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).
"software entities … should be open for extension, but closed for modification."
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.
Interface segregation principle
"many client-specific interfaces are better than one general-purpose interface."
Dependency inversion principle
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