7. Modular design（モジュール 設計）

Modular design is an important technique for designing good quality program systems.

In this part, you will learn: （内容）The definition of module （モジュールの定義）Module strength or cohesion （モジュールの強度 又は結合力 ）Module coupling （モジュール連結）Inter-module communication （モジュールの間の交信） Information hiding （情報隠蔽）

7.1 The definition of module （モジュールの定義）

There is no unique definition of module. Module is just a notion that represents a program unit with the following features:

A module provides functional services. For example, given an input, a module performs a function or behavior.A module can be replaced by another module implementing the same functional specification, with little or no impact on the entire program.

The idea of modular design is similar to that of designingother products, such as cars, houses, computers, and so on.

Specifically, a module can be interpreted as one of the following program units:

A procedure in Pascal.

A function in C.

An operation in general.

A process in DFDs.

A method in Java.

A unit that groups operations.

A class (e.g., a Java class).

These are primitive understanding ofmodule.

These areadvancedunderstanding ofmodule.

Examples

(1) Primitive notion of module: a Java method:

int Swap(int x, int y)

{

int a = 0; //local variable declaration

a = x; //statement-1

x = y; //statement-2

y = a; //statement-3

}

(2) Advanced notion of module:

module A;

variable declarations;

operation-1;

operation-2;

operation-3;

…

operation-n

end-module

A program unit that groups operationstogether.

A Java class: class Calculator { int reg; public Calculator() { initialize reg; } //constructor of the class public int Add(int i) { reg = reg + i; } public int Subtract(int i) { reg = reg – i; } public int Multiply(int i) { reg = reg * i; } }

7.2 Module strength or cohesion（モジュールの強度 又は結合力

）We use the primitive notion of module in the discussions from now on in this part, since we will discuss the related issues based on the advanced notion of module later in this course.

Description: module strength or cohesion is a measure of the extent to which the statements or functions in a module belong together. （モジュールの強度又は結合力は、文又は操作が同じモジュールに所属する程度を測る尺度である）

In general, if statements are related closely in performing a single task or common tasks, they should belong to the same module. （同じタスクを完成するために必要な文又は操作が同じモジュールに所属する訳）

However, since there is no unique interpretation of the meaning of a single task and common tasks, there is no precise standard to define what statements should be used in the same module.

There are some guidelines for designing modules.

Guidelines for designing modulesモジュール設計の指針）

The statements are all necessary to perform one particular task within the overall program structure. For example, Add, Subtract, and Mutiply in class Calculator can be individual modules. The statements encompass a variety of small unrelated tasks, but since they must be performed at the same time, for convenience they are grouped in a single module. For example, the initialization of global variables.The statements perform two or more related tasks that are always performed together and therefore have been placed in the same module. For example, writing data to files, at the end of an execution of a program.

7.3 Module coupling （モジュール連結）

Description: module coupling is a measure of the interdependence between modules, within a program system.

In general, the stronger the module coupling is, the more possible one of the modules is affected by changing the other module. For example, the small families and big family systems can help explain this issue.

In general, module coupling through global variables are stronger than those that couple through arguments (parameters).

Exampleclass Calculator {

int reg;

void Calculator() {

initialize reg;

} //constructor of the class

int Add(int i) {

reg = reg + i;

}

int Subtract(int i) {

reg = reg – i;

}

int Multiply(int i) {

reg = reg * i;

}

}

class Calculator {

int reg;

void Calculator() {

initialize reg;

} //constructor of the class

int Add(int reg, int i) {

reg = reg + i;

}

int Subtract(int reg, int i) {

reg = reg – i;

}

int Multiply(int reg, int i) {

reg = reg * i;

}

}

Use thesame globalvariable reg.

Use nocommonglobalvariable

7.4 Inter-module communication （モジュールの間の交信）

A program is usually composed of related modules.

A module is related to other modules in the manner that either it is invoked (or called) by other modules or it invokes (or calls) other modules.

