获取参数化类T的扩展Enum的实际枚举类<?>

Question

I have a class:

public class MultipleSorting<T extends Enum<?>> {
    private T criteriaType;

    public Class<T> getCriteriaClass() {
        Field field = ReflectionUtils.getField(getClass(),"criteriaType");
        ReflectionUtils.makeAccessible(field);
        return (Class<T>)field.getType();           
    }
}

This class is get instantiated as:

public abstract class MultiSortPageableController<T extends MultiSortPageableController<?,?>, U extends Enum<?>> {
    private MultipleSorting<U> multipleSorting;

    public MultiSortPageableController() {
        super();

        multipleSorting = new MultipleSorting<U>();
    }
}

The actual value of U is passed from the child class of MultiSortPageableController which is:

public abstract class AbstractArticleSearchController<T extends AbstractArticleSearchController<T>> extends MultiSortPageableController<T,ArticleSortField> {

}

The ArticleSortField is an Enum .

I was expecting the method getCriteriaClass of MultipleSorting would return ArticleSortField from a method of MultiSortPageableController . But it is returning java.lang.Enum .

I am unable to figure it out why it is not returning the actual enum and how can I make it so. Any pointer would be very helpful to me. I need to get ArticleSortField .

Purpose:

I two requirement:

1. To get the actual class of enum type (say ArticleSortField.class )
2. To list enum value. If I have the enum class, then I could invoke class..getEnumConstants() .

Answer 1

Java compiler removes information about generics, therefore when you use reflection you get no information about the declared type, other than Enum. This process is called type erasure .

How about passing the type down, via the constructor, like this:

public class MultipleSorting<T extends Enum<?>> {
    private Class<T> criteriaType;

    MultipleSorting(Class<T> criteriaType) {
        this.criteriaType = criteriaType;
    }

    public Class<T> getCriteriaClass() {
        return criteriaType;
    }
}

public abstract class MultiSortPageableController<T extends MultiSortPageableController<?, ?>, U extends Enum<?>> {
    private MultipleSorting<U> multipleSorting;

    public MultiSortPageableController(Class<U> criteriaType) {
        super();

        multipleSorting = new MultipleSorting<U>(criteriaType);
    }
}

public abstract class AbstractArticleSearchController<T extends AbstractArticleSearchController<T>> extends MultiSortPageableController<T, ArticleSortField> {
    public AbstractArticleSearchController() {
        super(ArticleSortField.class);
    }
}