实例化泛型类型时发出警告

Question

I have a snippet code for instantiation of a generic type as following:

public class GenericMessageMapper<I ,X>
{
   public X xFromI(I imf)
   {
      // do thing by imf argument
      Class<X> clazz = (Class<X>)((ParameterizedType)
                                   this.getClass()
                                       .getGenericSuperclass())
                                       .getActualTypeArguments()[1];
      X x = clazz.newInstance(); 
   }
}

Above code worked fine but MyEclipse show a warning on the code(a yellow under line on preparing clazz variable line) by this message : Type safety: Unchecked cast from Type to Class<X>

I temporary add following annotation in above of xFromI method:

@SuppressWarnings("unchecked")

What is reason of this warning and what is solution?

Answer 1

A warning is just that. A warning. Sometimes warnings are irrelevant, sometimes they're not. They're used to call your attention to something that the compiler thinks could be a problem, but may not be.

In the case of casts, it's always going to give a warning in this case. If you are absolutely certain that a particular cast will be safe, then you should consider adding an annotation like this just before the line:

@SuppressWarnings (value="unchecked")

refer Type safety: Unchecked cast

Answer 2

It only means that the type of the class is only known at runtime, so a situation could arise where you think you are supposed to get a class of type X but actually it is of other type (it doesn't know it in compile time since X can be of any type).

Basically you can ignore this warning.

Answer 3

ftom2 is right - in usual situation you should perform instance check ( instanceof ) before cast in case of this warning. But in your case this is impossible because generics in Java was implemented by erasure. Class<X> is not reified type so it is impossible to use it for instance check and only way to turn off the warning is to use @SuppressWarnings(value = "unchecked") . To learn more about generics you can use this book.