1. Chapter 2 Elementary Programming

2. Motivations
In the preceding chapter, you learned how to create, compile, and run a Java program. Starting from this chapter, 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.
Java comes with 8 primitive data types that split into four categories to handle simple data values: Integers, floating points, characters, truth values; Integers hold number values that cannot have a fractional part. There are four different types: byte, short, int, & long

3. Objectives
To write Java programs to perform simple calculations (§2.2).
To obtain input from the console using the Scanner class (§2.3).
To use identifiers to name variables, constants, methods, and classes (§2.4).
To use variables to store data (§§ ).
To program with assignment statements and assignment expressions (§2.6).
To use constants to store permanent data (§2.7).
To declare Java primitive data types: byte, short, int, long, float, double, and char (§§2.8.1).
To use Java operators to write numeric expressions (§§2.8.2–2.8.3).
To display current time (§2.9).
To use short hand operators (§2.10).
To cast value of one type to another type (§2.11).
To compute loan payment (§2.12).
To represent characters using the char type (§2.13).
To compute monetary changes (§2.14).
To represent a string using the String type (§2.15).
To become familiar with Java documentation, programming style, and naming conventions (§2.16).
To distinguish syntax errors, runtime errors, and logic errors and debug errors (§2.17).
(GUI) To obtain input using the JOptionPane input dialog boxes (§2.18).

4. Listing 2.3 - Computing Average
1 import java.util.Scanner; // Scanner is in the java.util package public class ComputeAverage { public static void main(String[] args) { // Create a Scanner object Scanner input = new Scanner(System.in); // Prompt the user to enter three numbers System.out.print("Enter three numbers: "); double number1 = input.nextDouble(); double number2 = input.nextDouble(); double number3 = input.nextDouble(); // Compute average double average = (number1 + number2 + number3) / 3; // Display result System.out.println("The average of " + number1 + " " +
number2 + " " + number3 + " is " + average); } }

5. Introducing Programming with an Example
Listing 2.1 Computing the Area of a Circle - This program computes the area of the circle.
public class ComputeArea { public static void main(String[] args) { double radius; // Declare radius double area; // Declare area // Assign a radius radius = 20; // New value is radius // Compute area
// Statement assigned to area and line end with ; area = radius * radius * ;
// Display results using print statement System.out.println("The area for the circle of radius " radius + " is " + area); } }
ComputeArea
Run

6. Trace a Program Execution
animation
Trace a Program Execution
allocate memory for radius
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); }
radius
no value
Note: the plus sign (+) in the program allows string to link or continues into a larger strings, which it can display in the output.

7. Trace a Program Execution
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); }
memory
radius
no value
area
no value
allocate memory for area

8. Trace a Program Execution
animation
Trace a Program Execution
assign 20 to radius
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); }
radius 20
area
no value

9. Trace a Program Execution
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); }
memory
radius 20
area
compute area and assign it to variable area

10. Trace a Program Execution
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); }
memory
radius 20
area
print a message to the console

11. Reading Input from the Console
1. Create a Scanner object (Scanner Class is a class in java.util, which allows the user to read values/data(e.g .numbers) of various types, which can be resolve from primitive (original) types and strings.)
Table Scanner input = new Scanner(System.in);
2. Use the methods next(), nextByte(), nextShort(), nextInt(), nextLong(), next Float(), 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 input = new Scanner(System.in);
double d = input.nextDouble();
ComputeAreaWithConsoleInput
ComputeAverage

12. Class, Object and Methods in Java
Class: Whatever we can see in this world all the things are a object.
And all the objects are categorized in a special group. That group is termed as a class (blueprint to create instances of itself).
Almost all the properties of the object should be matched with it's own class.
Methods enable a class object's behavior
example: car (class), shape, size, color (attributes), car convertible (Object), the convertible is open and closes is the behavior of the class (methods).

13. 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.

14. Variables
Variables are used to store values to be used later in a program.
// 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;

15. 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;

16. Assignment Statements
After variable declared, Assignment statement can be used (=);
The syntax is: variable = expressions;
x = 1; // Assign 1 to x;
radius = 1.0; // Assign 1.0 to radius;
a = 'A'; // Assign 'A' to a;

17. Constants
Value of the variable may not be change during the execution for a program.
/* A constant must be declared and initialized in the same statement. */
/* The word final is a Java keyword fro declaring a constant. */
//A Constants are named in uppercase
final datatype CONSTANTNAME = VALUE;
final double PI = ;
final int SIZE = 3;

18. Numerical Data Types
Primitive Data types (The Java Tutorials Link)

19. Numeric Operators

20. Integer Division +, -, *, /, and %
5 / 2 yields an integer 2. (when both operands of a division are integers, the result of the division is an integer and the fraction will be truncated).
5.0 / 2 yields a double value 2.5 (when operands of a division are floating-point number, the result of the division is an integer).
5 % 2 yields 1 (the remainder of the division),
5 percent of 2 is 1

