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Help understanding a generics and collections question

L K Kakani
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Joined: Aug 30, 2010
Posts: 32
This is one of the questions from the SCJP6 book mock exam A




Why is the answer "null b" ? In the explanation it was given that because Map m2 is not typed we get null accessing m2.get("a") but b for m2.get(2). Why not null for both?

Thanks

Kakani.


SCJP 6.0(88%)
Piotr Nowicki
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Joined: Jul 13, 2010
Posts: 610

The mangle method is returning a new map which switch the key with value. In other words, it converts (String => Integer) mapping into (Integer => String).

The m2 map is something like:

1 => "a"
2 => "b"

That's why m2.get("a") returns null (there is no such key in the map) and m2.get(2) returns "b" as it's the actual value for this key.

The generic code would look like this:



and the result remains unchanged.


OCP Java SE 6 Programmer, OCM Java SE 6 Developer, OCE Java EE 6 JSPSD, OCE Java EE 6 EJBD, OCE Java EE 6 JPAD, Spring 3.0 Core Professional.
L K Kakani
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Joined: Aug 30, 2010
Posts: 32
Thanks a lot Pedro. So it has got nothing to do with the type of Map m2 it's just because of using a value in get() method instead of key.
Piotr Nowicki
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Joined: Jul 13, 2010
Posts: 610

That's what I think :-)

No problem mate!
 
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