7.15. Case Study - The Loan Class

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7.15. Case Study: The Loan Class

Let us use the Loan class as an example to demonstrate the creation and use of classes. Loan has the data fields: annualInterestRate , numberOfYears , loanAmount , and loanDate , and the methods getAnnualInterestRate , getNumberOfYears , getLoanAmount , getLoanDate , setAnnualInterestRate , setNumberOfYears , setLoanAmount , getMonthlyPayment , and getTotalPayment , as shown in Figure 7.23.

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Figure 7.23. The Loan class models the properties and behaviors of loans.

Note

An object can contain another object. The relationship between the two is called composition . In this example, a Loan object contains a Date object.

The UML diagram in Figure 7.23 serves as the contract for the Loan class. Throughout the book, you will play the role of both class user and class writer. The user can use the class without knowing how the class is implemented. Assume that the Loan class is available. Let us begin by writing a test program that uses the Loan class in Listing 7.8.

Listing 7.8. TestLoanClass.java

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 1   import   javax.swing.JOptionPane; 2 3   public class   TestLoanClass { 4  /** Main method */  5   public static void   main(String[] args) { 6  // Enter yearly interest rate  7 String annualInterestRateString = JOptionPane.showInputDialog( 8   "Enter yearly interest rate, for example 8.25:"   ); 9 10  // Convert string to double  11   double   annualInterestRate = 12 Double.parseDouble(annualInterestRateString); 13 14  // Enter number of years  15 String numberOfYearsString = JOptionPane.showInputDialog( 16   "Enter number of years as an integer, \nfor example 5:"   ); 17 18  // Convert string to int  19   int   numberOfYears = Integer.parseInt(numberOfYearsString); 

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 20 21  // Enter loan amount  22 String loanString = JOptionPane.showInputDialog( 23   "Enter loan amount, for example 120000.95:"   ); 24 25  // Convert string to double  26   double   loanAmount = Double.parseDouble(loanString); 27 28  // Create Loan object  29 Loan loan = 30    new   Loan(annualInterestRate, numberOfYears, loanAmount)  ; 31 32  // Format to keep two digits after the decimal point  33   double   monthlyPayment = 34 (   int   )(  loan.getMonthlyPayment()  *   100   ) /   100   .     ; 35   double   totalPayment = 36 (   int   )(  loan.getTotalPayment()  *   100   ) /   100   .     ; 37 38  // Display results  39 String output =   "The loan was created on "   + 40 loan.getLoanDate().toString() +   "\nThe monthly payment is "   + 41 monthlyPayment +   "\nThe total payment is "   + totalPayment; 42 JOptionPane.showMessageDialog(   null   , output); 43 } 44 } 

The main method reads interest rate, payment period (in years), and loan amount; creates a Loan object; and then obtains the monthly payment (lines 33 “34) and total payment (lines 35 “36) using the instance methods in the Loan class. Figure 7.24 shows the output of a sample run of the program.

Figure 7.24. The program creates a Loan instance with the annual interest rate, number of years, and loan amount, and displays the loan date, monthly payment, and total payment by invoking the methods of the instance.

The Loan class can be implemented in Listing 7.9.

Listing 7.9. Loan.java

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 1   public      class   Loan  { 2   private double   annualInterestRate; 3   private int   numberOfYears; 4   private double   loanAmount; 

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 5   private   java.util.Date loanDate; 6 7  /** Default constructor */  8   public   Loan() { 9   this   (   7.5   ,   30   ,   100000   ); 10 } 11 12  /** Construct a loan with specified annual interest rate,  13  number of years and loan amount  14  */  15    public   Loan(   double   annualInterestRate,   int   numberOfYears,  16    double   loanAmount)  { 17   this   .annualInterestRate = annualInterestRate; 18   this   .numberOfYears = numberOfYears; 19   this   .loanAmount = loanAmount; 20 loanDate =  new  java.util.Date(); 21 } 22 23  /** Return annualInterestRate */  24   public double   getAnnualInterestRate() { 25   return   annualInterestRate; 26 } 27 28  /** Set a new annualInterestRate */  29   public void   setAnnualInterestRate(   double   annualInterestRate) { 30   this   .annualInterestRate = annualInterestRate; 31 } 32 33  /** Return numberOfYears */  34   public int   getNumberOfYears() { 35   return   numberOfYears; 36 } 37 38  /** Set a new numberOfYears */  39   public void   setNumberOfYears(   int   numberOfYears) { 40   this   .numberOfYears = numberOfYears; 41 } 42 43  /** Return loanAmount */  44   public double   getLoanAmount() { 45   return   loanAmount; 46 } 47 48  /** Set a newloanAmount */  49   public void   setLoanAmount(   double   loanAmount) { 50   this   .loanAmount = loanAmount; 51 } 52 53  /** Find monthly payment */  54   public double   getMonthlyPayment() { 55   double   monthlyInterestRate = annualInterestRate /   1200   ; 56   return   loanAmount * monthlyInterestRate / (   1   - 57 (Math.pow(   1   / (   1   + monthlyInterestRate), numberOfYears *   12   ))); 58 } 59 60  /** Find total payment */  61   public double   getTotalPayment() { 62   return   getMonthlyPayment() * numberOfYears *   12   ; 63 } 

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 64 65  /** Return loan date */  66   public   java.util.Date getLoanDate() { 67   return   loanDate; 68 } 69 } 

From a class developer's perspective, a class is designed for use by many different customers. In order to be useful in a wide range of applications, a class should provide a variety of ways for customization through constructors, properties, and methods.

The Loan class contains two constructors, four get methods, three set methods, and the methods for finding monthly payment and total payment. You can construct a Loan object by using the no-arg constructor or the one with three parameters: annual interest rate, number of years, and loan amount. When a loan object is created, its date is stored in the loanDate field. The getLoanDate method returns the date. The three get methods, getAnnualInterest , getNumberOfYears , and getLoanAmount , return annual interest rate, payment years, and loan amount, respectively. All the data properties and methods in this class are tied to a specific instance of the Loan class. Therefore, they are instance variables or methods.

Recall that the java.util.Date can be used to create an instance to represent current date and time in §7.5.1, "The Date Class." The Loan class contains the accessor method for loanDate , but no mutator method for it. Does this mean that the contents of loanDate cannot be changed? See Review Question 7.24.

Important Pedagogical Tip

The UML diagram for the Loan class is shown in Figure 7.23. Students should begin by writing a test program that uses the Loan class even though they don't know how the Loan class is implemented. This has three benefits:

• It demonstrates that developing a class and using a class are two separate tasks .

• It makes it possible to skip the complex implementation of certain classes without interrupting the sequence of the book.

• It is easier to learn how to implement a class if you are familiar with the class through using it.

For all the examples from now on, you may first create an object from the class and try to use its methods and then turn your attention to its implementation.