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Ch3

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  • 1. 1 Elementary Programming
  • 2. 2 Motivations  In the preceding chapter, you learned  how to create, compile, and run a Java program.  you will learn how to solve practical problems programmatically.  Through these problems, you will learn Java primitive data types and related subjects, such as variables, constants, data types, operators, expressions, and input and output.
  • 3. 3 Introducing Programming with an Example Listing 2.1 Computing the Area of a Circle public class ComputeArea { public static void main(String[] args) { double radius; // Declare radius double area; // Declare area radius = 20; // Assign a radius // Compute area area = radius * radius * 3.14159; // Display results System.out.println ("The area for the circle of radius " + radius + " is " + area); } }
  • 4. 4 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 * 3.14159; // Display results System.out.println("The area for the circle of radius " + radius + " is " + area); } } no valueradius allocate memory for radius animation
  • 5. 5 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 * 3.14159; // Display results System.out.println("The area for the circle of radius " + radius + " is " + area); } } no valueradius memory no valuearea allocate memory for area animation
  • 6. 6 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 * 3.14159; // Display results System.out.println("The area for the circle of radius " + radius + " is " + area); } } 20radius no valuearea assign 20 to radius animation
  • 7. 7 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 * 3.14159; // Display results System.out.println("The area for the circle of radius " + radius + " is " + area); } } 20radius memory 1256.636area compute area and assign it to variable area animation
  • 8. 8 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 * 3.14159; // Display results System.out.println("The area for the circle of radius " + radius + " is " + area); } } 20radius memory 1256.636area print a message to the console animation
  • 9. 9 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.  Java is case sensitive  Upper case letters are the same as lower case letters  Examples  Name is not the same as name
  • 10. 10 Variables The area is 3.14159 for radius 1.0 The area is 12.56636 for radius 2.0 public class DisplayRadius { public static void main(String[]args) { double radius; //Declare variable radius double area; //Declare variable area // Compute the first area radius = 1.0; //assign value to variable radius area = radius * radius * 3.14159; //compute value of area System.out.println("The area is " + area + " for radius " + radius); // Compute the second area radius = 2.0; //assign value to variable radius area = radius * radius * 3.14159; //compute value of area System.out.println("The area is " + area + " for radius "+radius); } }
  • 11. 11 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; • Declarations are used to • tell the compiler how much memory space is needed for the value that will be assigned to the variable • tell the compiler the type of data that will be referenced by that variable
  • 12. 12 Assignment Statements x = 1; // Assign 1 to x; radius = 1.0; // Assign 1.0 to radius; a = ‘D'; // Assign ‘D' to a; • Assignment statements • Assign a value to the variable • That value is stored in the memory location associated with that variable
  • 13. 13 Declaring and Initializing in One Step  int x = 1;  double d = 1.4; • In Jave, you can declare a variable and store a value in that variable in one statement • It is usually a good idea to initialize the variable when it is declared
  • 14. 14 Constants  Constant  Once a value is assigned it cannot be changed  final is the keyword used to declare a variable a constant  Format  final datatype CONSTANTNAME = VALUE;  Examples  final double PI = 3.14159;  final int SIZE = 3;  Note  Convention is to use all upper case letters for the name of the constant variable
