Software Engineering: OO Basics and Principles

OO Basics:

• Abstraction
• Encapsulation
• Polymorphism
• Inheritance

OO Principles:

• Encapsulate what varies
• Favor composition over inheritance
• Program to interfaces, not implementations
• Strive for loosely-coupled designs between objects that interact
• Classes should be open for extension but closed for modification
• Depend on abstraction. Don't depend on concrete classes

Design Patterns: Creational - Singleton

Singleton Pattern:

• a class manages the creation of its one and only instance through a static variable
• no Public constructors, constructors are declared as Private
• a Singleton object is not instantiated using the new() operator but calling a static method to request for an instance
• may implement "lazy instantiation", if an instance is not needed then it doesn't get created
• "eager instantiation" may be implemented if the overhead of creation and the runtime aspects of the Singleton is not burdensome
• Singletons are meant to be used sparingly

Lazy Instantiation:

in Java:

public class Singleton {
private static Singleton uniqueInstance;

private Singleton() {}

public static Singleton getInstance() {
if (uniqueInstance == null) {
uniqueInstance = new Singleton();
}
return uniqueInstance;
}
}

Eager Instantiation:

in C#:

public sealed class Singleton
{
static readonly Singleton uniqueInstance = new Singleton();

Singleton() {}

public static Singleton getInstance {
get { return uniqueInstance; }
}
}

in Java:

public class Singleton {
private static Singleton uniqueInstance = new Singleton();

private Singleton() {}

public static Singleton getInstance() {
return uniqueInstance;
}
}

C# Notes: Value Types

3 General Value Types:

1. Struct / User-defined Types
2. Enum Types
3. Simple / Built-in Types

Notes on Value Types:

• contains the actual data instead of just a reference to the data.
• stored in the stack memory
• an operation to a copy of a variable doesn't affect the other copies
• derived implicitly from System.ValueType
• has an implicit default constructor that initialises the default value
• may contain a NULL value