Skip to main content

Maven Only: type parameters of X cannot be determined

This is a rare error that compiling with Eclipse does not catch, but maven compile goal will. In the case in question, There is a Generic return type as follows, as discussed in java forums

private <A> A getValue(){
    return getOtherValue(); //<--  Error occurs here
}

It compiles out an error:

type parameters of <X>X cannot be determined; no unique maximal instance exists for type variable X with upper bounds int,java.lang.Object

The intriguing question is, why does this happen only in maven? Why does it not happen in Eclipse? Few in the java forum have suggested that this is a bug in the compiler. However, an impatient programmer looking for a solution around it, Just cant get it. There is an unanswered stack overflow post here. The java forum is also confusing for a novice.

The simple answer here is, the getValue() method (or the getOtherValue, or the method it calls) is, at some place, not really Strongly typed and the compiler is left to guess wildly about what type the return value actually is. To overcome it, the whole call chain need to infer generics or simply perform an unchecked type cast. The solution will be strongly typed after the call in the chain.

private <A> A getValue(){
    return (A) getOtherValue(); //<--  No Error now
}

Popular posts from this blog

Powered By

As it goes, We ought to give thanks to people who power us. This page will be updated, like the version page , to show all the tools, and people this site is Powered By! Ubuntu GIMP Firebug Blogger Google [AppEngine, Ajax and other Apis] AddtoAny Project Fondue jQuery

Decorator for Memcache Get/Set in python

I have suggested some time back that you could modularize and stitch together fragments of js and css to spit out in one HTTP connection. That makes the page load faster. I also indicated that there ways to tune them by adding cache-control headers. On the server-side however, you could have a memcache layer on the stitching operation. This saves a lot of Resources (CPU) on your server. I will demonstrate this using a python script I use currently on my site to generate the combined js and css fragments. So My stitching method is like this @memize(region="jscss") def joinAndPut(files, ext): res = files.split("/") o = StringIO.StringIO() for f in res: writeFileTo(o, ext + "/" + f + "." + ext) #writes file out ret = o.getvalue() o.close() return ret; The method joinAndPut is * decorated * by memize. What this means is, all calls to joinAndPut are now wrapped (at runtime) with the logic in memize. All you wa...

Faster webpages with fewer CSS and JS

Its easy, have lesser images, css and js files. I will cover reducing number of images in another post. But If you are like me, You always write js and css in a modular fashion. Grouping functions and classes into smaller files (and Following the DRY rule, Strictly!). But what happens is, when you start writing a page to have these css and js files, you are putting them in muliple link rel=style-sheet or script tags. Your server is being hit by (same) number of HTTP Requests for each page call. At this point, its not the size of files but the number server roundtrips on a page that slows your page down. Yslow shows how many server roundtrips happen for css and js. If you have more than one css call and one js call, You are not using your server well. How do you achieve this? By concatinating them and spitting out the content as one stream. So Lets say I have util.js, blog.js and so.js. If I have a blog template that depends on these three, I would call them in three script tags. Wh...