  • 15. 15 Numerical Data Types Name Range Storage Size byte –27 (-128) to 27–1 (127) 8-bit signed short –215 (-32768) to 215–1 (32767) 16-bit signed int –231 (-2147483648) to 231–1 (2147483647) 32-bit signed long –263 to 263–1 64-bit signed (i.e., -9223372036854775808 to 9223372036854775807) float Negative range: 32-bit IEEE 754 -3.4028235E+38 to -1.4E-45 Positive range: 1.4E-45 to 3.4028235E+38 double Negative range: 64-bit IEEE 754 -1.7976931348623157E+308 to -4.9E-324 Positive range: 4.9E-324 to 1.7976931348623157E+308
  • 16. 16 Numeric Operators Name Meaning Example Result + Addition 34 + 1 35 - Subtraction 34.0 – 0.1 33.9 * Multiplication 300 * 30 9000 / Division 1.0 / 2.0 0.5 % Remainder 20 % 3 2
  • 17. 17 Integer Division +, -, *, /, and % 5 / 2 yields an integer 2. 5.0 / 2 yields a double value 2.5 5 % 2 yields 1 (the remainder of the division)
  • 18. 18 Remainder Operator  Remainder is very useful in programming.  For example  an even number % 2 is always 0  an odd number % 2 is always 1.  So you can use this property to determine whether a number is even or odd.  Suppose today is Saturday and you and your friends are going to meet in 10 days. What day is in 10 days? You can find that day is Tuesday using the following expression: Saturday is the 6th day in a week A week has 7 days After 10 days The 2nd day in a week is Tuesday (6 + 10) % 7 is 2
  • 19. 19 Problem: Displaying Time Write a program that obtains hours and minutes from seconds. public class DisplayTime { public static void main(String[] args) { int seconds = 500; int minutes = seconds / 60; int remainingSeconds = seconds % 60; System.out.println(seconds + " seconds is " + minutes + " minutes and " + remainingSeconds + " seconds"); } } 500 seconds is 8 minutes and 20 seconds
  • 20. 20 NOTE  Calculations involving floating-point numbers are approximated because these numbers are not stored with complete accuracy. For example,  System.out.println(1.0 - 0.1 - 0.1 - 0.1 - 0.1 - 0.1);  displays 0.5000000000000001, not 0.5, and  System.out.println(1.0 - 0.9);  displays 0.09999999999999998, not 0.1.  Integers are stored precisely. Therefore, calculations with integers yield a precise integer result.
  • 21. 21 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 = 1000000;  double d = 5.0;
  • 22. 22 Integer Literals  An integer literal can be assigned to an integer variable as long as it can fit into the variable.  A compilation error would occur if the literal were too large for the variable to hold.  For example, the statement byte b = 1000 would cause a compilation error, because 1000 cannot be stored in a variable of the byte type.  An integer literal is assumed to be of the int type, whose value is between  -231 (-2147483648) to 231 –1 (2147483647).  To denote an integer literal of the long type, append it with the letter L or l.  L is preferred because l (lowercase L) can easily be confused with 1 (the digit one).
  • 23. 23 Floating-Point Literals  Floating-point literals are written with a decimal point.  By default, a floating-point literal is treated as a double type value.  For example, 5.0 is considered a double value, not a float value.  You can make a number a float by appending the letter f or F  You make a number a double by appending the letter d or D.  For example, you can use 100.2f or 100.2F for a float number, and 100.2d or 100.2D for a double number.
  • 24. 24 Scientific Notation  Floating-point literals can also be specified in scientific notation  For example, 1.23456e+2, same as 1.23456e2, is equivalent to 123.456, and 1.23456e-2 is equivalent to 0.0123456.  E (or e) represents an exponent and it can be either in lowercase or uppercase.
  • 25. 25 Arithmetic Expressions ) 94 (9 ))(5(10 5 43 y x xx cbayx + ++ ++− − + is translated to (3+4*x)/5 – 10*(y-5)*(a+b+c)/x + 9*(4/x + (9+x)/y)
  • 26. 26 How to Evaluate an Expression Though Java has its own way to evaluate an expression behind the scene, the result of a Java expression and its corresponding arithmetic expression are the same. Therefore, you can safely apply the arithmetic rule for evaluating a Java expression. 3 + 4 * 4 + 5 * (4 + 3) - 1 3 + 4 * 4 + 5 * 7 – 1 3 + 16 + 5 * 7 – 1 3 + 16 + 35 – 1 19 + 35 – 1 54 - 1 53 (1) inside parentheses first (2) multiplication (3) multiplication (4) addition (6) subtraction (5) addition Appendix C, page 690, has a chart of operator precedence.
