How could I write Java code that is executed like javascript code used together with the eval function?
What I would like to achieve would be something like this:
Look into BeanShell or Groovy. Both will give you reasonable solutions--but those solutions rely on my interpretation of your problem, which may be flawed.
I've used the JavaScript engine shipped with Java 6 and it works quite well. The performance of the engine is very very decent.
Check the page http://java.sun.com/javase/6/docs/technotes/guides/scripting/programmer_guide/index.html
For completeness sake, the Compiler API is coming in the next version of Java..
I believe the compiler API is present in the current Java SE 6. See the javadoc.
See this blog post for details and an example.