null strategy with enums/design patterns

[blondy khosa]

  public enum Strategy {
 
    DOCALCULATIONS {
 
        public double getCost() {
            return 0; 
        }
        },
    DOANOTHER {
 
        public double getCost() {
            return 2;
        }
        };
   
    public abstract  double getCost  
    
}  

public class TestStrategy{
private Strategy getStrategy(){
if(true){
return Strategy.DOCALCULATIONS;}
if(false){
return Strategy.DOANOTHER;}
return null; // my code will not work...I want to return a null strategy but //I don't how to do it...I have just started working with enums....please help with an example of code on how to do it
}
}