  • 27. 27 Problem: Converting Temperatures Write a program that converts a Fahrenheit degree to Celsius using the formula: )32)((9 5 −= fahrenheitcelsius public class FahrenheitToCelsius { public static void main(String[] args) { double fahrenheit = 435; // Say 100; double celsius = (5.0 / 9) * (fahrenheit - 32); System.out.println("Fahrenheit " + fahrenheit + " is " + celsius + " in Celsius"); } } Fahrenheit 435.0 is 223.88888888888889 in Celsius
  • 28. 28 Shortcut Assignment Operators Operator Example Equivalent += i += 8 i = i + 8 -= f -= 8.0 f = f - 8.0 *= i *= 8 i = i * 8 /= i /= 8 i = i / 8 %= i %= 8 i = i % 8
  • 29. 29 Increment and Decrement Operators Operator Name Description ++var preincrement The expression (++var) increments var by 1 and evaluates to the new value in var after the increment. var++ postincrement The expression (var++) evaluates to the original value in var and increments var by 1. --var predecrement The expression (--var) decrements var by 1 and evaluates to the new value in var after the decrement. var-- postdecrement The expression (var--) evaluates to the original value in var and decrements var by 1. ++var and --var: performs the arithmetic operation first than uses the value. var++ and var– uses the value first than performs the arithmetic operation.
  • 30. 30 Increment and Decrement Operators, cont. int i = 10; int newNum = 10 * i++; int newNum = 10 * i; i = i + 1; Same effect as int i = 10; int newNum = 10 * (++i); i = i + 1; int newNum = 10 * i; Same effect as newNum will have the value 100, i will have the value 11 newNum will have the value 110, i will have the value 11
  • 31. 31 Increment and Decrement Operators, cont. Using increment and decrement operators makes expressions short, but it also makes them complex and difficult to read. Avoid using these operators in expressions that modify multiple variables, or the same variable for multiple times such as this: int k = ++i + i.
  • 32. 32 Assignment Expressions and Assignment Statements Prior to Java 2, all the expressions could be used as statements. Since Java 2, only the following types of expressions can be statements: variable op= expression; // Where op is +, -, *, /, or % ++variable; variable++; --variable; variable--; • Expressions perform operations on data and move data around. • All statements except blocks are terminated by a semicolon. Blocks are denoted by open and close curly braces.
  • 33. 33 Numeric Type Conversion Consider the following statements: byte i = 100; long k = i * 3 + 4; double d = i * 3.1 + k / 2; • Some of the statements have mixed types • byte types are mixed with long types • byte and long types are mixed with double type byte type long type
  • 34. 34 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.
  • 35. 35 Type Casting Implicit casting double d = 3; (type widening) Explicit casting int i = (int)3.0; (type narrowing) int i = (int)3.9; (Fraction part is truncated) What is wrong? int x = 5 / 2.0; converts the 5 to 5.0, performs division to get 2.5 then tries to store a double in an int, but it won’t fit. You will get a compile error. byte, short, int, long, float, double range increases 8-bits 16-bits 32-bits 64-bits 32-bits 64-bits Will store 3.000000000 Will store 3
  • 36. 36 Problem: Keeping Two Digits After Decimal Points Write a program that displays the sales tax with two digits after the decimal point. public class SalesTax { public static void main(String[] args) { double purchaseAmount = 197.55; double tax = purchaseAmount * 0.06; System.out.println("Sales tax is $" + tax); System.out.println("Sales tax is $" + (int)(tax * 100) / 100.0); } } Sales tax is $11.853 Sales tax is $11.85
  • 37. 37 Character Data Type char letter = 'A'; (ASCII) char numChar = '4'; (ASCII) char letter = 'u0041'; (Unicode) char numChar = 'u0034'; (Unicode) Four hexadecimal digits. NOTE: The increment and decrement operators can also be used on char variables to get the next or preceding Unicode character. For example, the following statements display character b. char ch = 'a'; System.out.println(++ch);
  • 38. 38 Unicode Format Java characters use Unicode, a 16-bit encoding scheme established by the Unicode Consortium to support the interchange, processing, and display of written texts in the world’s diverse languages. Unicode takes two bytes, preceded by u, expressed in four hexadecimal numbers that run from 'u0000' to 'uFFFF'. So, Unicode can represent 65535 + 1 characters. Unicode u03b1 u03b2 u03b3 for three Greek letters
  • 39. 39 Problem: Displaying Unicodes Write a program that displays two Chinese characters and three Greek letters. import javax.swing.JOptionPane; public class DisplayUnicode { public static void main(String[] args) { JOptionPane.showMessageDialog(null, "u6B22u8FCE u03b1 u03b2 u03b3", "u6B22u8FCE Welcome", JOptionPane.INFORMATION_MESSAGE); } }
  • 40. 40 Escape Sequences for Special Characters 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
  • 41. 41 Appendix B: ASCII Character Set ASCII Character Set is a subset of the Unicode from u0000 to u007f 1 2
  • 42. 42 ASCII Character Set, cont. ASCII Character Set is a subset of the Unicode from u0000 to u007f 1 2
  • 43. 43 Casting between char and Numeric Types int i = 'a'; // Same as int i = (int)'a'; char c = 97; // Same as char c = (char)97;
  • 44. 44 The String Type The char type only represents one character. To represent a string of characters, use the data type called String. For example, String message = "Welcome to Java"; • String is actually a predefined class in the Java library just like the System class and JOptionPane class. • The String type is not a primitive type. It is known as a reference type. • Any Java class can be used as a reference type for a variable. • Reference data types will be thoroughly discussed in Chapter 7, “Objects and Classes.” • For the time being, you just need to know how to declare a String variable, how to assign a string to the variable, and how to concatenate strings.
