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OO Design Principle
Li-Wei Cheng

2. OO design principles - 1
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 A good OO design must be:
 Loose coupling
 Each of its components has, or makes use of, little or no
knowledge of definitions of other separate components.
 High cohesion
 How strongly-related the responsibilities of a single module
are.
 If the methods that serve the given class tend to be similar
in many aspects, then the class is said to have high
cohesion.

3. OO design principles - 2
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 Single Responsibility Principle
 Every class only has single responsibility.
 The responsibility is the only reason to change class.
 Ex:
A module that compiles and print the report.
Such a module can be changed for two reasons.
Thus, the module should be divided into two modules,
one for compiles and one for print the report.
Keeps each class/module has single responsibility.

4. OO design principles - 3
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 Dependency Inversion Principle
 High-level modules should not depends on low level
modules. Both should depends on abstraction.
 Abstraction should not upon on details. Details should
depends upon abstraction.
 Applying the dependency inversion principle can also
be seen as applying the Adapter Pattern.
 Liskov Substitution Principle
 If S is a subtype of T, then objects of type T in a
program may be replaced with objects of type S
without altering any of the desirable properties of that
program (e.g. correctness).

5. OO design principles - 4
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 Open-Closed Principle
 Software entities (classes, modules, functions, etc.) should
be open for extension, but closed for modification.
 Code obey this rule needs no efforts on code review,
unit tests, and other procedure to qualify it when it do
extension.

6. OO design principles - 5
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 Open-Closed Principle
 It can be used in two ways: (both use inheritance)
 Meyer’s Open/Closed Principle (OLD)
 Reuse class through inheritance
 Derived subclass might or might not have the same interface as
the original class
 The existing implementation is closed to modifications, and new
implementations need not implement the existing interface
 Polymorphic Open/Closed Principle (NEW, Powerful)
 Inheritance from abstract base class
 Interface specifications can be reused through inheritance but
implementation need not be
 The existing interface is closed to modifications and new
implementations must, at a minimum, implement that interface

7. OO design principles - 6
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 Interface Separate Principle
 Clients should not forced to implement interfaces they
don’t use
 Instead of one fat interface many small interfaces are
preferred based on groups of methods, each one
serving one sub-module
 Similar to high cohesion

8. OO design principles - 7
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 Law of Demeter (Principle of Least Knowledge)
 A specific case of loose coupling.
 Use only one dot. (Don’t use a.getB().method())
 Each unit should have only limited knowledge about
other units: only units “closely” related to the current
unit.
 Each unit should only talk to its friends; don’t talk to
strangers.
 Only talk to your immediate friends.

9. OO design principles - 8
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 SOLID = 5 OO Design Principles
 Single Responsibility Principle
 Open/Closed Principle
 Liskov Substitution Principle
 Interface Separate Principle
 Dependency Inversion Principle
 Law of Demeter (Opt)