Can you initialize a derived class with an instance of a baseclass

Question

By that I mean a baseclass, which was not itself initialised using an instance of the derived class. i.e. lets assume it is not an abstract class.

class GeomObj{
  Colour x;
}

class Triangle extends GeomObj{
  largestAngle y;
}

GeomObj u;

//now is the following allowed?Taking into account that u was not initialized using an instance of Triangle in the first place

Triangle v = (Triangle)u;

score 2 · Accepted Answer · answered Apr 13 '15 at 16:56

2

No because a GeomObj is not a triangle. But the inverse works:

Triangle u;
GeomObj v = (GeomObj)u;

answered Apr 13 '15 at 16:56

JFPicard

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It's like with dogs and pubs: each pub (subcalss) is a dog (superclass), but not each dog is a pub. =) – Turing85 Apr 13 '15 at 16:58
I was just wondering why it can't be converted like int to double when using typecast, where simply a new object is created with the additional parameters. and thanx for the answer :) – Marie Simm Apr 13 '15 at 18:15

score 0 · Answer 2 · answered Apr 13 '15 at 18:47

0

Furthermore, you can say that every Triangle is GeomObj, but not every GeomObj is a Triangle. So the compiler won't allow you to do that.

answered Apr 13 '15 at 18:47

Rafał P

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Can you initialize a derived class with an instance of a baseclass

2 Answers2