COMP 250. Lecture 32. polymorphism. Nov. 25, 2016

Transcription

1 COMP 250 Lecture 32 polymorphism Nov. 25,

2 Recall example from lecture 30 class String serialnumber Person owner void bark() {print woof } : my = new (); my.bark();?????? extends extends class void hunt () void bark() {print aowwwuuu } class void fight () void bark() {print Arh! Arh! Arh! }

3 Recall example from lecture 30 class String serialnumber Person owner void bark() {print woof } : my = new (); my.bark(); aowwwuuu extends extends class void hunt () void bark() {print aowwwuuu } class void fight () void bark() {print Arh! Arh! Arh! }

4 Polymorphism poly = multiple morph = form We will look at sub-type polymorphism. More general discussion about this in higher level courses e.g. COMP 302.

5 Polymorphism Compile time: Suppose a reference variable has a declared type: class C. C var; Runtime: That reference variable can reference any of class C or that extends class C.

6 Polymorphism Compile time: Suppose a reference variable has a declared type: abstract class A. A var; Runtime: That reference variable can reference any whose class extends abstract class A.

7 Polymorphism Compile time: Suppose a reference variable has a declared type: interface I. I var; Runtime: That reference variable can reference any whose class implements interface I.

8 boolean b; Object obj; : if ( b ) obj = new float[23]; else obj = new (); System.out.print( obj ); // calls tostring()

9 How does polymorphism work? How does all this class relationship stuff work in a running program? (Sketch only.)

10 Java Class Descriptors A is an that contains all the information about a class that is used in a running program. String LinkedList

11 Class Descriptors class name fields (names, types) methods (names, parameters, implementation) reference to superclass.

12 Q: Each is an instance of a class. So, if class descriptors are s, then what class(es) are they instances of? String LinkedList other s

13 Q: Each is an instance of a class. So, if class descriptors are s, then what class(es) are they instances of? A: the Class class String LinkedList other s

14 class Object boolean equals( Object ) int hashcode( ) String tostring( ) Object clone( ) Class getclass() extends class Class Class getsuperclass() Method[ ] getmethods( ) Field[ ] getfields( ) String getname( ) : class extends (automatic) class Animal : class : class : extends extends

15 All s inherit the Object.getClass() method. This method the for that. String getclass() getclass() LinkedList getclass()

16 All s inherit the Object.getClass() method. This method the for that. String getclass() Class getclass() Object getclass() getclass() LinkedList getclass()

17 class Object boolean equals( Object ) int hashcode( ) String tostring( ) Object clone( ) Class getclass() extends class Class Class getsuperclass() Method[ ] getmethods( ) Field[ ] getfields( ) String getname( ) : class extends (automatic) class Animal : class : class : extends extends

18 getsuperclass() getsuperclass() Object getsuperclass()

19 Object getsuperclass() getclass() getsuperclass() getclass()

20 Object my = new (); System.out.println( my ); getsuperclass() getsuperclass() getclass()

21 Object Test What else do we need to run a program? e.g. Consider running a Test class which has a main() method. other s

22 Object Call Stack goforwalk() anothermethod() somemethod() Test.main() Test other s

23 Object Call Stack my = new (); // in main() method Test () Test.main() my other s

24 Object Call Stack my = new (); // after instruction is done. Test Test.main() my other s

25 Object Call Stack my = new (); my.bark() TestProgram bark() this Test.main() my Look for the bark() method in the other s

26 Class Descriptors Methods are here Object Call Stack Local variables and parameters of methods are here Objects Object instance fields are here bark() TestProgram Test.main() other s