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Why is a return type erased [message #997607]

Mon, 07 January 2013 10:26

Ed Merks

Messages: 33140
Registered: July 2009

Senior Member

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<meta http-equiv="content-type" content="text/html; charset=ISO-8859-1">
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I'd open a bugzilla, but Javac and JDT agree that the
following code contains
an error
so it must behave this way for a reason (that's inexplicable to me): 
 
public class TestRawInheritance 
{ 
  class Base<T> 
  { 
    // 
  } 
 
  class Derived extends Base 
  { 
    // 
  } 
 
  class Consumer<T1> 
  { 
    <T> T1 foo(Base<T> base) 
    { 
      return null; 
    } 
 
    T1 bar(Base base) 
    { 
      return null; 
    } 
 
    T1 test() 
    { 
      foo(new Derived()); 
      T1 result = foo(new Derived()); 
      result = bar(new Derived()); 
      return result; 
    } 
  } 
  
  public void test() 
  { 
    Boolean result = new Consumer<Boolean>().foo(new
Derived()); 
  } 
} 
 
Specifically the line "T1 result = foo(new Derived());" is marked
with the error "Type mismatch: cannot convert from Object to T1",
but it seems counter intuitive that a method that returns T1 ought
to ever
to be interpreted such that it returns Object other than in a
context where
Consumer itself is a raw type.   I assume it's because Derived
itself
is a raw type with respect to Base, but why
does that affect the return type of this method? And
why isn't the call to bar an error? 
 
Also, why is this not an error? 
 
 public void test() 
 { 
   Boolean result = new Consumer<Boolean>().foo(new
Derived()); 
 } 
 
EMF generates code similar to this, i.e., the XyzSwitch class, and
I only just noticed these errors in an
ugly test model... 

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Ed Merks
Professional Support: https://www.macromodeling.com/

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Re: Why is a return type erased [message #997610 is a reply to message #997607]

Tue, 08 January 2013 11:15

Dani Megert

Messages: 3802
Registered: July 2009

Senior Member

On 07.01.2013 11:26, Ed Merks wrote:
> I'd open a bugzilla, but Javac and JDT agree that the following code
> contains an error so it must behave this way for a reason (that's
> inexplicable to me):
>
> public class TestRawInheritance
> {
> class Base<T>
> {
> //
> }
>
> class Derived extends Base
> {
> //
> }
>
> class Consumer<T1>
> {
> <T> T1 foo(Base<T> base)
> {
> return null;
> }
>
> T1 bar(Base base)
> {
> return null;
> }
>
> T1 test()
> {
> foo(new Derived());
> T1 result = foo(new Derived());
> result = bar(new Derived());
> return result;
> }
> }
>
> public void test()
> {
> Boolean result = new Consumer<Boolean>().foo(new Derived());
> }
> }
>
> Specifically the line "T1 result = foo(new Derived());" is marked with
> the error "Type mismatch: cannot convert from Object to T1", but it
> seems counter intuitive that a method that returns T1 ought to ever to
> be interpreted such that it returns Object other than in a context
> where Consumer itself is a raw type. I assume it's because Derived
> itself is a raw type with respect to Base, but why does that affect
> the return type of this method? And why isn't the call to bar an error?
This looks like a bug in the inference code (both Oracle and Eclipse
compiler). It gets confused by the raw type. As a workaround you can
- add a cast
- parametrize it with <Object>
- use the diamond (1.7 and beyond)

Please file a bug against JDT Core and the Oracle JDK.
>
> Also, why is this not an error?
Here, the inference already knows T1=Boolean and hence doesn't fail i.e.
doesn't try do infer it.
>
> public void test()
> {
> Boolean result = new Consumer<Boolean>().foo(new Derived());
> }
>
> EMF generates code similar to this, i.e., the XyzSwitch class, and I
> only just noticed these errors in an ugly test model...
And ugly generated code ;-).

Dani

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Re: Why is a return type erased [message #997755 is a reply to message #997610]

Tue, 08 January 2013 17:14

Ed Merks

Messages: 33140
Registered: July 2009

Senior Member

I've opened https://bugs.eclipse.org/bugs/show_bug.cgi?id=397675 if
anyone is interested. I reported an Oracle JDK bug as well with the
response "Your report has been assigned an internal review ID of
2421492". I'll add the Oracle link to the bugzilla when I get one.

On 08/01/2013 12:15 PM, Daniel Megert wrote:
> On 07.01.2013 11:26, Ed Merks wrote:
>> I'd open a bugzilla, but Javac and JDT agree that the following code
>> contains an error so it must behave this way for a reason (that's
>> inexplicable to me):
>>
>> public class TestRawInheritance
>> {
>> class Base<T>
>> {
>> //
>> }
>>
>> class Derived extends Base
>> {
>> //
>> }
>>
>> class Consumer<T1>
>> {
>> <T> T1 foo(Base<T> base)
>> {
>> return null;
>> }
>>
>> T1 bar(Base base)
>> {
>> return null;
>> }
>>
>> T1 test()
>> {
>> foo(new Derived());
>> T1 result = foo(new Derived());
>> result = bar(new Derived());
>> return result;
>> }
>> }
>>
>> public void test()
>> {
>> Boolean result = new Consumer<Boolean>().foo(new Derived());
>> }
>> }
>>
>> Specifically the line "T1 result = foo(new Derived());" is marked
>> with the error "Type mismatch: cannot convert from Object to T1", but
>> it seems counter intuitive that a method that returns T1 ought to
>> ever to be interpreted such that it returns Object other than in a
>> context where Consumer itself is a raw type. I assume it's because
>> Derived itself is a raw type with respect to Base, but why does that
>> affect the return type of this method? And why isn't the call to bar
>> an error?
> This looks like a bug in the inference code (both Oracle and Eclipse
> compiler). It gets confused by the raw type. As a workaround you can
> - add a cast
> - parametrize it with <Object>
> - use the diamond (1.7 and beyond)
>
> Please file a bug against JDT Core and the Oracle JDK.
>>
>> Also, why is this not an error?
> Here, the inference already knows T1=Boolean and hence doesn't fail
> i.e. doesn't try do infer it.
>>
>> public void test()
>> {
>> Boolean result = new Consumer<Boolean>().foo(new Derived());
>> }
>>
>> EMF generates code similar to this, i.e., the XyzSwitch class, and I
>> only just noticed these errors in an ugly test model...
> And ugly generated code ;-).
>
> Dani

Ed Merks
Professional Support: https://www.macromodeling.com/

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