Why Zserio templates are not implemented by Java generics?

[mikir]

We have identified the following problems if Zserio templates would be implemented by Java generics:

1. Primitives types which can be used as template parameters (similarly to C++)

   Consider the following easy Zserio template code:

   struct MyTemplate<TYPE>
   {
       TYPE x;
   };
   

   If template parameter is a primitive Zserio type (uint8, string, bool), there is no way how to deduce corresponded write and read methods from them. For example, primitive types uint16 and varuin32 have the same Java mapping to int type but different coding to the bit stream. Therefore for each possible primitive type, there would have to be a corresponded class wrapper available in the runtime library, e.g. UInt8TemplateParameter, BoolTemplateParameter, etc..

2. Expressions with template parameter

   Consider the following Zserio template code:

   struct Coordinate2D<TYPE>(TYPE xOffset, TYPE yOffset)
   {
       TYPE x;
       TYPE y;
       function TYPE getX()
       {
           return x + xOffset; // expression with template parameter
       }
   
       function TYPE getY()
       {
           return y + yOffset; // expression with template parameter
       }
   };
   

   If there are some expressions with template arguments, Zserio must check the correctness of such expression during each template instantiation. However, Java generic parameters must be an objects with defined interface. There is no way how to define common interface for all possible template parameters which could be used for expressions.

3. Instantiation of template parameter

   There is one specific problem in Java generics. There is no way how to instantiate template parameter. Something like the following command is not possible:

   TYPE object = new TYPE();
   

   Such instantiation is necessary during reading. This can be solved to pass created empty object in read method or using reflections. But this will complicate things for applications even more.