Coupling and Cohesion are two fundamental concepts for measuring the quality of software design. They are inversely related — good design has high cohesion and low coupling.

• Cohesion — How focused is a single module/class? (internal)
• Coupling — How dependent are modules/classes on each other? (external)

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Cohesion refers to how closely related the responsibilities of a single module/class are.

• Class has a single, well-defined purpose
• All methods relate to that purpose
• Easy to understand and maintain

• Class does unrelated things
• Methods are not related to each other
• Hard to understand, name, and maintain

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Coupling refers to how dependent one module/class is on another.

• Classes interact through well-defined interfaces
• Changes in one class rarely affect others

• Classes know too much about each other
• Changes in one class ripple through the system

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Goal	Description
Aim for High Cohesion	Each class should have one clear responsibility
Aim for Low Coupling	Classes should communicate through minimal interfaces
Use DI	Reduces coupling significantly
Program to Interfaces	Reduces implementation coupling
Single Responsibility	Naturally leads to high cohesion

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Concept	Good Design	Bad Design
Cohesion (internal)	High (focused purpose)	Low (does everything)
Coupling (external)	Low (loosely connected)	High (tightly connected)
Goal	High Cohesion + Low Coupling

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