21. Remainder Operator
Remainder is very useful in programming. For example, an even number % 2 is always 0 and 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:

22. Problem: Displaying Time
Write a program that obtains hours and minutes from seconds.
DisplayTime
Run
import java.util.Scanner; public class DisplayTime { public static void main(String[] args) { Scanner input = new Scanner(System.in); // Prompt the user for input System.out.print("Enter an integer for seconds: "); int seconds = input.nextInt(); int minutes = seconds / 60; // Find minutes in seconds int remainingSeconds = seconds % 60; // Seconds remaining System.out.println(seconds + " seconds is " + minutes " minutes and " + remainingSeconds + " seconds"); } }

23. NOTE
Calculations involving floating-point numbers are approximated because these numbers are not stored with complete accuracy. For example,
System.out.println( );
displays , not 0.5, and
System.out.println( );
displays , not 0.1. Integers are stored precisely. Therefore, calculations with integers yield a precise integer result.

24. 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;

25. 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 ( ) to 231–1 ( ). 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).

26. 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, and 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.

27. Scientific Notation
Floating-point literals can also be specified in scientific notation, for example, e+2, same as e2, is equivalent to , and e-2 is equivalent to
E (or e) represents an exponent and it can be either in lowercase or uppercase.
e2, is equivalent to x 10²=

28. Arithmetic Expressions
is translated to
(3+4*x)/5 – 10*(y-5)*(a+b+c)/x + 9*(4/x + (9+x)/y)

29. 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.

30. Problem: Converting Temperatures
Write a program that converts a Fahrenheit degree to Celsius using the formula:
Exercise Listing 2.5: Type the program from the textbook
FahrenheitToCelsius
Run

31. 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.
ShowCurrentTime
Run

32. Shortcut Assignment Operators
Very often the value of a variable is used, modified, and then re-assigned back to the same variable. See below example and equivalent.
The += called the addition assignment operator.
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

33. Increment & Decrement Operators
The ++ ad -- operators can be used in prefix or suffix, as shown below
Example: int i = 3, j = 3;
i++; // i becomes 4
j--; // j becomes 2
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.

34. Assignment Statements
Prior to Java 2, all the expressions can be used as statements. Since Java 2, only the following types of expressions can be statements:
variable op= expression; // Where op is +, -, *, /, or %
++variable; increment var by 1 and use the new var value
variable++; increment var by 1 and use the original var value
--variable; decrement var by 1 and use the new var value
variable--; decrement var by 1 and use the original var value

35. Increment and Decrement Operators, cont.
10 * 10 (original value of i ) is 100
i ++ means Increment after
i is increment by 1 after, and the old value of i is returned and used in the multiplication, Thus newNum becomes 100.
i is increment by 1 before, and the new value of i is returned and used in the multiplication, Thus newNum becomes 110.

36. Numeric Type Conversion
Binary operations with two operands: if an integer and a floating
number are involved in a binary operation, Java automatically
converts the integer to a floating-point value.
Example: 3 * 4.5 is the same as 3.0 * 4.5.
Consider the following statements:
byte i = 100;
long k = i * 3 + 4;
double d = i * k / 2;

37. Conversion Rules
When performing a binary operation involving two operands of different types, Java automatically converts the operand based on the following rules:
  So, 3*4.5 is the same as 3.0*4.5.
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.

38. Type Casting Implicit casting double d = 3; (type widening)
A character (char) can be cast into any numeric type and vice versa. When an integer is cast into a char, only is lower 16 bits of data are used, the other part is ignored.
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;

39. Problem: Keeping Two Digits After Decimal Points
Write a program that displays the sales tax with two digits after the decimal point.
import java.util.Scanner; public class SalesTax { public static void main(String[] args) { Scanner input = new Scanner(System.in); System.out.print("Enter purchase amount: "); double purchaseAmount = input.nextDouble(); double tax = purchaseAmount * 0.06; System.out.println("Sales tax is " + (int)(tax * 100) / 100.0); } }
SalesTax
Run

40. 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.
ComputeLoan
Run

41. Character Data Type char letter = 'A'; (ASCII)
Four hexadecimal digits.
char letter = 'A'; (ASCII)
char numChar = '4'; (ASCII)
char letter = '\u0041'; (Unicode)
char numChar = '\u0034'; (Unicode)
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);

42. Unicode & ASCII code
Computers use binary numbers internally as character is stored in a computer as a sequence of 0’s & 1’s.
Mapping the character to its binary called encoding.
Java supports Unicode, as encoding scheme characters established.

