[ajug-members] correct interface programming

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am i saying this correctly?, that which is below...  -barclay

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if i program to an interface, i do not have to worry about the number of 
interfaces that are implemented, do i?  but do i not have to use all methods 
in an implemented interface?  i do have to- it's a contract. correct? and 
because you can only extend one class, you have no contract there. correct?  
can someone see what my question is?

a tree; a tree can grow, a tree can provide shade; and a tree can provide 
lumber for table-tennis.

so my tree class is designed...

then if i am implementing a 'tree'- it depends on which implementation i use 
thus have a contract to use certain methods
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