Generic Type Information at Runtime in Java

I’ve been playing around with Hadoop lately, and it seems that this is a situation when the lack of runtime information about generics really hurts.

The bad news: java isn’t going to be changing that anytime soon.

The good news: you don’t need to wait for them to do it.

       Here’s how.

1. Define Useful Generic Base

The secret here is to add a method per generic type to expose it’s type. Here’s an example

public interface UsefulMap<In1, Out1>
{
  public Class<In1> getIn1Type();
  public Class<Out1> getOut1Type();
}

2. Implement

Now when you implement, you will have to provide the runtime information.

public class MyMap implements UsefulMap<Double, String>
{
  @Override
  public Class<Double> getIn1Type()
  {
    return Double.class;
  }
  @Override
  public Class<String> getOut1Type()
  {
    return String.class;
  }
}

The nice part is, if you forget or change it, it won’t compile.

  public Class<Double> getIn1Type()
  {
    return Integer.class; <- Type mismatch: cannot convert from 

                             Class<Integer> to Class<Double>
  }

Once you do this, there are a bunch of convenience functions you can write that will make your setup much easier & safer.

For example, you can now write

HadoopJob.Create()         

         .WithMapper(new MyMapper())

         .AndReducer(new MyReducer())

         .Start();