Lab #2 Java s Basic Elements and Loan Calculator

Transcription

1 Lab #2 Java s Basic Elements and Loan Calculator Objectives: Introduce Java s primitive data types, assignment, arithmetic operators, and the String class. Walkthrough examples of the above. Problem Statement I: Write a Java program that extracts the digits in a four-digit number. 1. Consider the six steps in the SDLC. Analyze the problem. We assume the user will input a 4-digit number (no input validation at this stage), such as 3948, and the program should output the digits. e.g., 3, 9, 4, and 8. Develop an algorithm. To get first digit is easy: divide by 1000 integer division truncates remainder. But we need to capture the remainder in order to get the other digits. The modulus function captures the remainder. So, divide by 1000 first, print the result, replace the number by the modulus of that division, divide it by 100, and continue in similar fashion. Write the code. Here it is. Compile and execute the program in command prompt. /**~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ * Class: ExtractDigits * File: ExtractDigits.java * Description: Extract and display the four digits of a * four-digit number * Environment: PC, Windows XP Pro, jdk6.18, NetBeans 6.8 * Date: 9/30/ javax.swing.joptionpane * History Log: Updated from 10/2/2001, 4/13/2005, 9/14/2006 import javax.swing.joptionpane; public class ExtractDigits public static void main(string[] args) /**~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ * Method: main() * Description: Displays 4 digits and accesses command-line * arguments void args are the command line strings * Date: 9/30/2010 // Java, unlike C/C++, does not need argument count (argc) if (args.length < 2) 1

2 System.out.println("usage: java ExtractDigits <month> <year>"); System.exit(1); //Unlike C,C++, args[0] is the first argument int month = Integer.valueOf(args[0]).intValue(); int year = Integer.valueOf(args[1]).intValue(); //first arg on command line if (month < 1 month > 12) //illegal month System.out.println("Month must be between 1 and 12"); System.exit(1); if (year < 1970) System.out.println("Year must be greater than 1969"); System.exit(1); //display results System.out.print("The month you entered is "); System.out.println(month); System.out.print("The year you entered is "); System.out.println(year); String input = JOptionPane.showInputDialog("Please enter a 4-digit number"); String output = ""; int number = Integer.parseInt(input); System.out.print("The digits of " + number + " are "); output += "The digits of " + number + " are "; System.out.print(number/ ", "); // display thousand's digit output += number/ ", "; number %= 1000; // number = 814 System.out.print(number/100 + ", "); // display hudred's digit output += number/100 + ", "; number %= 100; // number = 14 System.out.print(number/10 + ", "); // display ten's digit output += number/10 + ", "; number %= 10; // number = 4 System.out.println(number + "."); output += number + "."; JOptionPane.showMessageDialog(null,output); System.exit(0); Test the program Compile and test the application with java ExtractDigits Document the program. Create a set of document files using javadoc ExtractDigits.java. Name all files that were generated. 2

3 2. Some detail comments are in order. Here are several notable points in the code above: The entire file consists of defining a public class, ExtractDigits, which has only the public static method main() every Java application must have this method (see my document Understanding Static.pdf for the keyword static). Two types of comments are used (there are three in all in Java): o Single line comments that begin anywhere with // o Javadoc comments that begin with /** and end with */ o The third kind of comment is the C/C++ style comment that begins with /* and ends with */ not seen in this example. Every lab and project that you do should have a class-level Javadoc comments for each class and method comments for each method, as shown above. Read my document JavaDocs.pdf on this requirement. The labs demonstrates the use of the String array variable args. Note that arrays in Java are 0-based (as in C/C++/C#/VB 2005) so args[0] contains 9 and args[1] reads and stores 2006 from the input line. Note also that the argument of main could be represented in two different ways: o public static void main(string[] args) o public static void main(string args[]) The second is more C/C++ like, but the first is preferred. There is nothing magical about the name args you could have named the argument mango, like this: o public static void main(string[] mango) In that case, however, any reference to args should be replaced with mango. Note that the 4-digit number is read via an InputDialog from the JOptionPane class hence the need for the line import javax.swing.joptionpane; At the heart of the calculations is the mod operator: % in Java. The mod operator returns the remainder in a division. Thus, 20 % 7 = 6 because 7 goes into 20 two times (that gives 14) with a remainder of 6. Also 5814 % 1000 = 814, 814 % 100 = 8, and so on. This is how the program extracts the digits. The division operator in Java, /, may seem bizarre. For example, 5814/1000 = 5, not Also 9/5 = 1, but 9.0/5=1.8. The logic is this: if both divisor and dividend are integers, / performs integer division with no remainder or decimals; if either one is a double or float, / performs real division with decimals as you might expect. Java allows for combined operators such as +=, -=, %=, *=, /=, and so on. Thus the lines output += number/100 + ", "; number %= 100; // number = 14 are just a short and more efficient version of 3