When a module invokes another module, the communication by passing data items may be necessary. The problem is how modules can communicate with each other, and what are advantages and disadvantages of each communication method.

Communication methods( 交信手段）

Communication by argumentsCommunication by global (or external) variables.

Let’s take the following program as an example to explain the meaning of these two methods.

Problem: input 10 non-zero integers and output the two numbers: (1) the number of integers that is greater than the average of the input integers; (2) the number of integers that is smaller than the average of the input integers. For example,

input: 5, 78, 56, 89, 12, 24, 75, 98, 34, 68 (average = 539 / 10 = 53.9);

output: count1 = 6 (the greater numbers: 78, 56, 89, 75, 98, 68)

count2 = 4 (the smaller numbers: 5, 12, 24, 34)

Two Java-based pseudocodeProgram 1:import java.io.*;public class CountNumbers { //there is no global variables here public static void main(String args[]) { int numbers[] = new int[10]; int count1 = 0, count2 = 0; double average = 0.0; for (int i = 0; i < numbers.length; i++) INPUT numbers[i]; average = CalculateAverage(numbers); count1 = CountGreater(average, numbers); count2 = CountSmaller(average, numbers); OUTPUT count1 and count2;}

double CalculateAverage(int a[]) //obtain average

{

int total = 0;

double ave = 0.0;

for (int j = 0; j < a.length; j++)

total = total + a[j];

ave = total / 10;

return ave;

}

int CountGreater(double ave, int a[]) { for (int k = 0; k < a.length; k++) if (a[k] > ave) count1 = count1 + 1; return count1; }

int CountSmaller(double ave, int a[]) { for (int k = 0; k < a.length; k++) if (a[k] < ave) count2 = count2 + 1; return count2; }}

Program 2:import java.io.*;public class CountNumbers { //there are four global variables here static int numbers[] = new int[10]; static double average = 0.0; static int count1 = 0, count2 = 0;

public static void main(String args[]) {for (int i = 0; i < numbers.length; i++) for (int l = 0; l < numbers.length; l++) numbers[l] = l +1; CalculateAverage(); CountGreaterSmaller(); //compute count1 and count2 System.out.println("Count1:" + count1); System.out.println("Count2:" + count2);}

static void CalculateAverage() { // Obtain average int total = 0; for (int j = 0; j < numbers.length; j++) total = total + numbers[j]; average = total / 10; } static void CountGreaterSmaller() { for (int k = 0; k < numbers.length; k++) if (numbers[k] > average) count1 = count1 + 1; else count2 = count2 + 1; } }

Advantages and disadvantages（利点と弱点）

Communication by global variables strengthens module coupling, therefore decreases the independence of modules. This is not a good situation from the modularity point of view.

However, the efficiency of communication may be high, because there is no need to copy any values when passing arguments to parameters of modules.

Communication by arguments reduces module coupling, therefore improve the independence of modules. This is usually regarded as a good quality of program, because the change of one module will produce less or no impact on other modules. But the efficiency of communication may be low, because there is a need to copy values when passing arguments to parameters of modules.

7.5 Information hiding( 情報隠蔽）

Information hiding is a quality that all of the

implementation details, including local variables

and algorithms, of a module are hidden from other

modules or parts of the program.

In other words, a module can only be used by other modules

through invocation or calling of that module to perform its

task, but cannot access the local variables and code of the

algorithm of the module.

Exampleimport java.io.*;public class CountNumbers { //there are four global variables here static int numbers[] = new int[10]; static double average = 0.0; static int count1 = 0, count2 = 0;

public static void main(String args[]) {for (int i = 0; i < numbers.length; i++) for (int l = 0; l < numbers.length; l++) numbers[l] = l +1; CalculateAverage(); CountGreaterSmaller(); //compute count1 and count2 System.out.println("Count1:" + count1); System.out.println("Count2:" + count2);}