泛型方法的泛型返回类型

Question

Consider a generic method definition like this:

public static <T> T getParameter(final Context context, final ContextParameter contextParameter, final Class<T> clazz) {
    return (T) context.get(contextParameter.name());
}

Works perfectly to get any type of object until you try a generic object like List<String>. The only way I could make it work was like this:

 final List<String> fileNames = 
             (List<String>) ContextUtils.getParameter(context,
                             ContextParameter.EOF_FILE_NAMES_TO_ZIP, List.class);

But that requires an extra cast, removing the purpose of having the method. Is there a way to be able to provide List<String>.class or something similar to the method? (I know that type erasure prevents List<String> to exist but maybe there is a way to do it).

Note: With making it work I mean without having a type safety warning. I'm aware that the code would work as it is but it is not type safe.

Answer 1

You can't do this in a type safe way without modifying the downstream code ( context.get() ) because the generic type is erased at run time. In fact, currently the clazz argument isn't being used at all: you could simply delete that argument and the method would perform identically since the cast to (T) is currently a no-op due to erasure. Your current code is not type safe even for non-genric classes.

So if you don't care about the safety, and just want to avoid having casts in the client code, you can just delete the last argument, and a suppress warnings once on the method.

If you want type safety, you can use super type tokens to keep the generic type information: Guava's TypeToken works and is easily available (IMO every Java project should use Guava already).

It will require downstream changes to your code, however - since you need to capture the type information when objects are added and check it when they come out. You didn't share the code for your Context class, so it's hard to say if that's possible, but in any case you'd want something like this:

static class TypedParam {
    final TypeToken<?> type;
    final Object object;
    private TypedParam(TypeToken<?> type, Object object) {
        this.type = type;
        this.object = object;
    }
}

public static class Context {
    Map<String, TypedParam> params = new HashMap<>();
    TypedParam get(String name) {
        return params.get(name);
    }
}

public static <T> T getParameter(final Context context, final String name, final TypeToken<T> type) {
    TypedParam param = context.get(name);
    if (type.isAssignableFrom(param.type)) {
        @SuppressWarnings("unchecked")
        T ret = (T)param.object;
        return ret;
    } else {
        throw new ClassCastException(param.type + " cannot be assigned to " + type);
    }
}

Basically you know the generic type of all the objects in your parameter map, and check with a "generic aware cast" at run time. I didn't include Context.put() above, but you'd need to capture the type info when the parameter was added.

You still have a @SuppressWarnings("unchecked") , but here it's provably type-safe since you are maintaining the type information (in the guts of many generic classes you'll find provably safe casts, so they can't always be avoided even in correct code).

Answer 2

As you already said, there is nothing like List<String>.class . So if you just want to read parameters without casts you can use something like this:

public static <T> T getParameter(final Map<String, Object> context, final String name) {
    return (T) context.get(name);
}

I assume the your context returns Object as well and you need to suppress type safety checks anyway for that one method.