how to implement Java equivalent of Function Pointers via abstract classes

[jon williams]

Im trying to to create a simple 4 function calculator using a jump table without switch case or if/else statements. I understand I can create the jump table via function pointers but I am kindda blanking out. I've started with the addition and subtraction part of the program but am trying to grasp the design/strategy to use. I am also getting an error when putting the method in the array, so far this is what i have:....I'm also trying to figure out how to initialize my concrete classes, there seems to be a problem...

      public class Calculator {

   public abstract class Functor{

    public abstract double Compute(double op1, double op2);
    }
    
    public class Addition extends Functor
    {
       
        public double Compute(double op1, double op2){
            System.out.println("addition= ");return op1 + op2;}
    }

    public class Subtraction extends Functor
    {
       
        public double Compute(double op1, double op2){ 
            System.out.println("subtraction= ");return op1 - op2;}
    }
       
    public static void main(String[] args) {

        Functor add = new Addition(); // problem here
        Functor minus = new Subtraction();  // problem here
        

	
    }
}

[Dheeraj Joshi]

You can not create directly an object of Addition and Subtraction classes.

Try creating something like this.

Calculator  c_obj = new Calculator ();
Functor add = null;
Functor minus = null;
	    	
add = c_obj.new Addition();
minus = c_obj.new Subtraction();

You can create inner class object by using outer class object.

Regards
Dheeraj Joshi