find the number of quarters in the remaining amount int numberOfQuarters = remainingAmount / 25; remainingAmount = remainingAmount % 25; // Find the number of dimes in the remaining amount int numberOfDimes = remainingAmount / 10; remainingAmount = remainingAmount % 10; // Find the number of nickels in the remaining amount int numberOfNickels = remainingAmount / 5; remainingAmount = remainingAmount % 5; // Find the number of pennies in the remaining amount int numberOfPennies = remainingAmount; remainingAmo unt Suppose amount is numberOfOneDol lars 2 numberOfOneQua rters numberOfOneQuart ers assigned animation

Trace ComputeChange int remainingAmount = (int)(amount * 100); // Find the number of one dollars int numberOfOneDollars = remainingAmount / 100; remainingAmount = remainingAmount % 100; // Find the number of quarters in the remaining amount int numberOfQuarters = remainingAmount / 25; remainingAmount = remainingAmount % 25; // Find the number of dimes in the remaining amount int numberOfDimes = remainingAmount / 10; remainingAmount = remainingAmount % 10; // Find the number of nickels in the remaining amount int numberOfNickels = remainingAmount / 5; remainingAmount = remainingAmount % 5; // Find the number of pennies in the remaining amount int numberOfPennies = remainingAmount; 37 6 remainingAmo unt Suppose amount is numberOfOneDol lars 2 numberOfQuart ers remainingAmount updated animation

Problem: Displaying Current Time 38 Write a program that displays current time in GMT in the format hour:minute:second such as 1:45:19. The currentTimeMillis method in the System class returns the current time in milliseconds since the midnight, January 1, 1970 GMT. (1970 was the year when the Unix operating system was formally introduced.) You can use this method to obtain the current time, and then compute the current second, minute, and hour as follows. ShowCurrentTime Run

Programming Style and Documentation Appropriate Comments Naming Conventions Proper Indentation and Spacing Lines Block Styles Java is case sensitive! 39

Appropriate Comments Summary before program:  What the program does  Your name  Class Section  Instructor  Date 40

Naming Conventions Choose meaningful and descriptive names. Variables and method names: ▫Lowercase for first word ▫Camelcase if several words Class names: ▫Capitalize first letter of each word Constants: ▫Capitalize all letters, use underscores to connect words. 41

Block Styles Chose next-line or end-of-line style for braces. 42

Programming Errors Syntax Errors ▫Detected by the compiler Runtime Errors ▫Causes the program to abort Logic Errors ▫Produces incorrect result 43

Syntax Errors public class ShowSyntaxErrors { public static void main(String[] args) { i = 30; System.out.println(i + 4); } 44

Runtime Errors public class ShowRuntimeErrors { public static void main(String[] args) { int i = 1 / 0; } 45

Logic Errors public class ShowLogicErrors { // Determine if a number is between 1 and 100 inclusively public static void main(String[] args) { // Prompt the user to enter a number String input = JOptionPane.showInputDialog(null, "Please enter an integer:", "ShowLogicErrors", JOptionPane.QUESTION_MESSAGE); int number = Integer.parseInt(input); // Display the result System.out.println("The number is between 1 and 100, " + "inclusively? " + ((1 < number) && (number < 100))); System.exit(0); } 46

Debugging Logic errors are called bugs. Debugging: process of finding and correcting. Narrow down segment of code where the bug is located:  hand-trace the program  insert print statements to see what’s happening  debugger utility (part of IDE) 47

Debugger A debugger can Execute a single statement at a time. Trace into or stepping over a method. Set breakpoints. Display variables. Display call stack. Modify variables. 48