CrewNotSelectedException factory example

public class CrewNotSelectedException extends Exception{

    private final String message;
    private static CrewNotSelectedException instance;

    public static CrewNotSelectedException factory(){
        if(instance==null){
            instance=new CrewNotSelectedException();
        }
        return instance;
    }

    private CrewNotSelectedException() {
        this.message = "Nevybratý člen posádky";
    }

    @Override
    public String getMessage(){
        return message;
    }
}