JCH
Java Community | Help. Code. Learn.•6d ago
Thorinwasher

Converting from raw class type to generic wildcard type

I'm doing this (fetch data for a static registry): 
private static <T extends BreweryKeyed> Map<BreweryKey, T> getFields(Class<T> tClass) {
    try {
        ImmutableMap.Builder<BreweryKey, T> outputBuilder = ImmutableMap.builder();
        for (Field field : tClass.getDeclaredFields()) {
            if (!Modifier.isStatic(field.getModifiers())) {
                continue;
            }
            Object staticField = field.get(null);
            if (tClass.isInstance(staticField)) {
                T tInstance = tClass.cast(staticField);
                outputBuilder.put(tInstance.key(), tInstance);
            }
        }
        return outputBuilder.build();
    } catch (IllegalAccessException e) {
        throw new RuntimeException(e);
    }
}
private static <T extends BreweryKeyed> Map<BreweryKey, T> getFields(Class<T> tClass) {
    try {
        ImmutableMap.Builder<BreweryKey, T> outputBuilder = ImmutableMap.builder();
        for (Field field : tClass.getDeclaredFields()) {
            if (!Modifier.isStatic(field.getModifiers())) {
                continue;
            }
            Object staticField = field.get(null);
            if (tClass.isInstance(staticField)) {
                T tInstance = tClass.cast(staticField);
                outputBuilder.put(tInstance.key(), tInstance);
            }
        }
        return outputBuilder.build();
    } catch (IllegalAccessException e) {
        throw new RuntimeException(e);
    }
}
The problem is that I don't know how to set a non raw type to tClass, for example this is not possible (compile issues): 
public static final Map<BreweryKey, StructureMeta<?>> STRUCTURE_META = (Map<BreweryKey, StructureMeta<?>>)getFields(StructureMeta.class);
public static final Map<BreweryKey, StructureMeta<?>> STRUCTURE_META = (Map<BreweryKey, StructureMeta<?>>)getFields(StructureMeta.class);
Any ideas?
3 Replies
JavaBot
JavaBot•6d ago
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Hey @Thorinwasher! Please use /close or the Close Post button above when your problem is solved. Please remember to follow the help guidelines. This post will be automatically marked as dormant after 300 minutes of inactivity.
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Thorinwasher
ThorinwasherOP•6d ago
Okay, doing this with a cast works, why is it necessary to do initially though? (I added a wrapper class around the map) 
    private static <T extends BreweryKeyed> Registry<? extends T> fromFields(Class<T> tClass) {
        try {
            List<T> tList = new ArrayList<>();
            for (Field field : tClass.getDeclaredFields()) {
                if (!Modifier.isStatic(field.getModifiers())) {
                    continue;
                }
                Object staticField = field.get(null);
                if (tClass.isInstance(staticField)) {
                    tList.add(tClass.cast(staticField));
                }
            }
            return new Registry<>(tList);
        } catch (IllegalAccessException e) {
            throw new RuntimeException(e);
        }
    }
    private static <T extends BreweryKeyed> Registry<? extends T> fromFields(Class<T> tClass) {
        try {
            List<T> tList = new ArrayList<>();
            for (Field field : tClass.getDeclaredFields()) {
                if (!Modifier.isStatic(field.getModifiers())) {
                    continue;
                }
                Object staticField = field.get(null);
                if (tClass.isInstance(staticField)) {
                    tList.add(tClass.cast(staticField));
                }
            }
            return new Registry<>(tList);
        } catch (IllegalAccessException e) {
            throw new RuntimeException(e);
        }
    }
Like do the <? extends T> instead of just <T>
JavaBot
JavaBot•5d ago
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