Tutorials : Designing Abstraction :

OCP compliant solution

To make the design OCP compliant, we need to make the following changes:

• The strong coupling between LoanRequestHandler and any Validator objects should be avoided.
• Individual validator objects implementing different business logics for approving a loan request should represent an abstract type and the LoanRequestHandler should use this abstract type rather than any specific sub-type to avoid any strong coupling.

Bearing these facts in mind, we come up with the following design. I will only show the skeleton of the modified design and the code inside each of them remains same as before. Only the design changes, not the implementation.

Listing 3

public class LoanRequestHandler {

	private int balance;
	private int period;
	/** Creates a new instance of LoanRequestHandler */
	public LoanRequestHandler(int balance, int period) {
		this.balance = balance;
		this.period = period;
	}

	public void approveLoan(Validator validator) {
		if(validator.isValid(balance))
		   //sanction the loan
		   System.out.println("Loan approved...");
		else
		   System.out.println("Sorry not enough balance...");
	}
}

The LoanRequestHandler now uses an interface type of object named Validator, which defines the method for approving the loan. Each individual Validator object will implement this method, with the object specific business logic within the method inherited from the defined Validator interface.

Listing 4

public interface Validator {

  public Boolean isValid(int balance);
}

Now each individual Validator object will implement this interface and make use of their individual business logic to approve or reject a loan request. Based on this the PersonalLoanValidator  object changes to the following.

Listing 5

public class PersonalLoanValidator implements Validator{

	/** Creates a new instance of PersonalLoanValidator */
	public PersonalLoanValidator() {
	}

	public boolean isValid(int balance) {
		if(balance>1000)
			return true;
		else
			return false;
	}
}

Now we will implement another Validator named BusinessLoanValidator.

public class BusinessLoanValidator implements Validator{
	/** Creates a new instance of PersonalLoanValidator */
	public PersonalLoanValidator() {
	}

	public boolean isValid(int balance) {
		if(balance>5000)
		   return true;
		else
		   return false;
	}
}

The BusinessLoanValidator only approves the loan request if the balance is more than 5000, whereas the PersonalLoanValidator will approve it if the balance is more than 1000. But the LoanRequestHandler remains unaffected by this change in the business logic change. So long it is supplied the correct Validator, it will operate correctly.

Clearly, this is a better design. Any new loan type can be handled by adding a new XXXValidator object, implementing the Validator interface. This is OCP compliant solution.

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