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CMSC 202

Design Basics:

Coupling

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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.

ClassA

ClassB

public class A { private B b; ...}

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Coupling (con’t)

• Coupling strength is based on the– quantity of module coupling points, and– the complexity of the coupling points.

A

C

E

B

D F

A

C

E

B

D F

A

C

E

B

D 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?

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The Low Coupling Situation

A

C

E

B

D F

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

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The Very High Coupling Situation

A

C

E

B

D F

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.

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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.

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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.

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ProgramProgram

Project 1 Design

Project1Class

CashRegisterClass

Driver

Stub

Stub: All code necessary to emulate the CashRegister class interfaceDriver: 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

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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 lockedSystem.out.println();cr2.unlock();System.out.println(cr2.toString()); // register is now unlockedSystem.out.println();cr2.lock();System.out.println(cr2.toString()); // register is locked againSystem.out.println();

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Sample CashRegister Driver (Partial, con’t)// Testing removeMoney()

System.out.println("***Testing removeMoney()***");CashRegister cr5 = new CashRegister();System.out.println(cr5.toString()); // register is lockedcr5.unlock(); // unlock registerSystem.out.println(cr5.toString()); // register is unlockedSystem.out.println("Success = " + cr5.addMoney(5, 6, 7, 8)); // add moneySystem.out.println(cr5.toString()); // successSystem.out.println("Success = " + cr5.removeMoney(-1, 1, 1, 1)); // negative $System.out.println(cr5.toString()); // failSystem.out.println("Success = " + cr5.removeMoney(4, 3, 2, 1)); // good $System.out.println(cr5.toString()); // successSystem.out.println("Success = " + cr5.removeMoney(10, 1, 1, 1)); // too much $System.out.println(cr5.toString()); // failcr5.lock(); // lock registerSystem.out.println("Success = " + cr5.removeMoney(1, 1, 1, 1)); // lockedSystem.out.println(cr5.toString()); // failSystem.out.println();

} // end main( )

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Sample Project1 Stub (Partial)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); 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); System.out.println(“tens = “ + tens); return (true);}// other stub methods

}

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A More Complex System Design

SystemSystem

ApplicationUI

Driver

Stub

BA

C

One or more classes

ED

F

One or more classes

Stub: all code necessary to emulate the application interfaceDriver: all code necessary to fully exercise the application

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GameGame

Application

Controller Class(e.g. Poker)

An Example from Later This Semester …

UI

Model Classes

CardDeck

Handother classes?

We write this

You write this

So, in this situation, who writes the application interface?

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Example 1: Evaluating Coupling

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

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Example 1: A Better Solution

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

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Example 2: Evaluating Coupling

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?

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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.

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Example 3: Evaluating Coupling

UI(main)

Course

Section Student

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

XBetter:

course.getStudentGPA(studentId)

section.getStudentGPA(studentId)

X XX

student.getTranscript()

transcript.getGPA()

X