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Define an array (or Map, or ENUM) of functionsTag(s): Language


Using an interface
First we define an interface. 
public interface Command {
  void exec();
}
then the implementation of the functions 
public class ACommandImpl implements Command {
  @Override
  public void exec() {
    System.out.println("This is Command type A");
  }
}


public class BCommandImpl implements Command {
  @Override
  public void exec() {
    System.out.println("This is Command type B");
  }
}


public class CCommandImpl implements Command {
  @Override
  public void exec() {
    System.out.println("This is Command type C");
  }
}
We put in an array an instance of each implementation. Then we are able to run the desired function by using the array index. 
public class TestCommand {
  public static void main(String[] args) {
    Command[] commands = new Command[3];
    commands[0] = new ACommandImpl();
    commands[1] = new BCommandImpl();
    commands[2] = new CCommandImpl();
    // no error checking!
    for (;;) {
      String s = javax.swing.JOptionPane.showInputDialog
          ("Command no ? (0,1 or 2)", new Integer(0));
      if (s == null) break;
      commands[Integer.parseInt(s)].exec();
    }
  }
}
To use a String instead of a number to access to desired function, use a Map. 
public class TestCommand {
  public static void main(String[] args) {
    java.util.Map <String, Command> commands =
      new java.util.HashMap<String, Command>();
    commands.put("A", new ACommandImpl());
    commands.put("B", new BCommandImpl());
    commands.put("C", new CCommandImpl());
    // no error checking!
    for (;;) {
      String s = javax.swing.JOptionPane.showInputDialog
        ("Command ID ? (A,B or C)", "A");
      if (s == null) break;
      commands.get(s.toUpperCase()).exec();
    }
  }
}
Using an enum
public interface Command {
  void exec();
}


enum Functions implements Command {
  A() {
    public void exec() {
      System.out.println("This is Command type A");
    }
  },
  B() {
    public void exec() {
      System.out.println("This is Command type B");
    }
  },
  C() {
    public void exec() {
      System.out.println("This is Command type C");
    }
  }
}
then 
public class TestCommand {
  public static void main(String[] args) {
    // no error checking!
    for (;;) {
      String s = javax.swing.JOptionPane.showInputDialog
        ("Command ID ? (A,B or C)", "A");
      if (s == null) break;
      Functions function = Functions.valueOf(s.toUpperCase());
      function.exec();
    }
  }
}
NOTE : Using this technique, it's possible to simulate "switch ... case" based on a string value.