• Post Reply Bookmark Topic Watch Topic
  • New Topic
programming forums Java Mobile Certification Databases Caching Books Engineering Micro Controllers OS Languages Paradigms IDEs Build Tools Frameworks Application Servers Open Source This Site Careers Other Pie Elite all forums
this forum made possible by our volunteer staff, including ...
Marshals:
  • Campbell Ritchie
  • Ron McLeod
  • Paul Clapham
  • Devaka Cooray
  • Tim Cooke
Sheriffs:
  • Rob Spoor
  • Liutauras Vilda
  • paul wheaton
Saloon Keepers:
  • Tim Holloway
  • Tim Moores
  • Mikalai Zaikin
  • Carey Brown
  • Piet Souris
Bartenders:
  • Stephan van Hulst

Generics

 
Ranch Hand
Posts: 53
Android C++ Java
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
I have a quick questions.
According to Java SCJP by Khalid Mughal:


List a = new ArrayList(); List < ?> b; List <? extends objects> c;

According to Java SCJP by khalid mughal (a very good book!):

a = b; // ok. Widening conversion. b = a; // ok too. No unchecked warning.

b = c; // ok c = b; // ok

c=a; // ok but now will issue a unchecked warning. // clause 1

I do understand that any raw types (example a) when assigned to any bounded wilcard references, a unchecked warning is issues (since the content in that raw type a could be anything).

My questions is since c is the highest upper bound (? extends objects), shouldn't a be able to assigned to c without that warning?
 
Ranch Hand
Posts: 1051
Eclipse IDE Firefox Browser
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator


Kim Ming Yap wrote:I have a quick questions.
According to Java SCJP by Khalid Mughal:


List a = new ArrayList(); List < ?> b; List <? extends objects> c;

According to Java SCJP by khalid mughal (a very good book!):

a = b; // ok. Widening conversion. b = a; // ok too. No unchecked warning.

b = c; // ok c = b; // ok

c=a; // ok but now will issue a unchecked warning. // clause 1

I do understand that any raw types (example a) when assigned to any bounded wilcard references, a unchecked warning is issues (since the content in that raw type a could be anything).

My questions is since c is the highest upper bound (? extends objects), shouldn't a be able to assigned to c without that warning?


I think when we try to mix the Generics with Non-Generics type then compiler issues warning.
a=b should give error because b is only a refernce type and
b=a doesnot issue warning because <?> could be anything whether Generics or Non-Generics.
b=c and c=b should also give error because none of both has been initialized

but when we do.
c=a; because now compiler knows that c is a Generics type but a is a non-generics,therefore when we try to mix Generics
and non-generics type then it will show compiles with warning.
Remeber:- compiles with warning will never be consider as a compilation failure.
a=c; is also not possible because c is also not been initialized.
 
Ranch Hand
Posts: 101
Eclipse IDE Java
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator

Kim Ming Yap wrote: List <? extends objects> c;


Did you mean List <? extends Object> c; ?

for the record:
 
Kim Ming Yap
Ranch Hand
Posts: 53
Android C++ Java
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
Yes Mark.

My point is as follows:

Object
|
List (Raw type) (eg. List a)
|
List <?> (eg. List b) = List <? extends objects> (eg. List c)

According to Java SCJP (Khalid Mughal):
Say List a = new ArrayList <Integer> ();

Just talk about narrowring conversion here:
1. b = a; // ok (unbounded wildcard)
2. c=b; // ok (unbounded wildcard here is actually same as ? extends objects>
But
3. c=a; // ok but with unchecked warning.

I do understand from that book that the general rule when mixing legacy code and generic code that any assignment of raw type to BOUNDED wildcard (eg. ? extends String> will results in unchecked warning. But this rule should not be true when ? extends objects (eg list c) is used since that is the highest upper bound (covering all subtypes).

So since a (raw type) can be assigned to b (unbounded wildcard) and b is c indeed (and object is the highest upper bound), i don't see any problem here.
My opinion is a can be assigned to c (c=a) without any unchecked warning generated (since ? extends objects is the highest upper bound which will cover all subtypes).

But the book is right. I tested c=a and it does generate a unchecked warning (which means type safety can be compromised).

That's the part i don't get it

 
Ranch Hand
Posts: 5575
Eclipse IDE Windows XP Java
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator

Mark Moge wrote:


Hmm. if you try to use add method, then you will get a warning
 
Once upon a time there were three bears. And they were visted by a golden haired tiny ad:
Gift giving made easy with the permaculture playing cards
https://coderanch.com/t/777758/Gift-giving-easy-permaculture-playing
reply
    Bookmark Topic Watch Topic
  • New Topic