Solid Principles: Open/closed principle

Previously we talked about the single responsibility principle. The open/closed principle is the second principle in the row regarding the solid principles acronym.

“Software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification”

By employing that principle the goal is to extend a module’s behaviour without modifying its source code.

Imagine a scenario of applying a discount to one of our products. A discount service will apply the discount specified and give back the discounted price.

Currently our system has only one kind of discount which applies to all adults.

package com.gkatzioura.solid.ocp;

import java.math.BigDecimal;
import java.math.RoundingMode;

public class Discount {

    public BigDecimal apply(BigDecimal price) {

        BigDecimal percent = new BigDecimal("0.10");
        BigDecimal discount = price.multiply(percent);
        return price.subtract(discount.setScale(2, RoundingMode.HALF_UP));
    }
}

And the discount service shall apply this discount to the price given.

package com.gkatzioura.solid.ocp;

import java.math.BigDecimal;

public class DiscountService {

    public BigDecimal applyDiscounts(BigDecimal price,Discount discount) {

        return discount.apply(price);
    }
}

However our company wants to offer a discount to seniors, thus we have the senior Discount.

package com.gkatzioura.solid.ocp;

import java.math.BigDecimal;
import java.math.RoundingMode;

public class SeniorDiscount {

    public BigDecimal apply(BigDecimal price) {

        BigDecimal percent = new BigDecimal("0.20");
        BigDecimal discount = price.multiply(percent);
        return price.subtract(discount.setScale(2, RoundingMode.HALF_UP));
    }
}

This makes things a little more complicated for the discount service since the service has to apply both the discount for adult and both the discount for seniors.

package com.gkatzioura.solid.ocp;

import java.math.BigDecimal;

public class DiscountService {

    public BigDecimal applyDiscounts(BigDecimal price,Discount discount) {

        BigDecimal discountPrice = price.add(BigDecimal.ZERO);
        discountPrice = discount.apply(discountPrice);
        return discountPrice;
    }

    public BigDecimal applySeniorDiscount(BigDecimal price,SeniorDiscount discount) {

        return discount.apply(price);
    }

}

By doing so we modified the discount service sourcecode to extend its behaviour. Also for every different discount that the sales department might come up with, the discount service will get extra methods.

In order to follow the open/closed principle we will create a discount interface.

package com.gkatzioura.solid.ocp;

import java.math.BigDecimal;

public interface Discount {

    BigDecimal apply(BigDecimal price);
}

The default discount will be renamed to the AdultDiscount and implement the discount interface.

package com.gkatzioura.solid.ocp;

import java.math.BigDecimal;
import java.math.RoundingMode;

public class AdultDiscount implements Discount {

    @Override
    public BigDecimal apply(BigDecimal price) {

        BigDecimal percent = new BigDecimal("0.10");
        BigDecimal discount = price.multiply(percent);
        return price.subtract(discount.setScale(2, RoundingMode.HALF_UP));
    }
}

The SeniorDiscount will also implement the Discount interface.

package com.gkatzioura.solid.ocp;

import java.math.BigDecimal;
import java.math.RoundingMode;

public class SeniorDiscount implements Discount {

    @Override
    public BigDecimal apply(BigDecimal price) {

        BigDecimal percent = new BigDecimal("0.20");
        BigDecimal discount = price.multiply(percent);
        return price.subtract(discount.setScale(2, RoundingMode.HALF_UP));
    }
}

Last but not least our DiscountService will be refactored in order to apply discounts based on the Discount interface.

package com.gkatzioura.solid.ocp;

import java.math.BigDecimal;

public class DiscountService {

    public BigDecimal applyDiscounts(BigDecimal price,Discount[] discounts) {

        BigDecimal discountPrice = price.add(BigDecimal.ZERO);

        for(Discount discount:discounts) {

            discountPrice = discount.apply(discountPrice);
        }

        return discountPrice;
    }
}

By this way the discount service will be able to apply different discounts without altering its source code.

The same principle can be applied to the Discount.
Supposing we want to have a basic discount to be applied extra when a discount is applied.

package com.gkatzioura.solid.ocp;

import java.math.BigDecimal;
import java.math.RoundingMode;

public class BasicDiscount implements Discount {

    @Override
    public BigDecimal apply(BigDecimal price) {

        BigDecimal percent = new BigDecimal("0.01");
        BigDecimal discount = price.multiply(percent);
        return price.subtract(discount.setScale(2, RoundingMode.HALF_UP));
    }
}

By extending the BasicDiscount class we are able to have more discounts with the behaviour of the BasicDiscount and also extend this behaviour without modifying the BasicDiscount sourcecode.

You can find the source code on github. The next principle is the liskov substitution principle.

Also I have compiled a cheat sheet containing a summary of the solid principles.
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