43. 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 characters.
Unicode \u03b1 \u03b2 \u03b3 for three Greek letters

44. Problem: Displaying Unicodes
Write a program that displays two Chinese characters and three Greek letters.
DisplayUnicode
Run

45. 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

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

47. ASCII Character Set, cont.
ASCII Character Set is a subset of the Unicode from \u0000 to \u007f

48. Casting between char and Numeric Types
int i = 'a'; // Same as int i = (int)'a';
char c = 97; // Same as char c = (char)97;

49. Problem: Monetary Units
This program lets the user enter the amount in decimal representing dollars and cents and output a report listing the monetary (pertaining to money) 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.
ComputeChange
Run

50. remainingAmount initialized
Trace ComputeChange
Suppose amount is 11.56
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;
remainingAmount
1156
remainingAmount initialized

51. numberOfOneDollars assigned
animation
Trace ComputeChange
Suppose amount is 11.56
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;
remainingAmount
1156
numberOfOneDollars 11
numberOfOneDollars assigned

52. remainingAmount updated
animation
Trace ComputeChange
Suppose amount is 11.56
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;
remainingAmount 56
numberOfOneDollars 11
remainingAmount updated

53. numberOfOneQuarters assigned
animation
Trace ComputeChange
Suppose amount is 11.56
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;
remainingAmount 56
numberOfOneDollars 11
numberOfOneQuarters 2
numberOfOneQuarters assigned

54. remainingAmount updated
animation
Trace ComputeChange
Suppose amount is 11.56
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;
remainingAmount 6
numberOfOneDollars 11
numberOfQuarters 2
remainingAmount updated

55. 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 & Scanner.
The String type is not a primitive type, more like 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 (link two or more information) strings.

56. String Concatenation
Two string can be concatenated (link together) with sign (+) operator. If one of the operands is a nonstring (e.g., a number), the nonstring value is converted into a string and link with other string. Here are some examples:
// Strings are concatenated
String message = "Welcome " + "to " + "Java";
The shorthand += operator can also be used for string concatenate.
 for example the following appends the string
message += “and Java is fun”;
So, the new message is “Welcome to Java and Java is fun”.

57. String Concatenation Cont..
System.out.println("The average of " + number1 + " " + number2 + " " + number3 + " is " + average);
// 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';
// s1 becomes SupplementB

58. Programming Style and Documentation
Appropriate Comments //comments
Naming Conventions
Proper Indentation and Spacing Lines
Block Styles

59. 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 the following:
your name,
class section, instructor,
program date
and a brief description at the beginning of the program.

60. 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.

61. 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

62. Proper Indentation and Spacing
Indent two spaces.
Spacing
Use blank line to separate segments of the code.

63. Block Styles
Use end-of-line style for braces.

64. Programming Errors Syntax Errors Runtime Errors Logic Errors
Programming errors are unavoidable even for expert programmers. Errors can be categorized into three types.
Syntax Errors
Detected by the compiler
Runtime Errors
Causes the program to abort
Logic Errors
Produces incorrect result

65. Syntax Errors
Errors that occur during the compilation, during coding construction such as mistyping a keyword, omitted some necessary punctuation or opening brace without closing brace are called syntax error or compiler errors.
Syntax error are usually easy to detect, because the compiler tells you where they are and what caused them.
public class ShowSyntaxErrors {
public static void main(String[] args) {
i = 30;
System.out.println(i + 4); }

66. Runtime Errors Runtime error cause a program to terminate abnormally.
They occur while a program is running if then environment detects an operation is impossible to carry out.
Example is:
input incorrect type of value (double value instead of integer value)
division by 0
public class ShowRuntimeErrors {
public static void main(String[] args) {
int i = 1 / 0; }

67. Logic Errors
Logic error occur when a program does not perform the way it was intended to. Error of this kind occur for many different reasons.
Example below is add to number1 to number2
// ShowLogicErrors.java: the program contains a logic error public class ShowLogicErrors { public static void main (String [] args) { // Add number1 to number2 int number1 = 3; int number2 = 3; number2 += number1 + number2; System.out.println ("number2 is " + number2); } }
Note: The program above have no syntax, runtime errors, but it doe not print the correct result for number2. See if you can find the error.

68. 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.

69. 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.

70. JOptionPane Input This book provides two ways of obtaining input.
Using the Scanner class (console input)
Using JOptionPane input dialogs (obtain from an input dialog box by invoke the
JOptionPane.showInputDialog methods
JOptionPane makes it easy to pop up a standard dialog box that prompts users for a value or informs them of something.
Class JOptionPane

71. Getting Input from Input Dialog Boxes
String input = JOptionPane.showInputDialog(
"Enter an input");

72. Getting Input from Input Dialog Boxes
String string = JOptionPane.showInputDialog(
null, “Prompting Message”, “Dialog Title”,
JOptionPane.QUESTION_MESSAGE);

73. 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:
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.

74. 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:
int intValue = Integer.parseInt(intString);
where intString is a numeric string such as “123”.

75. 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:
double doubleValue =Double.parseDouble(doubleString);
where doubleString is a numeric string such as “123.45”.

76. Problem: Computing Loan Payments Using Input Dialogs
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.
ComputeLoanUsingInputDialog
Run