Software component should be closed for modification, but open for extensions

Closed for modification:
New features added to the software component, should not have to modify the existing software component.

Open for extension:
A software component should be expandable to add new features or to add new behavior to it.

Let's understand OCP with an example

Consider that an Insurance company, ABC, provides Health Insurance to its customers.
The company gives discounts on the premiums to its loyal customers.

InsurancePremiumDiscountCalculator class

/**
 * Responsible for calculating the insurance premium for the customer
 */
public class InsurancePremiumDiscountCalculator {
    public int calculatePremiumDiscountPercent(HealthInsuranceCustomerProfile customer) {
        if (customer.isLoyalCustomer()) {
            return 20;
        }
        return 0;
    }
}

HealthInsuranceCustomerProfile class

public class HealthInsuranceCustomerProfile {

    public boolean isLoyalCustomer() {
        // logic to return if the given customer is loyal
        return new Random().nextBoolean();///This is just for returning true or false randomly
    }
}

Note: The above code works fine for now, but let's say in the future the company ABC also starts giving Vehicle insurance as well to its customers then, we will have to make significant changes in the code as below.

VehicleInsuranceCustomerProfile class needs to be created for customers taking vehicle insurance

public class VehicleInsuranceCustomerProfile {
    public boolean isLoyalCustomer() {
        /// logic to return if the given customer is loyal
        return new Random().nextBoolean();///This is just for returning true or false randomly
    }
}

InsurancePremiumDiscountCalculator class needs to be modified by following the ways

/**
 * Responsible for calculating the insurance premium for the customer
 */
public class InsurancePremiumDiscountCalculator {
    public int calculatePremiumDiscountPercent(HealthInsuranceCustomerProfile customer) {
        if (customer.isLoyalCustomer()) {
            return 20;
        }
        return 0;
    }
    public int calculatePremiumDiscountPercent(VehicleInsuranceCustomerProfile customer) {
        if (customer.isLoyalCustomer()) {
            return 20;
        }
        return 0;
    }
}

Note: The above changes are not following OCP because we are modifying the existing code to accommodate the new feature (Vehicle insurance)

Revamping the code as per OCP

InsurancePremiumDiscountCalculator class should be designed like this to accommodate new features

/**
 * Responsible for calculating the insurance premium for the customer
 */
public class InsurancePremiumDiscountCalculator {
    public int calculatePremiumDiscountPercent(CustomerProfile customer) {
        if (customer.isLoyalCustomer()) {
            return 20;
        }
        return 0;
    }
}

CustomerProfile interface for all types of customers(Health Insurance, Vehicle insurance,etc)

public interface CustomerProfile {
    public boolean isLoyalCustomer(); 
}

HealthInsuranceCustomerProfile class

public class HealthInsuranceCustomerProfile implements CustomerProfile {
    @Override
    public boolean isLoyalCustomer() {
        return new Random().nextBoolean();
    }
}

VehicleInsuranceCustomerProfile class

public class VehicleInsuranceCustomerProfile implements CustomerProfile {
    @Override
    public boolean isLoyalCustomer() {
        return new Random().nextBoolean();
    }
}

Now, the above software component follows OCP(Open Close Principle)

Let's try adding another feature of Home Insurance

For this new feature addition, we won't have to modify the existing code, we will just have to extend it by adding a new class called HomeInsuranceCustomerProfile

HomeInsuranceCustomerProfile class

public class HomeInsuranceCustomerProfile implements CustomerProfile {
    @Override
    public boolean isLoyalCustomer() {
        return new Random().nextBoolean();
    }
}

And that is it, it works like a charm.

Key takeaways:

Ease of adding new features.
Leads to the minimal cost of development and testing.
Open close principle often requires decoupling, which, in turn, automatically follows Single Responsibility Principle (SRP).

Important:

Solid principles are all intertwined and interdependent.
Solid principles are more effective when they are used together.