Question

I have configured a Maven build for a very small GWT project I’m having. Compiling/running from Eclipse causes no problem, but when I execute Maven build, I’ve got compilation issues in one single place (this is GWT ‘client-side’ code, but I’m not sure if that’s the case). Maven/compile output:

[..]SmartTable.java:[63,52] ')' expected
[..]SmartTable.java:[63,53] ')' expected
[..]SmartTable.java:[63,69] ';' expected
[..]SmartTable.java:[63,71] not a statement
[..]SmartTable.java:[63,77] ';' expected
[..]SmartTable.java:[63,79] not a statement

.. where this specific line defined as following:

final Comparator<T> comparator = ((SmartTable<T>.ComparableColumn) column).comparator;

As you have guessed, I’m iterating over all columns defined for this generics-class (which I call ‘SmartTable’), and getting a comparator (of course, if column instanceof SmartTable.ComparableColumn) for further operations.
Classes ‘Column’ and ‘ComparableColumn’ are nested in SmartTable, and their headers looks as following:

public abstract class Column {
    // private String id;
    private String caption;
    private int width;
    private Filter<T> filter;
...
public class ComparableColumn extends Column {
    private Comparator<T> comparator;
...

When it gets compiled from Eclipse, I have no problems in packaging WAR (without ‘clean’ – simply package which will work since Eclipse configured to place .class files in the same target directory used by Maven) and deploying/running it correctly.

I’ve already tried changing maven-compiler-plugin and configuration (setting values 1.5, 1.6; and 1.4 – for sake of experiment to see if it will start complain on generics in general), but that wasn’t of help. Eclipse project compiler compliance is default (1.6). It seems that I have no other compilation issues with the other generics code.

Tried to make it as short as possible, but not sure if I managed to do it well 🙂

EDIT: More of code by demand

public class SmartTable<T> extends FlexTable {
    private List<Column> columns = new ArrayList<Column>();
    ...
    private Comparator<T> currentComparator;
...

    public void init(Comparator<T> defaultComparator) {
        this.currentComparator = defaultComparator;
        ...
        int index = 0;
        for (Column column : columns) {
            ...
            if (column instanceof SmartTable.ComparableColumn) {
                @SuppressWarnings("unchecked")
                final Comparator<T> comparator = ((SmartTable<T>.ComparableColumn) column).comparator;
            ...
    }
...

    public abstract class Column {
        // private String id;
        private String caption;
        private int width;
        private Filter<T> filter;
        private List<WidgetCreator<T>> widgetCreators;

        public Column(/* String id, */String caption, int width, Filter<T> filter, List<WidgetCreator<T>> widgetCreators) {

...

    public class SimpleColumn extends Column {
        public SimpleColumn(/* String id, */String caption, int width, Filter<T> filter, List<WidgetCreator<T>> widgetCreators) {
            super(/* id, */caption, width, filter, widgetCreators);
        }

...

    public class ComparableColumn extends Column {
        private Comparator<T> comparator;

        public ComparableColumn(/* String id, */String caption, int width, Filter<T> filter,
                List<WidgetCreator<T>> widgetCreators, Comparator<T> comparator) {
            super(/* id, */caption, width, filter, widgetCreators);
            this.comparator = comparator;
        }

...

}

EDIT2: Actual problem seems to be very concentrated 🙂

public class SmartTable2<T> {
    public void init() {
        Column c = new ComparableColumn();

        final Comparator<T> comparator = ((SmartTable2<T>.ComparableColumn) c).comparator;
    }

    class Column {

    }

    class ComparableColumn extends Column {
        Comparator<T> comparator;
    }
}

EDIT3: Some thoughts, hopefully closer to the solution
.. so this doesn’t compile simply with javac, throwing exactly same compilation errors. So what I’ve learned just now: Eclipse uses it’s own internal Java compiler – which is part of JDT Core (hope I have not misunderstood it); so might it be the case that JDT Compiler is able to compile such a syntax, while Sun’s JDK javac is not??? (oops, Oracle not Sun)

Answer 1

Try this syntax:

Comparator<T> comparator = (Comparator<T>) ((SmartTable.ComparableColumn) column).comparator;

EDIT:
The cast is useless:

Comparator<T> comparator = ((SmartTable.ComparableColumn) column).comparator;

Does this resemble your scenario ?

public class Main {

    public static <T> void main(String[] args) throws Exception {
        SmartTable<T>.Column column = new SmartTable<T>().new Column();
        Comparator<T> comparator = ((SmartTable.ComparableColumn) column).comparator;

    }

    static class SmartTable<T> {
        class Column {

        }
        class ComparableColumn extends Column {
            Comparator<T> comparator;
        }
    }

}

This compiles fine with javac (with unchecked warnings, of course), while trying to compile the cast to (SmartTable.ComparableColumn) results in exactly the errors in your post.

EDIT (of all edits):
Your question seems to have been asked before: Casting to an inner class with generics and the answer seems to be a compiler bug: https://bugs.java.com/bugdatabase/view_bug?bug_id=6665356