Software Module Coupling Basics

1. CMSC 202 Design Basics: Coupling

2. Coupling • The degree to which software modules rely upon one another. • If module A is coupled to module B, module A has a dependency on module B. • For example, if class A below has an instance variable of class type B (composition), • A is coupled to B, • A depends on B, but • B is not coupled to A and • B does not depend on A. public class A { private B b; ... } Class A Class B

3. Coupling (con’t) • Coupling strength is based on the • quantity of module coupling points, and • the complexity of the coupling points. A A A B B B C C C D E F D E F D E F Low coupling Higher coupling Very high coupling For each example above, what are the concerns when the module C must be modified or replaced?

4. The Low Coupling Situation This is a “has a” dependency (i.e. composition) public class A { . . . private B bVar = new B( ); private C cVar = new C( ); // instance variable of type C . . . private void aMethod1( ) { . . . int x = cVar.cMethod1( ); // call to a method in class C . . . } // other methods which may call methods in class C } public class C { . . . private D dVar = new D( ); // instance variable of type D private E eVar = new E( ); // instance variable of type E . . . public int cMethod1( ) { // called from class A F fVar = new F( ); . . . // code that uses fVar } // other methods that are called by methods in class A // other methods that call methods in classes D and E } A B C D E F Low coupling • C must be aware of: • Which of its methods are used by class A • What it needs from classes D, E, and F (instance variables and methods) This is a “uses a” dependency

5. The Very High Coupling Situation It’s now much more complex to make modifications to class C. A B • C must be aware of: • Which of its methods are used by classes A, B, and D • What it needs from classes B, D, E, and F (instance variables and methods) C D E F

6. Coupling (con’t) • Want weak (loose) coupling • Cannot have zero coupling, so our goals are to • minimize coupling, • weaken (loosen) coupling, and • most importantly, to control coupling. • Every coupling point is intentional. • Every coupling point has a well-defined interface.

7. So, What’s a Module? • Any “chunk” of a software system or program • program • function • method • class • cluster of functions • cluster of classes • other … • Coupling must be controlled at every level of a software system or program.

8. Project 1 Design Program Stub Project1 Class CashRegister Class Test Script (Driver) Code that tests Project1 class functionality and error conditions Driver Stub: All code necessary to emulate the CashRegister class interface Driver: All code necessary to fully exercise the CashRegister class (can be a separate class or CashRegister’s main( )

9. Sample CashRegister Driver (Partial) • public static void main( ) { • // Testing constructor and toString() • System.out.println("***Testing constructor() and toString()***"); • CashRegister cr1 = new CashRegister(); • System.out.println(cr1.toString()); • System.out.println(); • // Testing unlock() and lock() • System.out.println("***Testing lock() and unlock()***"); • CashRegister cr2 = new CashRegister(); • System.out.println(cr2.toString()); // register is locked • System.out.println(); • cr2.unlock(); • System.out.println(cr2.toString()); // register is now unlocked • System.out.println(); • cr2.lock(); • System.out.println(cr2.toString()); // register is locked again • System.out.println();

10. Sample CashRegister Driver (Partial, con’t) • // Testing removeMoney() • System.out.println("***Testing removeMoney()***"); • CashRegister cr5 = new CashRegister(); • System.out.println(cr5.toString()); // register is locked • cr5.unlock(); // unlock register • System.out.println(cr5.toString()); // register is unlocked • System.out.println("Success = " + cr5.addMoney(5, 6, 7, 8)); // add money • System.out.println(cr5.toString()); // success • System.out.println("Success = " + cr5.removeMoney(-1, 1, 1, 1)); // negative $ • System.out.println(cr5.toString()); // fail • System.out.println("Success = " + cr5.removeMoney(4, 3, 2, 1)); // good $ • System.out.println(cr5.toString()); // success • System.out.println("Success = " + cr5.removeMoney(10, 1, 1, 1)); // too much $ • System.out.println(cr5.toString()); // fail • cr5.lock(); // lock register • System.out.println("Success = " + cr5.removeMoney(1, 1, 1, 1)); // locked • System.out.println(cr5.toString()); // fail • System.out.println(); • } // end main( )

11. Sample Project1 Stub (Partial) • package proj1; • publicclass CashRegister { • . . . • publicboolean addMoney(int twenties, int tens, int fives, int ones) { • System.out.println(“addMoney called”); • System.out.println(“twenties = “ + twenties); • System.out.println(“tens = “ + tens); • System.out.println(“tens = “ + tens); • return (true); • } • publicboolean removeMoney(int twenties, int tens, int fives, int ones) { • System.out.println(“removeMoney called”); • System.out.println(“twenties = “ + twenties); • System.out.println(“tens = “ + tens); • System.out.println(“tens = “ + tens); • return (true); • } • // other stub methods • }

12. A More Complex System Design System Stub Application UI D E A B F C One or more classes One or more classes Driver Stub: all code necessary to emulate the application interface Driver: all code necessary to fully exercise the application

13. An Example from Later This Semester … Game You write this Application UI Controller Class (e.g. Poker) Model Classes Deck Card We write this Hand other classes? So, in this situation, who writes the application interface?

14. Example 1: Evaluating Coupling • This code is very strongly coupled to any other internal or external code that calls it. • Yes, there is only one coupling point between the caller and someMethod( ). But it is very strong (complex). • The calling code must be aware of the meaning of all flag values. • someMethod must be careful if it adds flag values, deletes flag values, or changes the meaning of any flag values. A Method public void someMethod(int flag) { . . . if (flag == 1) . . . else if (flag == 2) . . . else if (flag == 3) . . . else . . . . . . } Relative to coupling, what’s a better way to implement this code?

15. Example 1: A Better Solution • Each of these methods is more cohesive than the method on the previous slide. They each perform one specific task. • Yes, the caller needs to know which method to call. But a change to one of these methods will not affect any of the other methods. • If new functionality is needed by the caller, a new corresponding method is written. The calling code then simply tests for the newly added flag value and calls the new method. • The number of coupling points has increased, but each is weak (loose). Replacement Methods public void someMethod1( ) { // method to call if (flag == 1) } public void someMethod2( ) { // method to call if (flag == 2) } . . . public void someMethodN( ) { // method to call if (flag == “N”) }

16. Example 2: Evaluating Coupling • Once again, this code is very strongly coupled to any other internal or external code that calls it. • Again, the coupling point between the caller and anotherMethod( ) is very strong (complex). • The calling code must be aware of the meaning of all return values. • anotherMethod( ) must be careful if it adds flag conditions, deletes flag conditions, or changes the meaning of any flag conditions. A Method public int anotherMethod( ) { . . . if (condition1) { . . . return (1); } else if (condition2) { . . . return (2); } else if (condition3) { . . . return (3); } else . . . . . . } Relative to coupling, what’s a better way to implement this code?

17. Example 2: A Better Solution • This one is not as easy to get around. • It’s common for predefined library functions/methods to return flags that may have many values. • If it’s possible to compute the condition to be tested outside of anotherMethod( ), then break up anotherMethod( ) into separate methods. • If the condition to be tested must be computed inside of anotherMethod( ), you just need to deal carefully with the coupling.

18. Example 3: Evaluating Coupling Problem: Create a program to track students registered for all sections of a course. Course UI (main) Better: But… more coupling! X X X course.getStudentGPA(studentId) Section Student Transcipt section.getStudentGPA(studentId) Date Suppose we want the GPA for a given student. From the UI class, we could: X X X course.getSection(sectionId).getStudent(studentId).getTranscript().getGPA()