JDK-7101374 : Incorrect "Type mismatch" reported for generic cast of class literals.
  • Type: Enhancement
  • Component: specification
  • Sub-Component: language
  • Affected Version: 7
  • Priority: P4
  • Status: Closed
  • Resolution: Duplicate
  • OS: windows_7
  • CPU: x86
  • Submitted: 2011-10-15
  • Updated: 2012-03-20
  • Resolved: 2011-10-31
Related Reports
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Description
FULL PRODUCT VERSION :
java version "1.7.0"
Java(TM) SE Runtime Environment (build 1.7.0-b147)
Java HotSpot(TM) 64-Bit Server VM (build 21.0-b17, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows [Version 6.1.7601]

A DESCRIPTION OF THE PROBLEM :
Attempting to compile this statement results in a "Type mismatch" error:

	Class<List<String>> stringListType = List.class;

The compiler error specifically states "cannot convert from Class<List> to Class<List<String>>". This is a distinctly false statement. The following adjustment compiles with warnings but no errors:

	Class<List<String>> stringListType = (Class<List<String>>) (Class<?>) List.class;

The code runs correctly, because the generic types are not present in the bytecode. So in fact the compiler "can convert from Class<List> to Class<List<String>>".

Type erasure causes all generic tokens on a variable declaration of type Class to be fully meaningless in regards to cast compatibility. Therefore, every possibly generic qualification of a variable declaration of type Class should be assignable from a class literal with a matching base type. For example, this statement should compile with no errors or warnings:

	Class<Complicated<Nested<Type<Parameters>>>> complicatedType = Complicated.class;

The compiler can do this. It is correct and also quite useful.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Attempt to compile the syntactically meaningful statement:

	Class<List<String>> stringListType = List.class;


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The statement should compile with no errors. This statement can be compiled, violates no rules, and causes no problems.
ACTUAL -
The compiler reports "Type mismatch: cannot convert from Class<List> to Class<List<String>>"


ERROR MESSAGES/STACK TRACES THAT OCCUR :
Type mismatch: cannot convert from Class<List> to Class<List<String>>

REPRODUCIBILITY :
This bug can be reproduced always.

CUSTOMER SUBMITTED WORKAROUND :
Add a painfully cumbersome pair of casts to the statement:

	Class<List<String>> stringListType = (Class<List<String>>) (Class<?>) List.class;