Question

You were asked by a bank to create a system for them. Your code has one main abstract class: ACCOUNT. Knowing that, discuss the SOLID code principles. Discuss the definition, uses, advantages, and disadvantages of each principle. Finally, explain how you would apply each principle to your ACCOUNT (and other classes if you need) project.

Please be as specific as possible. Provide as much detail as possible.

Answer 1

The basic required operations are

1. Create new account
2. Deposit transaction
3. With drawl transaction
4. Printing of statements(transactions)
5. Balance Inquiry.

Background:

Many years ago monolithic approach was used for software development.

Monolithic approach means that a single large program, there was no modularity.

So , fixing bugs, maintenance was difficult.

Solution came in the form of code modularity. it means that develop separate modules and finally combine them to build a complete project.

Modular approach offers following benefits.

a)errors can be found easily.

b)new functionality(module) can be added easily in the existing system.

c)maintenance is easy.

As stated in the problem, the main abstract class is ACCOUNT

This class can have both type of methods(abstract and non abstract methods).

Object of abstract class can not be created. Abstract class can not be instantiated.

It’s child class can be instantiated.

It means there will be a need of inheritance. There may be child classes of ACCOUNT abstract class.

SOLID code principles are

S is single responsibility principle (SRP)

O open closed principle (OCP)

L Liskov substitution principle (LSP)

I interface segregation principle (ISP)

D Dependency inversion principle (DIP)

single responsibility principle (SRP):- says that one class should take only one responsibility.

It gives modular approach of development and makes code maintenance easy.

Hence there should be separate classes for

1. Create new account
2. Deposit transaction
3. With drawl transaction
4. Printing of statements(transactions)
5. Balance enquiry

open closed principle (OCP) A class should be open for extension but closed for modification.

It means, if there is a need to full fill new requirement, then derive a new class from the existing class for the new requirement and do not modify the existing class(old class) to full fill the new requirement.

Other wise every time whenever there is a new requirement, the existing class will be modified, and it makes very difficult to manage the code.

So,In the given problem, apart from the anticipated operations, if new operation is required, then instead of modifying the existing class, new child class should be derived from the suitable class(base class).

Liskov substitution principle (LSP)

sub classes must be substitutable for the super class.

if X is a subtype of Y, then objects of type Y may be replaced with objects of type X without altering any of the desirable properties of the program .

it means there should be upcasting.

Code that follows LSP is loosely dependent to each other and offers code reusability.

As per the given problem, new operations(such as transfer of money from one account to another account ) can be easily implemented.

interface segregation principle (ISP):-

client should not be forced to depend on methods it does not use.

Client which has relevant methods should follow the particular interface, because in case of interface,

The implementing class is forced to follow the signature of abstract method along with return type.

There are so many restrictions.

So, why put unnecessary restrictions?

interface segregation principle allows flexibility in the code maintenance and addition of new functionalities.

Dependency inversion principle (DIP)

Software modules should not be dependent on each other.

If required, then good programming practice is to minimize the dependency between modules.

Because, if there is strong dependency among modules, then modification in one module causes the modification in other module, and it makes program maintenance difficult.

As per the given problem there may be various types account holders, such as saving account, current account and so on. And both types of account may have different types of requirements. these operations should not dependent on each other.