— Scott McNealy, Sun Microsystems Chairman, President and ceo

works
()

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inheritance and 
polymorphism
you are here
4
169
superclass
Doctor
worksAtHospital
treatPatient ()
Adds one new
instance variable
Adds one new method
subclasses
Overrides the inherited
treatPatient() method
Adds one new method
Surgeon
treatPatient ()
makeIncision()
FamilyDoctor
makesHouseCalls
giveAdvice ()
one instance variable
one method
Sharpen your pencil
How many instance variables does 
Surgeon have?
How many instance variables does 
FamilyDoctor have?
How many methods does Doctor have?
How many methods does Surgeon have?
How many methods does FamilyDoctor 
have?
Can a FamilyDoctor do treatPatient()?
Can a FamilyDoctor do makeIncision()?
public class Doctor {
boolean worksAtHospital;
void treatPatient() {
// perform a checkup
}
}
public class FamilyDoctor extends Doctor {
boolean makesHouseCalls;
void giveAdvice() {
// give homespun advice
}
}
public class Surgeon extends Doctor{
void treatPatient() {
// perform surgery
}
void makeIncision() {
// make incision (yikes!)
}
}
I inherited my 
procedures so I didn’t 
bother with medical school.
Relax, this won’t hurt a bit. 
(now where did I put that 
power saw...)
An inheritance example:

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170
 
chapter 7
What do these six types have in 
common? This helps you to abstract 
out behaviors. (step 2)
How are these types related? This 
helps you to define the inheritance