GraphQL Java should use generics where possible

[bbakerman]

DataFetchers today do no use generics. So you get code like

    public DataFetcher exampleFetcher() {
    return environment -> {
        GraphContext context = (GraphContext) environment.getContext();
        return supplierMethodCall(context);
    };
}

The ACTUAL return type is Object so its hard to maintain. But the shape of fetched objects are really important in GraphQL.

Where as

public DataFetcher<FooObject> exampleFetcher() {
    return environment -> {
        GraphContext context = environment.getContext();
        return supplierMethodCall(context);
    };
}

Tells a maintainer what they need to know. It must return a FooObject and hence so must the method call etc..

In fact this could be applied to a number of places in GraphQL Java. DataFetchingEnvironment is another place where we should do this. Its neater looking and tells people what they are getting.

         // where it would be defined as  <T> T getContext();
        GraphContext context = environment.getContext();
        return supplierMethodCall(context);

There are a number of places in GraphQL where one is casting into specific types. Where that occurs the API should be T defined.

You already do it on DataFetchingEnvironment.getArg

    public <T> T getArgument(String name) {
        return (T) arguments.get(name);
    }

I suggest it is done on the source and context attributes as well.

[dminkovsky]

How about this as a start? #262

[dminkovsky]

Also c.f. #91

[bbakerman]

fixed