Real World Analogies and Code Example

In the previous lesson we defined the Open/Closed Principle: software should be open for extension and closed for modification. Now we apply it to a piece of code.

The Starting Point

Here is an Invoice class that prints an invoice and applies a discount.

There is nothing wrong with this class yet. It handles the one case the business has. The Open/Closed Principle says nothing about code that never changes.

The problem appears when a second kind of invoice arrives.

The Change That Exposes the Problem

Sales now needs international invoices

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Keshav Garg

· 2 years ago

 public class Invoice {
    private double amount;

    public Invoice(double amount) {
        this.amount = amount;
    }

    public double getAmount() {
        return amount;
    }

    // This method handles generating basic invoices
    public void generateInvoice() {
        System.out.println("Generating basic invoice for amount: " + amount);
    }

    // This method handles applying discounts on the invoice
    public void applyDiscount() {
        System.out.println("Applying discount on invoice: " + amount);
    }
}

public class InternationalInvoice extends Invoice {
	@Override
	public void generateInvoice() {
			System.out.println("Generating International invoice for amount: " + amount);
	}
}

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