1. Software Design Principles
August 19, 2005

2. Software design principles
The single-responsibility principle
The open-closed principle
The Liskov substitution principle
The dependency inversion principle
The interface segregation principle

3. The single-responsibility principle
“A class should have only one reason to change.” -- Robert Martin
A responsibility = a reason to change
Separate coupled responsibilities into separate classes

4. The single-responsibility principle
Example:
Often we need to sort students by their name, or ssn. So one may make Class Student implement the Java Comparable interface.
class Student implements Comparable { …
int compareTo(Object o) { … } };
Student is a business entity, it does not know in what order it should be sorted since the order of sorting is imposed by the client of Student.
Worse: every time students need to be ordered differently, we have to recompile Student and all its client.
Cause of the problems: we bundled two separate responsibilities (i.e., student as a business entity with ordering) into one class – a violation of SRP

5. The single-responsibility principle
Register
Add(Course d, Student s);
Student
SSN
Name
getSSN()
getName()
major
getMajor()
int compareTo()
Comparable
int compareTo()
When a new requirement needs to sort students in a different order, Student, Register, and AClient all need to be recompiled, even Register has nothing to do with any ordering of Students.
AClient
op() { ;}
It invokes
Collections.sort(aListofStudents);

6. The single-responsibility principle
Register
Add(Course d, Student s);
Comparator
int compare(Object o1, Object o2)
Student
SSN
Name
getSSN()
getName()
major
getMajor()
StudentByName
int compare(Object o1, Object o2)
ClientA
op() { ;}
StudentBySSN
int compare(Object o1, Object o2)
The solution is to separate the two responsibilities into two separate classes and use another version of Collections.sort().
ClientB
op() { ;}
It invokes
Collections.sort(aListofStudents, StudentBySSN);

7. The single-responsibility principle
Computational
Geometry
Application
Rectangle
+draw():void
+area():integer
Graphical
Application
GUI
Class Rectangle may be forced to make changes from two different unrelated sources. One is from the Computational Geometry Application (CGA). E.g., adding area function for length and width of type double. The other is from Graphical Application (GA). E.g., add draw() in Windows XP to the existing draw in X Windows. A change from either of the two source would still cause the other application to recompile.

8. The single-responsibility principle
Computational
Geometry
Application
Graphical
Application
Geometric
Rectangle
+area():double
Graphic
Rectangle
+draw():void
GUI
Package CGA is no longer dependent on graphical side of Rectangle and thus it becomes independent of package GUI. Any change caused by graphical application no longer requires CGA to be recompiled.
However, any changes from the CGA side may cause GA to be recompiled.

9. The single-responsibility principle
Computational
Geometry
Application
Graphical
Application
Rectangle
-double length
-double width
+getLength():dobule
+getWidth():dobule
Geometric
Rectangle
+area():double
Graphic
Rectangle
+draw():void
GUI
Class Rectangle contains the most primitive attributes and operations of rectangles. Classes GeometricRectangle and GraphicRectangle are independent of each other. A change from either side of CGA or GA, it would not cause the other side to be recompiled.
NOTE: this does not violate LSP, since Rectangle does not have any client.

10. The open-closed principle
“Software entities (classes, modules, functions, etc,) should be open for extension, but closed for modification.” – R. Martin
To make a class open for extension, closed for modification, program the class to interfaces (or abstract classes), not implementation (concrete classes).

11. The open-closed principle
Employee
+int EmpType
Faculty
+getOffice()
Staff
+getDept()
Secretary
+getTypeSpeed()
Engineer
+getEngTYpe()
void printEmpRoster(Employee[] emps) {
for (int i; i<emps.size(); i++) {
if (emps[i].empType == FACULTY)
printfFaculty((Faculty)emps[i]);
else if (emps[i].empType ==STAFF)
printStaff((Staff)emps[i]);
else if (emps[i].empType == SECRETARY)
printSecretary((Secretary)emps[i]); }
What if we need to add Engineer??

12. The open-closed principle
Employee
+printInfo()
Faculty
+printInfo()
Staff
+printInfo()
Secretary
+printInfo
Engineer
+printInfo()
void printEmpRoster(Employee[] emps) {
for (int i; i<emps.size(); i++) {
emps[i].printInfo(); }
When Engineer is added, printEmpRoster() does not even need to recompile. PrintEmpRoster() is open to extension, closed for modification.

