Implement a String like a command in java

Question

I'm trying to implement a String like a command in Java:

public static void main(String[] args) {
   String command = "System.out.println("hello word");";
   ...
}

I need to convert the command String to a command line, in order to execute output "hello word" in the console. I want to using to this technology to improve code:

SimpleDateFormat sdf = new SimpleDateFormat("MMMM");
String month = sdf.format(calIndex.getTime()); 
switch (month) { 
   case "January": 
       leaveDetailCLateLEarly.february += 10;
       break; }
...

The above is quite verbose. I want to write something shorter:

String command = "leaveDetailCLateLEarly."+ month + "+= 10"

Where leaveDetailCLateLEarly is an object and having as an attribute the 12 months. (ie January, February, etc...) Then I have command string: 'leaveDetailCLateLEarly.month= 10' where the month value is able to change (January, February, March, ... ). When project runs, the String will convert to a command line to implement the desired functionality.

Why do you want to do this? I ask because while it can be done it is very complicated and you are likely to be better off doing this another way. — Peter Lawrey, Aug 28 '13 at 10:05
This is called evaluation, and there is no such feature in java. — Levente Kurusa, Aug 28 '13 at 10:05
a double quote within a string should be specified by an escape sequence. — nj-ath, Aug 28 '13 at 10:06
Looks like you want to use (or embed) a scripting language. You could use BeanShell, it looks just like Java. — Thilo, Aug 28 '13 at 10:06
The downvotes are a bit mean. It's a clear enough question, there's just no real way of doing what the OP wants. — NickJ, Aug 28 '13 at 10:10
@NickJ I agree, except there are at least three ways I can think of. ;) — Peter Lawrey, Aug 28 '13 at 10:11
@PeterLawrey I've just read you answer - definitely not something to be taken lightly. I agree with re-thinking the problem and finding another way. — NickJ, Aug 28 '13 at 10:18
What you want does not exist. This is the closest I could find: http://stackoverflow.com/questions/2605032/using-eval-in-java — NickJ, Aug 28 '13 at 10:07

score 5 · Answer 1 · answered Aug 28 '13 at 10:10

Here is a library I wrote many years ago to do this using the Compiler API. Essence JCF It compiles and load any String as a class file. (Including nested classes) without touching the disk.

You can also use Beanshell to dynamically run Java code. This is used by many Java Debuggers, but is not a fully featured version of Java.

Another approach is to use groovy which which supports all the Java code and runs dynamically.

None of these approaches is trivial, or widely used. I suggest you rethink why you need to do this and try a different approach.

Barbe Rouge · Answer 2 · 2013-08-28T20:03:08.620

0

It am not sure really what you are asking. Do you want that if the user inputs the string "command" the "hello world" message is displayed? If so, try this:

Scanner scanner = new Scanner(System.in);
String input ="";
do{
  System.out.println("Command?:");
  input = scanner.nextLine();
while(!input.equals("command"));
System.out.println("Hello world!");

edited Aug 28 '13 at 20:03

answered Aug 28 '13 at 10:10

Barbe Rouge

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It may be theoretically possible to write a Java interpreter in Java but it's a huge task and not a worthy goal. Then what happens if you get it to interpret itself? Will the universe implode? – NickJ Aug 28 '13 at 10:22

Implement a String like a command in java

2 Answers2