code: public Writable newWritable(short writableTypeKey) {
          Constructor<? extends Writable> c = constructorMap.get(writableTypeKey);
          if (c == null) {
              throw new IllegalStateException("Unknown writable key: " + writableTypeKey);
          }
          try {
              return c.newInstance();
          } catch (Exception e) {
              throw new RuntimeException("Could not create new Writable instance");
          }
      }
docs: Create a new writable instance (using reflection) given the specified key
  
  @param writableTypeKey Key to create a new writable instance for
  @return A new (empty/default) Writable instance