This model of a Music Shop structure demonstrates:

• unit testing as part of a TDD approach
• composition to realistically illustrate the type of association between accessory and instrument classes
• appropriate use of packages for organisation of code.
• use of interfaces to abstract out behaviours and distinguish between accessory and instrument classes
• abstract classes to keep the code DRY
• meaningfully named methods and variables to ensure clean code
• method overriding to reveal the polymorphic possibilities of the interface choice