javagenericscompiler-errorseclipse-oxygen

Possible eclipse bug

I'm trying to get my head around a generic-type problem. But it seems like eclipse is complaining while there isn't a valid complaint.

Consider the following methods

public static <FR extends FilterResult, T> List<? super WrappedFilterResult<? super T, FR>> filter(String check, Collection<T> elements, Function<? super T, String> converter, Filter<? extends FR> filter, ACComparator<? super WrappedFilterResult<? super T, ? super FR>> comparator)
{
    // eclipse says 'filter' doesn't accept these arguments
    return filter(check, elements, new ArrayList<>(), converter, filter, comparator);
    
    // doing a self call will result in the same error?
    // return filter(check, elements, converter, filter, comparator);
    
    // calling without returning doesn't solve it either?
    // filter(check, elements, converter, filter, comparator);
    // return null;
}

// no complaints here
public static <FR extends FilterResult, T, C extends Collection<? super WrappedFilterResult<? super T, FR>>> C filter(String check, Collection<T> elements, C result, Function<? super T, String> converter, Filter<? extends FR> filter, ACComparator<? super WrappedFilterResult<? super T, ? super FR>> comparator)
{
    // content
}

For the first method eclipse is complaining it cannot call the filter method because the method is not applicable for the arguments. But even if I do a self call it will complain.

Thinking it might be the return type I eliminated it by only calling and returning null, but sadly that doesn't solve anything either.

Sorry for the complicated method declaration but I have more similar methods with the same kind/amount of parameters working with no problems. So I have no clue why this wouldn't work.

Info:

I hope its something minor I fail to see atm, any help is appreciated.

Thanks in advance

Edit

The class declarations if someone needs them

public static class FilterResult {}
public interface Filter<FR extends FilterResult> {}
public static class WrappedFilterResult<T, FR extends FilterResult> extends FilterResult {}
public interface ACComparator<FR extends FilterResult> {}

Submitted to Bugzilla

Solution

  • I assume it is an Eclipse JDT compiler bug, though I have not consulted the JLS and also don't really feel like digging around in it.

    The reasons for my assumption is threefold:

    1. Your code compiles successfully under javac 8u112

    2. I would expect that a method calling itself with its own parameters should compile.

    3. Earlier, I also ran into a case where Eclipse compiler disagreed with other compilers.

    MCVE to reproduce your issue:

    public static class FilterResult {}
public static class WrappedFilterResult<T, FR extends FilterResult> extends FilterResult {}
public interface ACComparator<FR extends FilterResult> {}

public static <FR extends FilterResult, T>
  void filter1(ACComparator<? super WrappedFilterResult<? super T, ? super FR>> comparator) {
    // both compile fine with normal Java compiler
    // but error with Eclipse JDT compiler (I'm using Eclipse 4.9.0)
    filter1(comparator);
    filter2(comparator);
}

public static <FR extends FilterResult, T>
  void filter2(ACComparator<? super WrappedFilterResult<? super T, ? super FR>> comparator) {
}

    Workaround:

    Pulling the offending type (ACComparator<...etc> in this case) into a generic type argument seems to get past this issue for Eclipse.

    public static
< FR extends FilterResult, T,
  A extends ACComparator<? super WrappedFilterResult<? super T, ? super FR>> // <-- here
>
void filterSuccess(A comparator) {
    // success!
    filter1(comparator);
    filter2(comparator);
}