Java, impossible cast Object to Float.....why?

Because prova is a Double, and Double is not a subtype of Float.

Either you could start with a float literal: 9.2f (in which case prova would actually be a Float) or, you could it like this:

Float prova2 = ((Double) prova).floatValue();

Because you are relying on autoboxing when you wrote

Object prova = 9.2;

If you want it to be a Float, try

Object prova = 9.2f;

Remember that java.lang.Float and java.lang.Double are sibling types; the common type is java.lang.Number

If you want to express a Number in whatever format, use the APIs, for example Number.floatValue()

9.2 is a double literal. Try 9.2f instead.

Object prova = 9.2f; // float literal is auto-boxed to a Float
System.out.println(prova);
Float prova2 = (Float) prova; // Float can be cast to Float, while Double cannot
System.out.println(prova2);

The error message (which you probably should have included in your question) explains it quite well also:

Exception in thread "main" java.lang.ClassCastException: 
    java.lang.Double cannot be cast to java.lang.Float

Because if you don't specify, it will be a double. If you want it to be a float, you need

Object prova = 9.2F;
System.out.println(prova);
Float prova2 = (Float) prova;
System.out.println(prova2);