Cohesion shows the relationship within the module. Coupling shows the relative independence between the modules. Cohesion shows the module's relative functional strength. While creating, you should aim for low coupling, i.e., dependency among modules should be less.A good design is the one that has low coupling. Coupling is measured by the number of relations between the modules. That is, the coupling increases as the number of calls between modules increase or the amount of shared data is large. Thus, it can be said that a design with high coupling will have more errors.

1. Software Design
Cohesion & Coupling
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2. Cohesion ?
❑ Degree of interaction within the module.
❑ Cohesion is a measure of the degree to which the
elements of module are functionally related.
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3. Cohesion
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4. class A {
private F1 f1;
private F2 f1;
public void m1() {
[...]
f1.m();
}
public void m2() {
[...]
f2.m();
}
}
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5. Types of Cohesion
Functional | Sequential (best)
Communicational
Procedural
Temporal
Logical
Coincidental (worst)
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6. 1. Coincidental Cohesion
❖ Def. ?
• module performs multiple completely unplanned,
unrelated or random actions.
❖ How could this happen?
• hard organizational rules about module size
❖ Why is this bad?
• degrades maintainability & modules are not
reusable
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7. 2. Logical Cohesion
❖ Def.
• modules defines same logical function and class
one of which is selected by calling module.
❖ Why is this bad?
• interface difficult to understand
• code for more than one action may be intertwined
• difficult to reuse
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8. 3. Temporal Cohesion
❖ Def. ?
• module performs series of actions related within the
same time.
❖ Why is this bad?
• actions weakly related to one another, but
strongly related to actions in other modules
• code spread out -> not maintainable or reusable
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9. 4. Procedural Cohesion
❖ Def. ?
• module performs series of actions in a sequential way.
❖ Why is this bad?
• actions are still weakly related to one another
• not reusable
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10. 5. Communicational Cohesion
❖ Def.
• module performs series of actions related by procedure to
be followed, but in addition all the actions shares the
same data.
❖ Why is this bad?
• still leads to less reusability -> break it up
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11. 6. Sequential Cohesion
❖ Def.
• When elements of module are grouped because the
output of one element serves as input to another element
and so on,
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12. 7. Functional Cohesion
❖ Def.
• module performs exactly one action
❖ Why is this good?
• more reusable
• corrective maintenance easier
fault isolation
reduced regression faults
• easier to extend product
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13. Coupling ?
❑ Degree of interaction between two or more
modules.
❑ A measure of the strength of the inter-connections
between system components.
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14. Coupling
A B
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15. Reasons of coupling
Duplication
Dependency
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16. class A {
private B b;
[..]
public void m() {
b.setZ(b.getX() + b.getY());
}
}
class B {
private int x, y, z;
int getX() { return x; }
int getY() { return y; }
void setZ(int z) { this.z = z; }
}
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17. Types of Coupling
Data (best)
Stamp
Control
Common
Content (worst)
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18. 1. Content Coupling
❖ Def.
• one module directly references contents of the other
❖ Why is this bad?
• almost any change to b requires changes to a
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19. 2. Common Coupling
❖ Def.
• two modules have write access to the same global data
❖ Why is this bad?
• resulting code is unreadable
modules can have side-effects
must read entire module to understand
• difficult to reuse
• module exposed to more data than necessary
cca
ccb
global variable
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20. 3. Control Coupling
❖ Def.
• one module passes an element of control to the other
❖ Why is this bad?
• modules are not independent
module b must know the internal structure of module a
affects reusability
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21. 4. Stamp Coupling
❖ Def.
• data structure is passed as parameter, but called module
operates on only some of individual components
❖ Why is this bad?
• affects understanding
not clear, without reading entire module, which fields of record are
accessed or changed
• unlikely to be reusable
other products have to use the same higher level data structures
• passes more data than necessary
e.g., uncontrolled data access can lead to computer crime
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22. 5. Data Coupling
❖ Def.
• every argument is either a simple argument or a data
structure in which all elements are used by the called
module
❖ Why is this good?
• maintenance is easier
• good design has high cohesion & weak coupling
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