Java Glossary : eval

Last updated 2004-06-28 by Roedy Green ©1996-2004 Canadian Mind Products

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eval

In many languages you can take a dynamically created String such as "6*(4+6^2)-cos(20)" and ask to have it evaluated, as if it were a miniature computer program. The function to do this often has a name such as eval. Java has no such function. What can you do? Here are four different approaches:

1. Compose a miniature Java program on the fly, one complete *.java file and invoke sun.tools.javac.Main instead of execing javac.exe. Use Class.forName or reflection on to then execute the dynamically compiled method.
2. Similar to the method above, but directly generate some byte codes for a miniature class.
   JASM on how
3. Write a parser using one of the compiler-compiler tools such as JavaCC.
4. Use a different language that supports dynamic scripting that is compatible with Java, e.g. Jpython or FESI.

For simple applications, all you need is a Hashtable that has lookup by String to give you a delegate object to execute some prewritten piece of code.

JASM ¤ scripting languages ¤ Spreadsheet Amanuensis student project

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