13. The open-closed principle
Three versions of SORT
sort(List list)
Elements of list must implement Comparable interface
sort(List list, StringComparator sc)
Elements of list are not required to implement Comparable
StringComparator orders objects of String only
Sort(List list, Comparator comp)
Comparator may compare objects of any type.
Open to extension since it can sort objects of any type at any order specified in the second parameter.

14. The Liskov substitution principle
“Subtypes must be substitutable for their base types.” – R. Martin
Demand no more, promise no less
Demand no more: the subclass would accept any arguments that the superclass would accept.
Promise no less: Any assumption that is valid when the superclass is used must be valid when the subclass is used.
Interface inheritance – The LSP should be conformed to.
Implementation inheritance – use composition instead of inheritance (in Java) or use private base classes (in C++).

15. The Liskov substitution principle
Implementation inheritance
When you use List to implement Queue (in Java), use composition, not inheritance.
The intention is that you use only List’s implementation
<foundation>
List
+insert()
+delete()
+find()
List
+insert()
+delete()
+find()
List
+insert()
+delete()
+find()
MyList
Queue
+enqueue()
+dequeue()
+isEmpty()
Queue
+enqueue()
+dequeue()
+isEmpty()
Queue
+enqueue()
+dequeue()
+isEmpty()

16. The Liskov substitution principle
class Square extends Rectangle {
public void setWidth(int width) {
super.setWidth(width);
super.setHeight(width); }
public void setHeight(int height) {
super.setHeight(height);
super.setWidth(height);
void clientOfRectangle(Rectangle r) {
r.setWidth(10);
r.setHeight(20);
print(r.area());
Rectangle r = new Square(…);
clientOfRectangle(r); // what would be printed?
Rectangle
-int width;
-int height
+getWidth()
+setWidth()
+getHeight()
+setHeight()
+area();
IS-A
Square
+getWidth()
+setWidth()
+getHeight()
+setHeight()

17. The Liskov substitution principle
Rectangle and Square
Invariant of Rectangle: width and height are independent of each other (which can be expected from the setWidth and setHeight operations)
Square violates the width-height-independence invariant of Rectangle

18. The Liskov substitution principle
There are cases in which the substitutability may not be needed
Generalization: we found that Faculty, Staff, Secretary and Engineer all have the same set of attributes and operations, so we created the Employee as a placeholder for those common properties.
For the system, there is no client for Employee
Thus, the four subclasses do not need to be substitutable
Actually, there is no way we can tell whether they are or not.
Employee
+printInfo()
Faculty
+printInfo()
Staff
+printInfo()
Secretary
+printInfo
Engineer
+printInfo()
generalization

19. The dependency inversion principle
“Abstraction should not depend on details. Details should depend on abstraction.” – R. Martin
High-level concepts are more stable than low-level implementation

20. The dependency inversion principle
Policy Layer
High-level modules make calls to low-level modules.
Mechanism Layer
Utility Layer
The upper-level layer is dependent upon lower-level layers.

21. The dependency inversion principle
Dependency Inversion: Lower-level layers is dependent upon upper-level layers.
Policy
Policy Layer
Mechanism
Mechanism Layer
<<interface>>
Policy
Service
Utility
<<interface>>
Mechanism
Service
Utility
Layer
Ownership Inversion: The client (upper-level layer) owns the interface, not the lower-level layers

22. The interface segregation principle
“Clients should not be forced to depend on methods that they do not use.” – R. Martin
When we bundle functions for different clients into one interface/class, we create unnecessary coupling among the clients.
When one client causes the interface to change, all other clients are forced to recompile.

23. Software design principles - summary
The single-responsibility principle
There is only one source that may the class to change
The open-closed principle
Open to extension, closed for modification
The Liskov substitution principle
A subclass must substitutable for its base class
The dependency inversion principle
Low-level (implementation, utility) classes should be dependent on high-level (conceptual, policy) classes
The interface segregation principle
A client should not be forced to depend on methods it does not use.