An official REPL is a good idea. There is an unofficial REPL for a long time (ten years or more?): BeanShell http://www.beanshell.org/ http://www.beanshell.org

An official REPL is a good idea. There is an unofficial REPL for a long time (ten years or more?): BeanShell http://www.beanshell.org/ http://www.beanshell.org/ Add bsh.jar and a little glue code and presto you have a shell into your application. There was a Java-based editor, jEdit, which integrated with Beanshell. I could do search-and-replace with replacement programmed in Beanshell. Aah, these were good times.