Xjyuk

I have the following Java code that uses a local class.

import java.util.Arrays;

public class X<T> 
 void m() 
 class Z 

 for (Object o : Arrays.asList(1, 2, 3))
 if (o instanceof Z) 
 

It does not compile with the following error message:

X.java:8: error: illegal generic type for instanceof
 if (o instanceof Z) 
 ^
1 error

I understand that the local class Z inherits the generic type signature of X<T>, being an inner class. The same kind of compilation error appears in this example, where Z is not local, but still inner:

import java.util.Arrays;

public class X<T> 
 class Z 

 void m() 
 for (Object o : Arrays.asList(1, 2, 3))
 if (o instanceof Z) // Compilation error
 

It can be worked around either by making Z non-inner / static:

import java.util.Arrays;

public class X<T> 
 static class Z 

 void m() 
 for (Object o : Arrays.asList(1, 2, 3))
 if (o instanceof Z) // Compiles now
 

Or by qualifying X.Z:

import java.util.Arrays;

public class X<T> 
 class Z 

 void m() 
 for (Object o : Arrays.asList(1, 2, 3)) 
 if (o instanceof X.Z) // Compiles now
 if (o instanceof X<?>.Z) // Also
 
 

But how can I qualify a local class, or work around this limitation, without changing the local class itself?

To me this seems to be an oversight or limitation in the Java language and I do not think it is possible.

The referenced type in an instanceof expression must be reifiable according to JLS 4.7, meaning that it must be expressed as a reifiable type by its fully qualified name. At the same time, JLS 6.7 states that local classes do not have a fully qualified name, they can therefore not be expressed as reifiable.

If you declare Z as generic, the instanceof operator treats Z as a raw type where all generic properties to it - in this case the enclosing class - are considered raw as well. (Similar to a generic methods of a raw type being considered as raw despite any generic signature. This is a measure to retain backwards compatiblity of type generification.) Since any raw type is reifiable, declaring Z to be generic will compile.

A possible workaround is to use reflection:

import java.util.Arrays;

public class X<T> 
 void m() 
 class Z 

 for (Object o : Arrays.asList(1, 2, 3))
 if (Z.class.isInstance(o)) 
 

Apparently, by making Z generic compilation succeeds. I expected that to require <T> as the type parameter, but you just have to make it generic, so anything will do

import java.util.Arrays;

public class X<T> 
 void m() 
 class Z<Anything> 

 for (Object o : Arrays.asList(1, 2, 3))
 if (Z.class.isInstance(o)) 
 

Proper solution would be qualify the local class, but I don't think you can. Either you refactor it to a private static class or that's probably the best you can get.

This should work either. Using reflection too. But seems a valid solution.

import java.util.Arrays;

public class X<T> 


 void m() 

 class Z2 
 

 for(Object o: Arrays.asList(1,2,3)) 
 if(Z2.class.isAssignableFrom(o.getClass()))