How can I replace a box of text with beamer?

I currently have this piece of text for a quiz:

\begin{frame}{Quiz}
\begin{block}{Frage}
types of the objects, so explicit casts don't have to be used
and the compiler can produce type safe code.

What implications have the generics for the runtime
performance of the  program which uses them?
\end{block}

\only<1>{
\begin{itemize}
\item With the generics the compiler can optimize the code for
used types. This and the omission of the casts are the
reasons why the code compiled with the generics is
\textbf{quicker} than the one compiled without.
\item The usage of generics has \textbf{no implications} for
the runtime performance of the compiled programs.
\item The improved flexibility and type safety means that the
compiler has to generate concrete implementation from
the generic template for each used type. This means
that applications start \textbf{a bit slower}.
\end{itemize}}
\only<2>{
The Java Virtual Machine and the copiled byte code are Generics
agnostic. The comiled byte code does not differ from byte code
compiled from sources which don't use the generics.
So using the generics has \textbf{no impact} on the runtime
performance of compiled Java code.
}
\end{frame}


I would like to present the question with the different possibilities to answer and on the next slides the different possibilities to answer should disappear, the rest should not move and the correct answer with an explanation should appear.

The current solution is not optimal, as the question moves. I've also tried visible instead of only, but with this solution the explanation doesn't appear at all.

(I know I could manually fix the "moving question problem" by adding space, but I don't want to do this for all slides.)

-
Did you try uncover<2>? –  Sigur Dec 15 '12 at 10:01
If I use uncover, the answer will be shown before I reveal it. I don't want my students simply to read the answer... –  moose Dec 15 '12 at 10:09
I know that there are 2 similar commands and one of them save space for the next one. Are you using transparency 100% to completely hide the items? –  Sigur Dec 15 '12 at 10:12
Have you tried replacing \only by \onslide? –  Brent.Longborough Dec 15 '12 at 10:16
@Sigur: With my the theme of my university, it fits on a single slide. –  moose Dec 15 '12 at 10:37

You may try the overprint environment (chap. 9.5, beamer user guide).

\documentclass{beamer}

\begin{document}
\begin{frame}{Quiz}
\begin{block}{Frage}
types of the objects, so explicit casts don't have to be used
and the compiler can produce type safe code.

What implications have the generics for the runtime
performance of the  program which uses them?
\end{block}

\begin{overprint}
\onslide<1>
\begin{itemize}
\item With the generics the compiler can optimize the code for
used types. This and the omission of the casts are the
reasons why the code compiled with the generics is
\textbf{quicker} than the one compiled without.
\item The usage of generics has \textbf{no implications} for
the runtime performance of the compiled programs.
\item The improved flexibility and type safety means that the
compiler has to generate concrete implementation from
the generic template for each used type. This means
that applications start \textbf{a bit slower}.
\end{itemize}
\onslide<2>
The Java Virtual Machine and the copiled byte code are Generics
agnostic. The comiled byte code does not differ from byte code
compiled from sources which don't use the generics.
So using the generics has \textbf{no impact} on the runtime
performance of compiled Java code.
\end{overprint}

\end{frame}
\end{document}

-
That's interesting ... onslide<1>{myContent} gives different results than onslide<1> myCodntent. –  moose Dec 15 '12 at 11:11
This is great! Exactly what I needed. There is one major drawback, though: it seems I can no longer use the handouts documentclass option if I use an overprint environment. Anyone know a way around this? –  William DeMeo Feb 7 '13 at 20:09
\documentclass[11pt,a4paper]{beamer}

\begin{document}

\begin{frame}{Quiz}
\begin{block}{Frage}
types of the objects, so explicit casts don't have to be used
and the compiler can produce type safe code.

What implications have the generics for the runtime
performance of the  program which uses them?
\end{block}

\uncover<1>{
\begin{itemize}
\item With the generics the compiler can optimize the code for
used types\ldots
\item The usage of generics has \textbf{no implications} for
the runtime performance of the compiled programs.
\item The improved flexibility and type safety means that the
compiler has to\ldots
\end{itemize}}
\uncover<2>{
The Java Virtual Machine and the copiled byte code are Generics
agnostic. The comiled byte code does not differ from byte code
compiled from sources \ldots
}
\end{frame}
\end{document}


Here is the sample above: beamer slide

-
It seems as if I hadn't expressed myself precisely. I'm sorry for that. I want the correct answer to be able to take the space where the possible answers were. I don't want the correct answer to appear below the possible answers. –  moose Dec 15 '12 at 10:36
Hum, OK, I see understand now. Sorry. –  Sigur Dec 15 '12 at 10:41