4 output = output + number/100 + ", "; number = number % 100; // number = 14 Note also that the + sign for strings is an overloaded operator for concatenation (but Java does not allow overloading of operators!!). 3. How would you change the program to allow input of any number of digits in a number? Problem Statement II: Write a Java program that calculates monthly payment for a loan, given the loan amount and annual interest rate. This is a totally separate problem from the above. Here is the code you should analyze the problem and write an algorithm before you code, checking with a calculator on known values. Note that two versions for reading the input are given: the older version using BufferedReader and InputStreamReader (commented out) with a separate method to do the reading, and the newer version using the Scanner class. /**~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ * Class: Ammortize * File: Ammortize.java * Description: Calculates monthly payment given * loan amount, interest, and years of loan * Environment: PC, Windows XP Pro, jdk6.0, NetBeans 5.5 * Date: 9/29/ java.io javax.text.numberformat * History Log: Updated from 10/2/2001, 4/13/2005, 6/29/2007 import java.io.*; import java.text.numberformat; import java.util.scanner; class Ammortize /**~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ * Method: main() * Description: Calculates monthly payment. Relies on two * member methods: inputfouble and payment void args are the command line strings * Date: 6/29/2007 public static void main (String args[]) double loanamount=0, interest=0, years=0; //create decimal format for currency NumberFormat moneyformat = NumberFormat.getCurrencyInstance(); // BufferedReader in = new BufferedReader(new InputStreamReader(System.in)); // loanamount = inputdouble ("Enter the loan amount in dollars > ", in); // interest = inputdouble ("Enter the interest rate in percent > ", in); // years = inputdouble ("Enter the number of years > ", in); 4

5 Scanner keyboard = new Scanner(System.in); System.out.print ("Enter the loan amount in dollars > "); loanamount = keyboard.nextdouble(); System.out.print ("Enter the interest rate in percent > "); interest = keyboard.nextdouble(); System.out.print ("Enter the number of years > "); years = keyboard.nextdouble(); System.out.print ("The payment is: "); System.out.println (moneyformat.format(payment(loanamount, interest, years))); // end of main () /**~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ * Method: inputdouble() * Description: Prints a prompt and reads a double * BufferedReader double String: prompt BufferedReader: in java.io.bufferedreader * Date: 6/29/2007 static double inputdouble (String prompt, BufferedReader in) boolean error; // error detector String input = ""; // read input variable double value = 0; // converted input to a double type do error = false; System.out.print(prompt); System.out.flush(); try input = in.readline(); catch(ioexception e) System.out.println ("An input error was caught"); System.exit (1); try value = Double.valueOf(input).doubleValue(); catch (NumberFormatException e) System.out.println("Please try again"); error = true; while(error); return value; // end of inputdouble() /**~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ * Method: payment() * Description: prints a prompt and reads a double * BufferedReader 5

6 double double: amount double: interest double: years * Date: 6/29/2007 /* * payment method does the magic calculation * double amount loan amount * double interest interest rate, as a percentage * double years number of years */ static double payment(double amount, double interest, double years) /* * call the exponentiation and natural log functions as * static methods from the Math class */ double top = amount * interest / 1200; //double bot = 1 - Math.exp (years*(-12) * Math.log (1 + interest/1200)); double bot = 1 - Math.pow((1 + interest /1200), years *(-12)); return (top / bot); // end of payment () Problem Statement III: Modify problem statement II in order to implement it as a JFrame (GUI based) in NetBeans. 1. Start NetBeans. Choose File > New Project. Alternatively, you can click the New Project icon in the IDE toolbar. 2. In the Categories pane, select the Java node. In the Projects pane, choose Java Application. Click Next. 6

7 3. Type Loan Calculator in the Project Name field and specify a path e.g. in your home directory as the project location. Ensure that the Set as Main Project checkbox is selected and deselect the Create Main Class checkbox if it is selected. Click Finish. 4. In the Projects window, right-click the Loan Calculator node and choose New > JFrame Form. Enter Loan as the class name and click Finish (leave Package empty we ll use the default package). 5. Select JFrame in the Inspector Window (Window->Navigaing->Inspector). Right-click on it, select Set Layout and choose GridLayout. 6. Select the GridLayout andset the Columns property to 2 and Rows property to 7 in the Properties Window. 7. Select JFrame in the Inspector Window and set the title property to Loan Calculator. 8. From the Swing Controls in the Pallete drag and drop a Label onto the JFrame. Change its name in the Inspector to counterjlabel and set the font property in the Properties Window to Tahoma 14. Set the text property to Calculation Counter and set the horizontalalignment property to RIGHT. 9. From the Swing Controls in the Pallete drag and drop a Text Field onto the JFrame. Change its name in the Inspector to counterjtextfield and set the text property in the Properties Window to Tahoma 14 Bold. Set the text property to 1 and uncheck the editable checkbox. 10. From the Swing Controls in the Pallete drag and drop a Label onto the JFrame. Change its name in the Inspector to principaljlabel and set the text property in the Properties Window to Tahoma 14. Set the text property to Loan Amount and set the horizontalalignment property to RIGHT. 11. From the Swing Controls in the Pallete drag and drop a Text Field onto the JFrame. Change its name in the Inspector to principaljtextfield and set the text property in the Properties Window to Tahoma 14. Set the text property to the empty string (delete current text) and set the horizontalalignment property to RIGHT. 7