  • 45. 45 String Concatenation // 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 Create a String type named message and store “Welcome to Java” at that location Create a String type named s and store “Chapter2” at that location Create a String type named s1 and store “SupplementB” at that location
  • 46. 46 String Concatenation (cont.) // add to message String message = "Welcome " + "to " + "Java"; Message = message + “! Good luck!!” Create a String type named message and store “Welcome to Java” at that location. Then add “! Good luck!!” to message and now “Welcome to Jave! Good luck!!” is stored at location message.
  • 47. 47 Obtaining Input This book provides two ways of obtaining input. 1. Using JOptionPane input dialogs (§2.11) Inputs from a GUI window. 2. Using the JDK 1.5 Scanner class (§2.16) Inputs from DOS window or lower pane in Eclipse workspace.
  • 48. 48 Getting Input Using Scanner 1. Need to import Scanner import java.util.Scanner; 2. Create a Scanner object Scanner scanner = new Scanner(System.in); 3. Use the methods next(), nextByte(), nextShort(), nextInt(), nextLong(), nextFloat(), nextDouble(), or nextBoolean() to obtain to a string, byte, short, int, long, float, double, or boolean value. For example, System.out.print("Enter a double value: "); Scanner scanner = new Scanner(System.in);
  • 49. 49 TestScanner  TestScanner.java import java.util.Scanner; // Scanner is in java.util public class TestScanner { public static void main(String args[]) { // Create a Scanner Scanner input = new Scanner(System.in); // Prompt the user to enter an integer System.out.print("Enter an integer: "); int intValue = input.nextInt(); System.out.println("You entered the integer " + intValue); // Prompt the user to enter a double value System.out.print("Enter a double value: "); double doubleValue = input.nextDouble(); System.out.println("You entered the double value " + doubleValue); // Prompt the user to enter a string System.out.print("Enter a string without space: "); String string = input.next(); System.out.println("You entered the string " + string); } }
  • 50. 50 Problem: Computing Loan Payments This program lets the user enter the interest rate, number of years, and loan amount and computes monthly payment and total payment. 12 )1( 11 × + − × arsnumberOfYe erestRatemonthlyInt erestRatemonthlyIntloanAmount
  • 51. 51 ComputeLoan.java import java.util.Scanner; public class ComputeLoan { public static void main(String[] args) { // Create a Scanner Scanner input = new Scanner(System.in); // Enter yearly interest rate System.out.print("Enter yearly interest rate, for example 8.25: "); double annualInterestRate = input.nextDouble(); // Obtain monthly interest rate double monthlyInterestRate = annualInterestRate / 1200; // Enter number of years System.out.print( "Enter number of years as an integer, for example 5: "); int numberOfYears = input.nextInt(); // Enter loan amount System.out.print("Enter loan amount, for example 120000.95: "); double loanAmount = input.nextDouble(); // Calculate payment double monthlyPayment = loanAmount * monthlyInterestRate / (1 - 1 / Math.pow(1 + monthlyInterestRate, numberOfYears * 12)); double totalPayment = monthlyPayment * numberOfYears * 12; // Format to keep two digits after the decimal point monthlyPayment = (int)(monthlyPayment * 100) / 100.0; totalPayment = (int)(totalPayment * 100) / 100.0; // Display results System.out.println("The monthly payment is " + monthlyPayment); System.out.println("The total payment is " + totalPayment); } }
  • 52. 52 Problem: Monetary Units 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.
