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Service Decoupling

• Guideline: Use interface extension to decouple services or restrict the semantic contract.
• Can decouple services from one another, to encourage clients code be as type-precise as possible
  
  
• List adds void add(int, Object), Object get(int), ListIterator listIterator(), etc...
• Can also build trees upwards:
  
  

Restricting Semantics

• Can't just slap interfaces together -- semantics must be compatible (though not necessarily identical)
• Subinterface semantics can be more concrete (more restrictive) than superinterface semantics
• Set doesn't declare any new methods compared to Collection
• Semantic contract of boolean add(Object):
  • Collection - Ensures that this collection contains the specified element.
  • Set - Adds the specified element to this set if it is not already present.