Dependencies among classes

The diagram below shows that class B depends of class A.  That means that if
class A's interface changes, class B might need to change.  The opposite isn't
true however.  If B's interface changes, there's no need for A to change. 

An example of this from real life would be if B represented a class involved in
providing a GUI and A represented a class that constitutes part of the
core application.   The GUI may have to change if the core application
changes, but the core application  doesn't need to worry about what changes are
made to the GUI.

    With classes dependencies exist for various reasons: 

• one class has another as part of its data,  e.g. if class B contains class A, then B depends on A.
• one class is a subtype of another, e.g. if B is a subtype of A, then B depends on A.
• one class sends a message to another, e.g.  if an object of type B sends a message to an object of type A, then B depends on A.
• one class mentions another as a parameter to an operation, e.g. if an operation in B has an object of type A as a parameter, then B depends on A.

Only changes to a class’s interface should affect any other class.