  • 53. 53 ComputeChange.java import java.util.Scanner; public class ComputeChange { public static void main(String[] args) { // Create a Scanner Scanner input = new Scanner(System.in); // Receive the amount System.out.print( "Enter an amount in double, for example 11.56: "); double amount = input.nextDouble(); 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; // Display results String output = "Your amount " + amount + " consists of n" + "t" + numberOfOneDollars + " dollarsn" + "t" + numberOfQuarters + " quartersn" + "t" + numberOfDimes + " dimesn" + "t" + numberOfNickels + " nickelsn" + "t" + numberOfPennies + " pennies"; System.out.println(output); } }
  • 54. 54 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; 1156remainingAmount remainingAmount initialized Suppose amount is 11.56
  • 55. 55 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; 1156remainingAmount Suppose amount is 11.56 11numberOfOneDollars numberOfOneDollars assigned animation
  • 56. 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; 56remainingAmount Suppose amount is 11.56 11numberOfOneDollars remainingAmount updated animation
  • 57. 57 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; 56remainingAmount Suppose amount is 11.56 11numberOfOneDollars 2numberOfOneQuarters numberOfOneQuarters assigned animation
  • 58. 58 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; 6remainingAmount Suppose amount is 11.56 11numberOfOneDollars 2numberOfQuarters remainingAmount updated animation
  • 59. 59 Problem: Displaying Current Time 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. Elapsed time Unix Epoch 01-01-1970 00:00:00 GMT Current Time Time System.CurrentTimeMills()
  • 60. 60 ShowCurrentTime.java public class ShowCurrentTime { public static void main(String[] args) { // Obtain the total milliseconds since the midnight, Jan 1, 1970 long totalMilliseconds = System.currentTimeMillis(); // Obtain the total seconds since the midnight, Jan 1, 1970 long totalSeconds = totalMilliseconds / 1000; // Compute the current second in the minute in the hour int currentSecond = (int)(totalSeconds % 60); // Obtain the total minutes long totalMinutes = totalSeconds / 60; // Compute the current minute in the hour int currentMinute = (int)(totalMinutes % 60); // Obtain the total hours long totalHours = totalMinutes / 60; // Compute the current hour int currentHour = (int)(totalHours % 24); // Display results System.out.println("Current time is " + currentHour + ":" + currentMinute + ":" + currentSecond + " GMT"); } }
  • 61. 61 Two Ways to Invoke the Method There are several ways to use the showInputDialog method. For the time being, you only need to know two ways to invoke it. One is to use a statement as shown in the example: import javax.swing.JOptionPane; //need to import the method //the import goes before public class. String string = JOptionPane.showInputDialog(null, x, y, JOptionPane.QUESTION_MESSAGE); where x is a string for the prompting message, and y is a string for the title of the input dialog box. The other is to use a statement like this: JOptionPane.showInputDialog(x); where x is a string for the prompting message.
  • 62. 62 Getting Input from Input Dialog Boxes String input = JOptionPane.showInputDialog( "Enter an input");
  • 63. 63 Getting Input from Input Dialog Boxes String string = JOptionPane.showInputDialog( null, “Prompting Message”, “Dialog Title”, JOptionPane.QUESTION_MESSAGE);
  • 64. 64 Converting Strings to Integers The input returned from the input dialog box is a string. If you enter a numeric value such as 123, it returns “123”. To obtain the input as a number, you have to convert a string into a number. To convert a string into an int value, you can use the static parseInt method in the Integer class as follows: String intString = JOptionPane.showInputDialog(null, “Enter an integer”, “Integer Example”, JOptionPane.QUESTION_MESSAGE); int intValue = Integer.parseInt(intString); where intString is a numeric string such as “123”.
  • 65. 65 Converting Strings to Doubles To convert a string into a double value, you can use the static parseDouble method in the Double class as follows: String doubleString = JOptionPane.showInputDialog(null, “Enter a double value”, “Double Example”, JOptionPane.QUESTION_MESSAGE); double doubleValue =Double.parseDouble(doubleString); where doubleString is a numeric string such as “123.45”.
