Evaluating Strings

[Felipe Rocha]

Hi there,
Is there eval() on JSP or Servlet as there is in PHP and JScript? How can i evaluate a string using JAVA ? The problem is like this:
I use a string ( that defines which function must be called) as parameter, i get it using getRequestParameter() and after that i want to call the function which has the same name as the string got , how can i do that ?
Thanks

[Bosun Bello]

The methods are available: equals(), startsWith(), endsWith(), etc. Check the API.

Bosun

[James Swan]

One way to do this is to use the reflection APIs (java.lang.reflect), these let you interrogate a class to get info about its members.
eg.

import java.lang.reflect.*;
public class MainClass {
    
    public static void main(String[] args) {
        
        if (args.length < 1) {
            System.out.println("usage: java MainClass <methodToRun>");
            System.exit(1);
        }

try {
            // load the class
Class c = Class.forName("TestClass");
            // get an instance
Object o = (TestClass)c.newInstance();
            // call the method passed in from cmd line
Method m = c.getDeclaredMethod(args[0],null);
m.invoke(o,null);
} catch (Exception e) {
e.printStackTrace();
}
    }
}
class TestClass {

TestClass() {
System.out.println("TestClass created...");
}

public void testMethod1() {
System.out.println("testMethod2() invoked...");
}


public void testMethod2() {
System.out.println("testMethod2() invoked...");
}
}

I have just started looking at the reflection APIs myself, so there might be better ways of doing this.
James.

[sundari swamy]

import java.lang.reflect.*;
public class MainClass {
    
    public static void main(String[] args) {
        
        if (args.length < 1) {
            System.out.println("usage: java MainClass <methodToRun>");
            System.exit(1);
        }

try {
            // load the class
Class c = Class.forName("TestClass");
            // get an instance
Object o = (TestClass)c.newInstance();
            // call the method passed in from cmd line
Method m = c.getDeclaredMethod(args[0],null);
m.invoke(o,null);
} catch (Exception e) {
e.printStackTrace();
}
    }
}

I feel the code given by James is a good start to work on with your requirements.
Few points I noted w.r.t James code , you also have to Prefix the Package name. And I guess the typecasting is also not required .(you have to hardcode the className for typecasting in this case. This wouldn't possibly help you to execute dynamically)

[Kris Nelson]

How many possible values can this parameter have ( i.e. how many functions are there )? If there's not too many, why couldn't you use an if statement with the necessary else if blocks?

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[Felipe Rocha]

Thanks a lot everybody . I used the reflect API and it worked great.
Thanks again