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Difference between the code snippets

 
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Please explain the difference between the following two code snippets and the outputs produced by them too.



 
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List<?> can be assigned a List of any type.
But List<T> must be assigned a List of type T. Since we do not know if at runtime, type T will be String or any other type, we cannot return ArrayList<String> from a method that is declared to return List<T>.
 
indra negi
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Not clear. Please explain the output and also why it results in that output. Also one more code snippet has to be added to the above list :


Please can someone explain the output of these three code snippets alongwith proper explanation. I am quite confused although I think I am clear about the last one but still please.
 
Harsh Pensi
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This code does not produce any output, because there is no main method
 
indra negi
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Well caught.

Actually I wan't to know whether the codes will compile or throw runtime exceptions. If compiler reports error then at which line and why.

 
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The above code will give compilation error "Type mismatch: cannot convert from ArrayList<String> to List<T>" reason being T might vary at run time and
would not be equal to return type List<T> always, the correct syntax would be as mentioned below



Regards
Salil Verma
 
Salil Vverma
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This code will give compilation error "Cannot instantiate the type ArrayList<?>" reason being. if meth3 is called from any function,by no means wild character (?) in the return statement would be replaced by a valid object. As we know that wild character (?) does not represents any specific object, the compilation error completely make sense.
The possible correction of this might be as follows -



Regards
Salil Verma

 
Salil Vverma
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This code is perfect, runs without any error so I hope it does not require any explanation

Regards
Salil Verma
 
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As my good friend Jim Yingst used to say:

"Hmmm, if only there was an inexpensive way to find out on my own if a certain bit of code would compile, and if so what the output would be... if only I could find out for myself..."
 
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