  • 66. 66 Dialog Box Input Conversion  Input from JOptionPane.showInputDialog is a String type  Need to convert the input into type required  byte byteValue = Byte.parseByte(intString);  int intValue = Integer.parseInt(intString);  long longValue = Long.parserLong(intString);  float floatValue = Float.parseFloat(intString);  double doubleValue =Double.parseDouble(doubleString);  Since the input value is a String, don’t need to convert the input into a String type
  • 67. 67 Problem: Computing Loan Payments Using Input Dialogs ComputeLoanUsingInputDialogComputeLoanUsingInputDialog RunRun Same as the preceding program for computing loan payments, except that the input is entered from the input dialogs and the output is displayed in an output dialog. 12 )1( 11 × + − × arsnumberOfYe erestRatemonthlyInt erestRatemonthlyIntloanAmount
  • 68. 68 ComputeLoanUsingInputDialog import javax.swing.JOptionPane; public class ComputeLoanUsingInputDialog { public static void main(String[] args) { // Enter yearly interest rate String annualInterestRateString = JOptionPane.showInputDialog( "Enter yearly interest rate, for example 8.25:"); // Convert string to double double annualInterestRate = Double.parseDouble(annualInterestRateString); // Obtain monthly interest rate double monthlyInterestRate = annualInterestRate / 1200; // Enter number of years String numberOfYearsString = JOptionPane.showInputDialog( "Enter number of years as an integer, nfor example 5:"); // Convert string to int int numberOfYears = Integer.parseInt(numberOfYearsString); // Enter loan amount String loanString = JOptionPane.showInputDialog( "Enter loan amount, for example 120000.95:"); // Convert string to double double loanAmount = Double.parseDouble(loanString); // Calculate payment double monthlyPayment = loanAmount * monthlyInterestRate / (1 - 1 / Math.pow(1 + monthlyInterestRate, numberOfYears * 12)); double totalPayment = monthlyPayment * numberOfYears * 12; // Format to keep two digits after the decimal point monthlyPayment = (int)(monthlyPayment * 100) / 100.0; totalPayment = (int)(totalPayment * 100) / 100.0; // Display results String output = "The monthly payment is " + monthlyPayment + "nThe total payment is " + totalPayment; JOptionPane.showMessageDialog(null, output); } }
  • 69. 69 Programming Style and Documentation  Appropriate Comments  Naming Conventions  Proper Indentation and Spacing Lines  Block Styles
  • 70. 70 Appropriate Comments Include a summary 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, due date, date submitted, program name and a brief description at the beginning of the program. /**************************************** * * Student Name: * Date Due: * Date Submitted: * Program Name: * Program Description: * ****************************************/
  • 71. 71 Naming Conventions  Choose meaningful and descriptive names.  Variables and method names:  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.
  • 72. 72 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, and use underscores to connect words. For example, the constant PI and MAX_VALUE
  • 73. 73 Proper Indentation and Spacing  Indentation  Follow Eclipse indention.  Spacing  Use blank line to separate segments of the code.
  • 74. 74 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
  • 75. 75 Programming Errors  Syntax Errors  Detected by the compiler  Runtime Errors  Causes the program to abort  Logic Errors  Produces incorrect result
  • 76. 76 Syntax Errors public class ShowSyntaxErrors { public static void main(String[] args) { i = 30; System.out.println(i + 4); } } Did not declare variable type for i
  • 77. 77 Runtime Errors public class ShowRuntimeErrors { public static void main(String[] args) { int i = 1 / 0; } } Cannot divide by 0
  • 78. 78 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); } } Does not print correct results for number2
  • 79. 79 Debugging Logic errors are called bugs. The process of finding and correcting errors is called debugging. A common approach to debugging is to use a combination of methods to narrow down to the part of the program where the bug is located. You can hand-trace the program (i.e., catch errors by reading the program), or you can insert print statements in order to show the values of the variables or the execution flow of the program. This approach might work for a short, simple program. But for a large, complex program, the most effective approach for debugging is to use a debugger utility.
  • 80. 80 Debugger  Debugger is a program that facilitates debugging. You can use a debugger to  Execute a single statement at a time.  Trace into or stepping over a method.  Set breakpoints.  Display variables.  Display call stack.  Modify variables.

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