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Generic Types extending Abstract implementing Interface

 
Sandra Bachan
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Chapter 7, Sierra/Bates, Quiz Question # 6



Which of the following changes (taken separately) would allow this code to compile? (Choose all that apply.)

A. Change the Carnivore interface to



B. Change the Herbivore interface to



C. Change the Sheep class to



D. Change the Sheep class to



E. Change the Wolf class to



F. No changes are necessary




I can understand why B is the answer. However, I cannot understand the explanation for D

And D doesn’t work, because in D we made Sheep extend Plant, now the Wolf class breaks because its munch(Sheep) method no longer fulfills the contract of Carnivore.


How is Wolf class broken?
 
Sandra Bachan
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The following code, which incorporates D does not compile:



Compiler outputs:

Eat.java:11: type parameter Sheep is not within its bound
class Wolf extends Animal implements Carnivore<Sheep> {
^
1 error


Please give a hint, thanks!

 
Faisal A Khan
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Hi Sandra,

By changing Sheep to extend Plant from Animal you have broken the interface contract of Carnivore when used in Wolf. How? By passing it a Sheep which doesn't extend an Animal anymore.

Please let me know if this helps.
 
Sandra Bachan
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@ Faisal: Now I get it. I changed Sheep to extend Animal and now it compiles.
 
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