If I understand you correctly, what you want is a runtime method
dispatch. The short answer to your question is "no it is not possible."
The medium answer is "well, yes it is, but it is generally not a good
idea to try and use."

The easiest way to do what you want to do is to manually set functions
up yourself:

class Foo {
   private String varA, varB, varC;
   // constructors, etc.

   public void setA(String newA) {varA = newA;}
   public void setB(String newB) {varB = newB;}
   public void setC(String newC) {varC = newC;}

   public void set(String variable, String value) {
       if ("A".equals(variable))
           setA(value);
       else if ("B".equals(variable))
           setB(value);
       else if ("C".equals(variable))
           setC(value);
       else
           throw new IllegalArgumentException("Variable "+variable+
               " not found!");
   }
}

If you really want the long answer, you probably don't. Use the previous
solution if at all feasible or not at all.

Still want it? Here's the reflection method:

class Foo { /* Defined similar to above sample, except w/o set */ }

class Test {
    public void foobar() {
        String variable = "Lastname";
        String value = "Stroustrup"; // Kudos if you get the reference!
        Foo bar = new Foo(); // assume it works

        Class<?> fooClass = Foo.class;
        try {
           Method m = fooClass.getMethod("set"+variable);
           m.invoke(bar,value);
        } catch (Exception e) { // See the APIs for all exceptions
            // Don't do this in real code.
        }
    }
}

I don't recommend that approach.