The document discusses several object-oriented (OO) design principles: loose coupling, high cohesion, the single responsibility principle, the dependency inversion principle, the Liskov substitution principle, the open-closed principle, the interface segregation principle, and the law of Demeter. It notes that good OO design aims for loose coupling between components and high cohesion within components, and discusses how each principle supports this goal.

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OO Design Principle
Li-Wei Cheng

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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.

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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.

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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).

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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.

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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

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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

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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.

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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)