Java generics: What is the compiler's issue here? (“no unique maximal instance”)

Question

I have the following methods:

public  T fromJson( Reader jsonData, Class clazz ) {
    return fromJson( jsonData, (Type)clazz );
}

public <         


        

Answer 1

The problem is the definition of the second method:

public  T fromJson( Reader jsonData, Type clazz ) {

There is no way for the compiler to tell what type T might have. You must return Object here because you can't use Type clazz (Type doesn't support generics).

This leads to a cast (T) in the first method which will cause a warning. To get rid of that warning, you have two options:

1. Tell the compiler the type. Use this (odd) syntax:

   this.fromJson( jsonData, (Type)clazz );
   

   Note that you need the this here because fromJson() alone is illegal syntax.

2. Use the annotation @SuppressWarnings("unchecked").