8 12. Continue in this fashion (you might want to copy and paste the JLabels, JButtons and the JTextFields) until you construct the UI to look like this: a. Note that you need to resize the JFrame. b. Set the mnemonic property of the Calculate JButton to C to enable the Alt-C combination as an accelerator key to fire the buton s event handler. Do the same for the Clear and Quit buttons. c. Note the names and the naming convention make sure yours are the same because the code refers to these controls by their names. d. Set the editable property of the counterjtextfield, paymentjtextfield and interestjtextfield to false (uncheck the editable check box) to make them read-only. 13. Declare a class instance variable counter and place it immediately after the class declation: int counter = 0; 14. Double-click on the Quit JButton and type the following code inside the event handler: // Exit the application System.exit(0); 15. Double-click on the Clear JButton and type the following code: // Clear the form and start afresh counter = 0; counterjtextfield.settext(""); principaljtextfield.settext(""); ratejtextfield.settext(""); yearsjtextfield.settext(""); paymentjtextfield.settext(""); interestjtextfield.settext(""); principaljtextfield.requestfocus(); 8

9 16. Right-click on the calculatejbutton and select Events->Action->actionPerformed (you could also double click on the calculatejbutton). This should take you to the code for the calculatejbutton event handler. 17. Inside the braces for this event handler type the following code and don t worry yet about the DecimalFormat error you need to import java.text. DecimalFormat coming up next. // Calculate payment on a loan try // get inputs double amount = Double.parseDouble(principalJTextField.getText()); double rate = Double.parseDouble(rateJTextField.getText()); double years = Double.parseDouble(yearsJTextField.getText()); boolean invalidinputs = (amount < 0 amount > rate < 0 rate > 100 years < 0 years > 100); if (invalidinputs) throw new NumberFormatException(); else // calculate payment using formula counter++; double payment = (amount * rate/1200)/(1 - Math.pow((1 + rate/1200), years * (-12))); double interest = years * payment * 12 - amount; DecimalFormat dollars = new DecimalFormat("$#,##0.00"); String result = dollars.format(payment); //display results counterjtextfield.settext(string.valueof(counter)); paymentjtextfield.settext(result); interestjtextfield.settext(dollars.format(interest)); catch(numberformatexception nume) JOptionPane.showMessageDialog(null, "Please enter a positive number for all required fields", "Input Error", JOptionPane.WARNING_MESSAGE); principaljtextfield.requestfocus(); principaljtextfield.selectall(); 18. Add the following three lines (plus comments) immediately after the initcomponents() in the default constructor in order to make the calculatejbutton default, add an icon to the form (required jpg image in the src folder) and set the cursor in the first input JTextField: // set calculatejbutton as default this.getrootpane().setdefaultbutton(calculatejbutton); // set icon for form this.seticonimage(toolkit.getdefaulttoolkit().getimage("src/javaconcepts.jpg")); principaljtextfield.requestfocus(); 9

10 19. Add the javadocs and import statements for the DecimalFormat, Toolkit, and for the JOptionPane class on the top of the source code but after the package statements. The entire top several lines should look like this: /**~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ * Class: Loan * File: Loan.java * Description: Calculate loan payment given loan amount, interest * rate, and time of loan * Environment: PC, Windows XP Pro, jdk6.18, NetBeans 6.5 * Date: 9/29/ javax.swing.jframe * History Log: Updated from 10/2/2001, 4/13/2005, 6/29/2007 *~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/ import java.awt.dimension; import java.awt.toolkit; import java.text.decimalformat; import javax.swing.*; 20. Choose Run > Run Main Project (F6). 21. If all is well, test the program against the following input/output: 22. Otherwise, debug it. 23. What can you do to improve this program? a. Disable the maximization of the form. b. Validate the input. c. Provide event handlers for at least the loan text field (so that pressing Enter calls the Calculate button event handler. d. Build a *.jar file (Shift-F11) in order to execute the program outside the NetBeans IDE. e. Create the Javadocs for the lab. 10