Polymorphism
Hsuan-Tien Lin
Deptartment of CSIE, NTU
OOP Class, May 5, 2009
H.-T. Lin (NTU CSIE) Polymorphism OOP(even) 05/05/2009 0 / 29
Polymorphism: The Motto
One Thing, Many Shapes...
brainstorm: where do we encounter this motto in the real
world?
One Variable, Many Values
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char a ;
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s w i t c h ( a ) {
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case 1 :
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r e t u r n 1 ∗ 1 ;
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case 2 :
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r e t u r n 2 ∗ 2 ;
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case 3 :
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r e t u r n 3 ∗ 3 ;
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. . .
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case 127:
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r e t u r n 127 ∗ 127;
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}
better ways?
mechanism? raw memory interpretation
H.-T. Lin (NTU CSIE) Polymorphism OOP(even) 05/05/2009 2 / 29
One Class, Many Instances
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S t u d e n t s ;
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i f ( s e q u a l s s t u d e n t 1 )
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show ( score1 ) ;
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e l s e i f ( s e q u a l s s t u d e n t 2 )
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show ( score2 ) ;
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. . .
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e l s e i f ( s e q u a l s s t u d e n t 1 0 0 )
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show ( score100 ) ;
better ways?
mechanism? abstraction
One Method Name, Many Parameter Lists
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/ / no o v e r l o a d i n g
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System . o u t . p r i n t S t r i n g ( " abc " ) ;
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System . o u t . p r i n t I n t ( 3 ) ;
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System . o u t . p r i n t D o u b l e ( 4 . 0 ) ;
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/ / o v e r l o a d i n g
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System . o u t . p r i n t ( " abc " ) ;
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System . o u t . p r i n t ( 3 ) ;
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System . o u t . p r i n t ( 4 . 0 ) ;
mechanism? signature by name + parameter types
H.-T. Lin (NTU CSIE) Polymorphism OOP(even) 05/05/2009 4 / 29
A Twist in Method Overloading
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/ / o v e r l o a d i n g
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System . o u t . p r i n t ( " abc " ) ;
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System . o u t . p r i n t ( 3 ) ;
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System . o u t . p r i n t ( 4 . 0 ) ;
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/ / a t w i s t ( o f course , n o t e x a c t l y workable
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S t r i n g ( " abc " ) . p r i n t ( ) ;
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I n t e g e r ( 3 ) . p r i n t ( ) ;
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Double ( 4 . 0 ) . p r i n t ( ) ;
mechanism? just write print() for every class (we’ll see)
Polymorphic Behavior
all cases above: some polymorphic behavior BUT almost no one calls them real “polymorphism”
Why Not?
H.-T. Lin (NTU CSIE) Polymorphism OOP(even) 05/05/2009 6 / 29
Polymorphic Behavior on Known Stuff
polymorphic behavior of known primitive-type variables
polymorphic behavior of known classes (extended types)
polymorphic behavior of known parameter types
Unknown Stuff: Future Extensions
Backward Compatibility inheritance hierarchy
Forward Advance
newly added variables/methods
H.-T. Lin (NTU CSIE) Polymorphism OOP(even) 05/05/2009 8 / 29
Polymorphism of Object Content
one advanced content, many compatible ways to
access
Polymorphism of Object References
one compatible reference, many advanced contents to point to
H.-T. Lin (NTU CSIE) Polymorphism OOP(even) 05/05/2009 10 / 29
Reference Upcast versus Reference Downcast
Upcast
simple (backward compatibility)
Downcast
need to check whether content fits (forward advance)
need RTTI (run-time type information/identification) to make
downcast work (where did we see it?)
Content/Reference Polymorphism
a possible mechanism: shared prefix
H.-T. Lin (NTU CSIE) Polymorphism OOP(even) 05/05/2009 12 / 29
Content/Reference Polymorphism
but not the only mechanism: how about chains?
Content/Reference Polymorphism: Summary
backward compatibility handled
forward advance: only via downcast (RTTI)
simpler mechanism for forward advance?
H.-T. Lin (NTU CSIE) Polymorphism OOP(even) 05/05/2009 14 / 29
Our Needs in Forward Advance
Our Needs in Forward Advance
new instance variables for advanced state
new instance methods to manipulate new variables give new meanings to existing instance variables give new meanings to existing instance methods
write an “updated but compatible” version of existing method
H.-T. Lin (NTU CSIE) Polymorphism OOP(even) 05/05/2009 16 / 29
Method Overriding (Virtual Function)
calls the updated version through an
upper-level reference
Method Invocation Polymorphism
one method (via upper-level reference), many possible extended behaviors
H.-T. Lin (NTU CSIE) Polymorphism OOP(even) 05/05/2009 18 / 29
Object.equals
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c l a s s C o o r d i n a t e extends O b j e c t {
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double x , y ;
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b o o l e q u a l s ( O b j e c t o ) {
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i f ( o i n s t a n c e o f C o o r d i n a t e ) {
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C o o r d i n a t e c = ( C o o r d i n a t e ) o ;
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r e t u r n ( c . x == t h i s . x && c . y == t h i s . y ) ;
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}
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r e t u r n f a l s e ;
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}
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}
Object.toString
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C o o r d i n a t e c = new C o o r d i n a t e ( ) ;
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System . o u t . p r i n t l n ( c ) ;
H.-T. Lin (NTU CSIE) Polymorphism OOP(even) 05/05/2009 20 / 29
Twist Revisited
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System . o u t . p r i n t ( " abc " ) ;
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System . o u t . p r i n t ( 3 ) ;
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System . o u t . p r i n t ( 4 . 0 ) ;
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/ / a t w i s t ( o f course , n o t e x a c t l y workable
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S t r i n g ( " abc " ) . p r i n t ( ) ;
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I n t e g e r ( 3 ) . p r i n t ( ) ;
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Double ( 4 . 0 ) . p r i n t ( ) ;
System.out.print(Object) can have polymorphic behavior by internally
calling the updated Object.print() (actually, Object.toString()) without
overloading
A Trip with java.lang.Object
H.-T. Lin (NTU CSIE) Polymorphism OOP(even) 05/05/2009 22 / 29
A Trip with java.lang.String
V-Table: A Possible Mechanism
again, not the only mechanism
H.-T. Lin (NTU CSIE) Polymorphism OOP(even) 05/05/2009 24 / 29
RTTI revisited
all we need is a link to the class area (stores name, vtable, etc.) where is the link? java.lang.Object
how to access? Object.getClass()
each area is an instance of java.lang.Class
An Advanced Use
How about creating an object with dynamic class name?!
see HW5
H.-T. Lin (NTU CSIE) Polymorphism OOP(even) 05/05/2009 26 / 29
