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As you memorized, you can't generalize more than one class. If you want to make sure a class has some operations, then you should use interfaces, because a class can implement many interfaces (a class $C$ implementing an interface $I$ means that $C$ is inheriting $I$).

An interface can only have

  • public static final attributes (=constants)
  • public methods (instance methods)
  • public static methods (class methods)
  • constructors not allowed
  • (in fact, an interface can have default methods and private methods too)


  • to create an abstract type
Machine machine;

Let's say Machine is an interface, it can't be instanced, but we could store inside this variable any object implementing Machine. We can also call on this object any methods defined in Machine.

  • methods do not have code

Most of the time, methods are empty, and it's up to the class implementing the interface to implements the method.

You can add a <<default>> before the method name if this method can be coded inside the interface, but since interfaces do not have attributes, it shouldn't be something you will do often.

Lollipop interface

If your interface is empty, you can use the lollipop interface


  • class1 is implementing interface1
  • class2 is using interface1

Common design


  • ATM is implementing Machine
  • Client is using (a) Machine