CareerCup

public class Calculator {
	private String name; //Good to have

	public Calculator(String name) {
		this.name = name;
	}

	//Basic Addition method
	public int add(int arg1, int arg2);

	//Overloaded Addition methods
	public double add(double arg1, double arg2);
	public float add(float arg1, float arg2);

	
	//Basic Subtraction method
	public int subtract(int arg1, int arg2);

	//Overloaded Subtraction methods
	public double subtract(double arg1, double arg2);
	public float subtract(float arg1, float arg2);

	//Similarly basic and overloaded multiply and division methods
	public int multiply(int arg1, int arg2);
	public double multiply(double arg1, double arg2);
	public float multiply(float arg1, float arg2);

	public int divide(int arg1, int arg2);
	public double divide(double arg1, double arg2);
	public float divide(float arg1, float arg2);

	public int modulus(int arg1, int arg2);
}

We can have a model view controller type architecture in a calculator
where
view can be the display
A controller and a model i.e a small memory storage to store last 10 commands.

If you are writting in C# 4.0 then you can use MEF. This way you wont need to change the code in order to add a new operation