Single responsibility principle

A class should have only a single responsibility (i.e. changes to only one part of the software's specification should be able to affect the specification of the class).

A class should have only one reason to change. Separate GUI and the business logic. Low coupling, high cohesion.

Open/closed principle

Software entities should be open for extension, but closed for modification.

However, realistically you don't know which bits need to be extensible.

Liskov substitution principle

Objects in a program should be replaceable with instances of their subtypes without altering the correctness of that program. See also design by contract.

Bird class with fly method. Duck is a bird, seagull is a bird, ostrich isn't a bird because it doesn't implement fly.

Interface segregation principle

Many client-specific interfaces are better than one general-purpose interface.

Use small interfaces not fat ones.

Dependency inversion principle

One should "depend upon abstractions, not concretions."


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