javac error: inconvertible types with generics?

Question

There are several other SO questions talking about generics compiling OK w/ Eclipse's compiler but not javac (i.e. Java: Generics handled differenlty in Eclipse and javac and Generics compiles and runs in Eclipse, but doesn't compile in javac) -- however this looks like a slightly different one.

I have an enum class:

public class LogEvent {
   public enum Type {
       // ... values here ...
   }
   ...
}

and I have another class with a method that takes in arbitrary objects of types descended from Enum:

@Override public <E extends Enum<E>> void postEvent(
    Context context, E code, Object additionalData) 
{
    if (code instanceof LogEvent.Type)
    {
        LogEvent.Type scode = (LogEvent.Type)code;
    ...

This works fine in Eclipse, but when I do a clean built with ant, I am getting a pair of errors, one on the instanceof line, the other on the casting line:

443: inconvertible types
    [javac] found   : E
    [javac] required: mypackage.LogEvent.Type
    [javac]         if (code instanceof LogEvent.Type)
    [javac]             ^

445: inconvertible types
    [javac] found   : E
    [javac] required: com.dekaresearch.tools.espdf.LogEvent.Type
    [javac]             LogEvent.Type scode = (LogEvent.Type)code;
    [javac]                                                  ^

Why does this happen, and how can I get around this problem so it will compile properly?

score 32 · Accepted Answer · answered Jan 28 '11 at 15:01

32

I don't know why it's happening, but a workaround is easy:

@Override public <E extends Enum<E>> void postEvent(
    Context context, E code, Object additionalData) 
{
    Object tmp = code;
    if (tmp instanceof LogEvent.Type)
    {
        LogEvent.Type scode = (LogEvent.Type)tmp;
    ...

It's ugly, but it works...

answered Jan 28 '11 at 15:01

Jon Skeet

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thanks. Why is that ugly? It's simple and has low runtime cost. – Jason S Jan 28 '11 at 16:07
3

@Jason S: It's ugly because I can't see why it's required. – Jon Skeet Jan 28 '11 at 16:10
This is a known bug, IIRC. Actually, there dozens of related bugs with none of them fixed yet. – maaartinus Jan 31 '11 at 11:06
1

I was about to throw myself out of the window ( hey I'm at ground floor :) Thanks a lot for this one... And the worst I think I've already been fighting for this one few years ago – Guillaume Jul 16 '12 at 18:13
1

I think this is the bug (or one of them): http://bugs.sun.com/view_bug.do?bug_id=6932571 – Jens Schauder Jun 10 '13 at 15:52
The key is to assign to an `Object` before the cast. That avoids triggering the bug. Thanks @JonSkeet! – barfuin Sep 16 '13 at 10:05

Sergei Tachenov · Answer 2 · 2011-01-28T16:59:42.160

8

Perhaps it is because you've declared E as something that extends Enum<E>. I can't say I understand it completely, but it looks like it limits the set of types to some subset that can't include LogEvent.Type for some reason. Or maybe it's just a bug in the compiler. I'd be happy if someone could explain it more clearly, but here is what you can do:

public <E extends Enum<?>> void postEvent(E code) 
{
    if (code instanceof LogEvent.Type)
    {
        LogEvent.Type scode = (LogEvent.Type)code;
        ...
    }
    ...

This works and it is more elegant than just casting to an Object.

edited Jan 28 '11 at 16:59

answered Jan 28 '11 at 15:22

Sergei Tachenov

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it works because "E extends Enum" refers to some specific subclass of Enum (which may not be compatible with LogEvent.Type). whereas "E extends Enum>" refers to any class which extends Enum, of which LogEvent.Type is a valid possibility. – jtahlborn Jan 28 '11 at 16:35
@jtahlborn: The `E extends Enum` is standard boilerplate for any class that properly extends Enum, which all `enum` classes do, and `LogEvent.Type` is an `enum` class, so it should work as well, but there seems to be a bug in `javac`. – Jason S Jan 28 '11 at 16:40
@jtahlborn, yes, think of it - does LogEvent.Type extend Enum? Surely it does. But it's exactly what we mean when we write "E extends Enum", with E = LogEvent.Type. So I agree that it looks like a bug in javac. In fact, "E extends Enum>" and "E extends Enum" should mean exactly the same thing because any enum type E can only extend Enum and nothing else. – Sergei Tachenov Jan 28 '11 at 16:53

score 4 · Answer 3 · answered May 03 '11 at 06:37

4

I had a similar problem and upgraded from jdk1.6.0_16 to jdk1.6.0_23 and it went away without any code changes.

answered May 03 '11 at 06:37

WW.

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score 0 · Answer 4 · answered Jan 28 '11 at 15:21

0

In order to use instanceof both operands have to inherit/implement the same class/interface.
E just can't be cast to LogEvent.Type

I don't know what your full method looks like, but this should solve your issue by using interfaces and not Generics.

public interface EventType { }
public class LogEvent  {
    public enum Type implements EventType {}
}

public void postEvent(Context context, EventType code, Object additionalData) {
    if(code instanceof LogEvent.Type) {
    }
}

answered Jan 28 '11 at 15:21

jonescb

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But I want Enum to be the base type, not EventType. I need my method to accept arbitrary Enum values. – Jason S Jan 28 '11 at 15:24
But why E can't be cast to LogEvent.Type? E is a subclass of Enum, and LogEvent.Type is a subclass of Enum as well. I understand that the problem part is that it's not just Enum, but Enum, I just can't understand what that means exactly. – Sergei Tachenov Jan 28 '11 at 15:24
The other SO questions on this topic seem to indicate that `javac` has a known bug with recursive generic bounds, so maybe it gets confused. With respect to the `>`, if you haven't seen it before: http://www.angelikalanger.com/GenericsFAQ/FAQSections/TypeParameters.html#FAQ106 – Jason S Jan 28 '11 at 16:01

javac error: inconvertible types with generics?

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