Design Principles

Question 1

What is coupling?

Answer 1

The degree of direct knowledge that one element has of another.

Question 2

What is tight coupling?

Answer 2

A class that knows more than it should about the way another class is implemented. Eg
When a java class has another class as an attribute and makes use of it.
// Java program to illustrate
// tight coupling concept
class Subject {
Topic t = new Topic();
public void startReading()
{
t.understand();
}
}
class Topic {
public void understand()
{
System.out.println(“Tight coupling concept”);
}
}

Question 3

What is loose coupling?

Answer 3

A class only knows what another class has through its exposed interface
Eg: Where a java class has a new instance of that class but it isn’t a field of that class
// Java program to illustrate
// tight coupling concept
class Volume
{
public static void main(String args[])
{
Box b = new Box(5,5,5);
System.out.println(b.volume);
}
}
class Box
{
public int volume;
Box(int length, int width, int height)
{
this.volume = length * width * height;
}
}

Question 4

What is Cohesion?

Answer 4

A method used to indicate the degree to which a class has a single, well focused purpose, The higher the cohesiveness of the class the better

Question 5

Aims of Coupling x Cohesion

Answer 5

Easier to develop
Less fragile
Easier to add new features
Easier to maintain

Question 6

Benefits of higher cohesion

Answer 6

Easier to maintain
Much less frequently changed
Classes are more usable than other as designed with well focused purpose.

Question 7

Single responsibility principle

Answer 7

Every class should have a single responsibility and that should be entirely encapsulated by the class. All services should be narrowly aligned with that responsibility

Question 8

God classes and God objects

Answer 8

God class: Keeps track of a lot of data and has lots of responsibility
God object: Object that knows or does too much

Question 9

Purpose of single responsibility

Answer 9

Organize code
More stable
Higher coupling

Question 10

Dependency inversion principle

Answer 10

Depend on abstractions not concretions. Program to interfaces not implementations. Program to the most abstract class possible

Question 11

Purpose of Dependency inversion principle

Answer 11

Concrete classes may change a lot whereas abstract classes usually rarely change.

Question 12

Interface segregation principle

Answer 12

Reduce the side effects of using larger interfaces by breaking application interfaces into smaller ones

Question 13

Rules of interface segregation principle

Answer 13

Don’t make large multipurpose interfaces.
Don’t make clients depend on interfaces that aren’t used

Question 14

Purpose of Interface segregation principle

Answer 14

When something is changed, changes for everyone else will be minimized.

Question 15

How to identify bloated interfaces?

Answer 15

1. See if your interface has too many methods.
2. See if they have low cohesion
3. Can apply to abstract classes

Question 16

What is the open closed principle?

Answer 16

Software entities should be open for extension but closed for modification. This uses abstraction and polymorphism

Question 17

Benefits of open closed principle;

Answer 17

Add new systems by creating new classes of the original interface without needing to modify our implementation

Question 18

Example of open closed

Answer 18

interface for payment
method: processPayment

class1: Revolut payment - implements process

class2: Paypal payment - implements process

class3: payment sys - uses either for options

Question 19

Violations of open closed (3)

Answer 19

1. Modifying existing code: Defeats the purpose of principle, introduces risk of breaking og functionality eg, rather than making new customercontroller, could have made different controller for sql impl
2. God classes: God classes tend to grow and become hard to extend and maintain
3. Conditional statements: Adding new conditional cases requires modifying existing code