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Need generic clarification  RSS feed

 
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Hi all,
K & B book for 310-055 i.e. java 5.
Chapter 7: Generics and collections,
Self test example 16.
Given the method declared as :

A programmer want to use this method like this:

Which pair of declarations could be placed at //INSERT DECLARATIONS HERE to allow the code to compile?
Answeres are as bellow according to book.
A. ArrayList<Integer> input = null;
List<Integer> output = null;
E.List<Number> input = null;
List<Number> output = null;
F.List<Integer> input = null;
List<Integer> output = null;

Actually in netbeans with java 1.6. any of the above does not compile and gives error as
Compiling 1 source file to /home/sharmila/First/build/classes
/home/sharmila/First/src/generics/Test.java:98: incompatible types
found : java.util.List<capture#17 of ? super java.lang.Integer>
required: java.util.List<java.lang.Integer>
output = process(input);
1 error
BUILD FAILED (total time: 0 seconds)
Is the problem with my interpretation or there is some error in book?
Please help for this. I am new to generics and have littlebit problems in interpreting generics.
Sharmila
 
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First of all, of course all the three correct answers (!) are incorrect, here is why (It’s might look exhaustive but necessary):
Whenever we use wildcard in our codes, compiler uses a technique known as “Wildcard Capture”. It basically is a way for compiler to deal with wildcard means that internally compiler converts the wildcard types to some anonymous type like this:
List<?> becomes List<capture#000 of ?>
List<? extends Number> becomes List<capture#000 of ? extends java.lang.Number>
List<? super Number> becomes List<capture#000 of ? super java.lang.Number>
And so on...
So now what is the capture of a wildcard compatible to? Means what kind of type a the capture of a wildcard can assign to?
Well, the answer is another wildcard! But never, ever a concrete type! So do you get the point?

The return value of the method is: List <? super E> and you never can assign it to a concrete type like List<Integer> and this is the case in the whole 3 method call which all are wrong! To fix the method call you should write:

<E extends Number> List<? super E> process(List<E> nums)

when you use the List<Integer> as the input the compiler infer the type E as Integer so the output should be compatible to <? super Integer> which is again <? super Integer>, so it should be:

ArrayList<Integer> inputA = null;
List<? super Integer> outputA = null;
outputA = DumpArray.process(inputA);

or in the second method call it should be:
List<Number> inputB = null;
List<? super Integer> outputB = null;
Or
List<? super Number> outputB = null;
outputB = process(inputB);

One last important point:
The wildcard capture is never compatible even to its bounds even if the bound is a final class!
Like this:
List<? extends String> lst1 = null
List<String> lst = lst2; //Compile error!

And unbounded wildcard capture (<?>) is compatible to nothing except itself!

That’s it! I have a K&B of SCJP 6 and could not find this question, Are you sure that they mentioned those as correct answer?!!
 
Sharmila Punde
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Thanks Morteza,
Very nice explaination.
I have K & B of SCJP 5 NOT 6.
Any way rest of the book is good with good explaination on each subject.
I love the way K & B explain the topics. Rest of the book is good.
Thanks again
 
Morteza Manavi-Parast
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No problem. Anyway if that's what the book says, then I guess the authors need some sort of Generics Clarification too!
 
Sharmila Punde
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Thanks Moretaza,
It was very good and technical and appropriate explaination
Thanks again
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