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During the development of a software, I need to execute a statement which is stored as a string. Say for example:

String test = " int a; ";

In the above example, I want to declare a variable a.

Another example is the execution of a mysql statement from a string: 

String sqlquery ="SELECT * FROM FULLDETAILS WHERE Classname = 'test' ";
            con = (Connection) DriverManager.getConnection("jdbc:mysql://localhost:3306/executable","root","mysql");
             st = (Statement) con.createStatement();
             rs = st.executeQuery(sqlquery);
                          ^

I want to achieve this for Java statements too. Is there a way?

EDIT :

The above given is an example, the real situation is much different (but the thing I basically need is the same).

The situation is that, I have a number of jpanels and I want to add some components to it, say a label, the content of the label is obtained from a database, and its only during runtime I can know, to which panel I should add the component. In this case, I cannot designate the panel by name, say if I have 10 jpanels, I thought I could use a string like:

String test = " jpanel " + i + " .add(jlabel1); " ;

will result in the value of test will be:

jpanel1.add(jlabel1); // provided i is a string with value of i is 1

now I want to execute this. Is there a way?

COD3BOY
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    Why would you want to do that? – yannis Dec 02 '11 at 07:45
  • Not simple. AFAIK, the standard JRE does not include a compiler. Additionally, all local variables may have other names. If this is possible, probably only by creating a new class from a string. – fuz Dec 02 '11 at 08:05
  • @YannisRizos please see edit! I'm not sure if I was successful in conveying the idea, but believe me I need this :D is there a way?? – COD3BOY Dec 02 '11 at 08:30
  • I did edit, but I'm not very good with Java and I can't really tell if the idea gets through... Anyone out there with better Java skills please review the question and edit accordingly... – yannis Dec 02 '11 at 08:46
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    I'm pretty confident you want to archive something that doesn't need some hypothetical `eval` magic at all, but it's hard to understand what exactly you want to do. The Map solution given by Volker seems plausible, though. – Erich Kitzmueller Dec 02 '11 at 09:29

2 Answers2

8

Maybe that's not what you want, but you may use Java's Reflection API carefully to call methods or something else based on (for example) strings.

SQL statements are far different, as they are not really executed by inside Java, they are processed by some additional SQL Server on the other side of the JDBC connection.

Somewhere, you should have an initial list of all your panels or create it at startup or whatever if I am right? You should go and find a datastructure to hold them (a HashMap seems suitable, for example), add all Panels and use a key to get them back and add labels as you want.

Rough example:

Map<Integer, Panel> panelMap = ...
for(int i = 1; i < 10; i++) {
    Panel panel = new Panel();
    panelMap.put(i, panel); 
}

panelMap.get(1).addLabel(labelForPanel1); (for all labels)

Variable names are more or less irrelevant when the program executes, they are all objects then. Hope that suits your needs a bit more...or maybe now I missed totally :)

Volker
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3

What you seem to want to do is something like an eval() function for Java. As far as I know, Java doesn't provide one, but it has support for scripting languages, so you might want to fall back to that.

I found a question on StackOverflow. See the answers there. Alternatively, check out JEXL. It's an Apache Project and I've used it successfully in a project before.

Community
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Raku
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