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Casting class types

 
koray guclu
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Posts: 59
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QUESTION:
Suppose we have two classes defined as follows
class ApBase extends Object implements Runnable
class ApBase extends Object implements Runnable
Given two variables created as follows:
ApBase aBase= new ApBase();
ApDerived aDer=new ApDerived();
Which of the following Java Code fragments will compile and execute without error?
a) Abject obj= aBase;
Runnable rn=obj;
b)Object obj= aBase;
Runnable rn=(Runnable)obj;
c)Object obj= aBase;
Observer ob=(Observer)aBase;
d)Object obj= aDer;
Observer ob2=obj;
*
Which statement is true? Can you give me a reasonable
exlanation for type casting of multiple inheritence like as in this example.
Thanks.
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Koray G��l�
(B.s. Computer engineer)
http://www.geocities.com/korayguclu/
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William Brogden
Author and all-around good cowpoke
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Posts: 13058
6
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That question looks familiar.
The basic rule is that the compiler goes by the type of the variable unless there is an explicit cast.
a) Object obj= aBase;
Runnable rn=obj; // compiler knows Object does not implement Runnable and objects.
b) Object obj= aBase;
Runnable rn=(Runnable)obj; // explicit cast overrides, runtime check performed.
Bill

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