Tag: java basics

• Abstractions should not depend upon details
• Details should not depend upon abstractions
• Important that higher level and lower level objects depend on the same abstract interaction
• This is not the same as Dependency Injection – which is how objects obtain dependent objects

Clients should not be forced to depend upon interfaces that they do not use.

• Make fine grained interfaces that are client specific
• Many client specific interfaces are better than one “general purpose” interface
• Keep your components focused and minimize dependencies between them
• Notice relationship to the Single Responsibility Principle?
• avoid ‘god’ interfaces

If it looks like a duck and quacks like a duck but it needs batteries, you probably have the wrong abstraction.

• By Barbara Liskov, in 1998
• Objects in a program would be replaceable with instances of their subtypes WITHOUT altering the correctness of the program.
• Violations will often fail the “Is a” test.
• A Square “Is a” Rectangle
• However, a Rectangle “Is Not” a Square

Software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification.

• Your classes should be open for extension
• But closed for modification
• You should be able to extend a classes behavior, without modifying it.
• Use private variables with getters and setters – ONLY when you need them.
• Use abstract base classes