parse error "public static class"

we encountered a parse error:

Encountered "public static class" at line 56, column 5.
Was expecting one of:
"boolean" ...
"byte" ...
"char" ...
"double" ...
"float" ...
"int" ...
"long" ...
"short" ...
"@" ...
"public" "public" ...
"public" "static" "public" ...
"public" "static" "static" ...
"public" "static" "protected" ...
"public" "static" "private" ...
"public" "static" "final" ...
"public" "static" "abstract" ...
"public" "static" "synchronized" ...
"public" "static" "native" ...
"public" "static" "transient" ...
"public" "static" "volatile" ...
"public" "static" "strictfp" ...
"public" "static" "@" ...
"public" "static" "enum" ...
"public" "protected" ...
"public" "private" ...
"public" "static" "<" ...
"public" "static" "void" ...
"public" "static" "boolean" ...
"public" "static" "char" ...
"public" "static" "byte" ...
"public" "static" "short" ...
"public" "static" "int" ...
"public" "static" "long" ...
"public" "static" "float" ...
"public" "static" "double" ...
"public" "static" <IDENTIFIER> ...
<IDENTIFIER> ...

the source it failed on is:

[...]
public interface Document {
/** document category description type */
@DescriptionType("org.edorasframework.filestore.core.documentCategoryDescriptionType")//$NON-NLS-1$
public static class Category {
}
[...]

the "public static class" line seems to be too unuasal