Understanding Modules, Coupling, Cohesion, and Interface Design Principles with Design Pattern Examples

Modules are logical divisions of a system that help decompose complex problems into simpler, manageable parts.

Coupling describes the degree of interdependence between modules; strong coupling makes modules less independent, while weak coupling promotes easier maintenance and testing.

Cohesion measures how closely related the responsibilities within a module are; high cohesion means a module focuses on a single purpose, improving robustness and reusability.

Good interface design follows the six SOLID principles: Single Responsibility, Open/Closed, Liskov Substitution, Dependency Inversion, Interface Segregation, and the Law of Demeter.

Examples of applying these principles include:

Facade Pattern – provides a unified interface to a set of subsystems, reducing client coupling.

Bridge Pattern – separates an abstraction from its implementation, as illustrated by JDBC's Driver and DriverManager.

1 // Developers work with JDBC API without worrying about specific driver implementations
2 Class.forName("com.mysql.jdbc.Driver");
3 Connection conn = DriverManager.getConnection(url, username, password);

Adapter Pattern – isolates third‑party libraries (e.g., log4j) behind a custom interface.

1 // Direct use of log4j creates strong coupling and is discouraged
2 org.apache.log4j.Logger.getRootLogger().info("info");
3 // A wrapper allows swapping the underlying logging framework
4 FRLoggerFactory.getLogger().info("info");