Question

I’ve read about non-static variable this cannot be referenced from a static context error but I don’t understand why I get it in my case (line return new CommandParser1(command);)?. I just create instance of class. That’s all. What is the problem?

public class ProtocolUtility {

    public static CommandParser createParser(String command) throws Exception {           
        switch (command) {
            case COMMAND_1:
                return new CommandParser1(command);                  
            case COMMAND_2:
               return new CommandParser2(command);
            default:
                return null;
        }
    }

   public abstract class CommandParser {

       protected String command;

       public String getCommand() {
          return command;
       }       
   }

   public  class CommandParser1 extends CommandParser {       
       public CommandParser1 (String command){
           //...
       }       
   }

   public  class CommandParser2 extends CommandParser {      
       public CommandParser2 (String command)  {
           //...
       }      
   }

}

Answer 1

CommandParser is an inner class which means it needs an instance of outer class (ProtocolUtility) to be created. Change its declaration to:

public static abstract class CommandParser {

Alternatively declare CommandParser in a separate .java file.

Your code would also work if createParser() wasn’t static as in this case an instance of ProtocolUtility you are currently in would be used as an outer instance.