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Design Basics: Coupling Version 9/10 CMSC 202. Coupling Version 9/10 2 The degree to which software modules rely upon one another. If module A is coupled.

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Presentation on theme: "Design Basics: Coupling Version 9/10 CMSC 202. Coupling Version 9/10 2 The degree to which software modules rely upon one another. If module A is coupled."— Presentation transcript:

1 Design Basics: Coupling Version 9/10 CMSC 202

2 Coupling Version 9/10 2 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. Class A Class B public class A { private B b;... }

3 Coupling (con’t) Version 9/10 3 Coupling strength is based on the quantity of module coupling points, and the complexity of the coupling points. A C E B DF A C E B DF A C E B DF Low couplingHigher couplingVery high coupling For each example above, what are the concerns when the module C must be modified or replaced?

4 The Low Coupling Situation Version 9/10 4 A C E B DF Low coupling 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 } 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 “has a” dependency (i.e. composition) This is a “uses a” dependency

5 The Very High Coupling Situation Version 9/10 5 A C E B DF 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) It’s now much more complex to make modifications to class C.

6 Coupling (con’t) Version 9/10 6 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? Version 9/10 7 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 Version 9/10 8 Program Project1 Class CashRegister Class Driver Stub 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( ) Test Script (Driver) Code that tests Project1 class functionality and error conditions

9 Sample CashRegister Driver (Partial) Version 9/10 9 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) Version 9/10 10 // 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) Version 9/10 11 package proj1; public class CashRegister {... public boolean addMoney(int twenties, int tens, int fives, int ones) { System.out.println(“addMoney called”); System.out.println(“twenties = “ + twenties); System.out.println(“tens = “ + tens); return (true); } public boolean removeMoney(int twenties, int tens, int fives, int ones) { System.out.println(“removeMoney called”); System.out.println(“twenties = “ + twenties); System.out.println(“tens = “ + tens); return (true); } // other stub methods }

12 Version 9/10 12 A More Complex System Design System Application UI Driver Stub BA C One or more classes ED F Stub: all code necessary to emulate the application interface Driver: all code necessary to fully exercise the application

13 Version 9/10 13 Game Application Controller Class (e.g. Poker) An Example from Later This Semester … UI Model Classes CardDeck Hand other classes? We write this You write this So, in this situation, who writes the application interface?

14 Example 1: Evaluating Coupling Version 9/10 14 public void someMethod(int flag) {... if (flag == 1)... else if (flag == 2)... else if (flag == 3)... else } 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. Relative to coupling, what’s a better way to implement this code? A Method

15 Example 1: A Better Solution Version 9/10 15 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”) } 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

16 Example 2: Evaluating Coupling Version 9/10 16 public int anotherMethod( ) {... if (condition1) {... return (1); } else if (condition2) {... return (2); } else if (condition3) {... return (3); } else } A Method 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. Relative to coupling, what’s a better way to implement this code?

17 Example 2: A Better Solution Version 9/10 17 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 Version 9/10 18 UI (main) Course SectionStudent Date course.getSection(sectionId).getStudent(studentId).getTranscript().getGPA() Transcipt Problem: Create a program to track students registered for all sections of a course. Suppose we want the GPA for a given student. From the UI class, we could: But… more coupling! X X X Better: course.getStudentGPA(studentId) section.getStudentGPA(studentId) XXX student.getTranscript() transcript.getGPA() X


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