Not sure whether to override or overload methods in interface

[Noam Ingalls]

Working on the parser for my interpreter, and with a lot of help I've come up with this idea on how to do it:

1. Split the source string into 2 parts with String.split(); get an array parts[] with two elements at [0] and [1].
2. If parts.length==1 then call directive.execute(); else if parts.length>1, String input=parts.[1]; call directive.execute(input)

The String input gets sent on to the appropriate class for tokenizing and handling. Now all the classes should be using directive.execute(input) except for one class (EndStatement), where input will be null and it only calls directive.execute(). I know I'm supposed to override or overload the method declared in my interface to do that, but I'm not sure how and reading up on the subject hasn't really helped me get my head around it either. Could someone help get me on track? Thanks!

Code for interface method and example classes

public interface Directive //Map keywords to an interface
	{	
	
		public void execute (String input);//how to override or overload this?
		
	}
		
	
	public class PrintlnStatement implements Directive//this class always has input
	{

		public void execute(String input)
		{	//@Override
		
			String[] printlntokens = input.trim().split("[\"]");
			for(int i=0; i<printlntokens.length; i++)
			{
				System.out.println(printlntokens);
			}
		}
	}
	
	
	public class EndStatement implements Directive//this class's input should be null
	{	//@Override
		public void execute(String input)
		{	
			String input=null;
			System.out.print("TPL finished OK [" + " x lines processed]");
			System.exit(0);
		}
	}

Code for parsing loop

for (String listString: myString)//iterate across arraylist
		{	if(listString.startsWith("#", 0))//ignore comments starting with #
				{
					continue;
				}	
			
			String[] parts=listString.trim().split("[\\s]+",2);//split String into 2 parts to get commands
			Directive directiveInfo= commandHash.get(parts[0]);//parts[0] should always be a command
			
			if(directiveInfo!=null)//check if command is valid
			{
				if (parts.length==1)//indicates there is only a command keyword string
				{
					String input=null;
					directive.execute();
				}//close(parts.length==1)
				
				else 
				{
					if (parts.length>1)//indicates that there is command+input
					{
						String input=parts[1];//get information for subsequent processing
						directive.execute(input);
						
					}//close (parts.length>1)
				}
			}
				else
				{
					System.out.println("No valid command given");
				}				
		}
	}//close parseCommands()

[Joanne Neal]

I'd just pass in an empty string if there are no arguments

directive.execute( (parts.length == 1) ? "" : parts[1] );

[Noam Ingalls]

How should I write that sentence for the case where parts>1 then?

[Joanne Neal]

Sorry. I only half read your original post. I thought you were just splitting it into two parts - the command and the rest of the line.

As you are splitting it up into multiple parts, I would just pass the whole array to your execute method and leave it to the individual implementing classes to interpret/validate the values.

String[] parts=listString.trim().split("[\\s]+",2);
Directive directiveInfo= commandHash.get(parts[0]);
directiveInfo.execute(parts);

Obviously you'll need to change the declaration of the execute method in your interface to accept a String[] instead of a String

[Noam Ingalls]

Hmm. Currently my PRINTLN command class only supports arguments in quotes. I'm trying to make PRINTLN support the equivalent of System.out.println() (effectively that command would be parsed as parts[0], with parts[1] being empty), but when I used an if-else statement to try and do that, it gave me exception errors, again it's ArrayIndexOutOfBounds. Is there some way to modify it?

This is what I tried:

public void execute(String[] parts)
		{	
			String input=parts[1];
			if(input==null);
			{
				System.out.println();
			}	
			else
			{
				String[] printlnTokens = input.trim().split("[\"]"); //treat everything after a double quote as a single token
				for(int i=0; i<printlnTokens.length; i++)
				{
					System.out.println(printlnTokens[i]);	
				}
			}
		}

>

[Jeff Verdegan]

This will tell you if the second element of an array is null, provided that array has at least two elements.

            String input=parts[1];  
            if(input==null);  

If you want to test whether an array has fewer than two elements (which is a completely different thing), you need to look at the array's length member.