SellOneItemTest.java

package ca.jbrains.pos.test;

import org.junit.*;
import static org.junit.Assert.*;
import java.util.*;

public class SellOneItemTest {

	private POSDisplay display = new POSDisplay();
	private Sale sale;

	public static class Sale {
		private POSDisplay display;
		private Map<String, String> pricesByBarcode = new HashMap<String, String> ();

		public Sale(POSDisplay d, Map<String, String> pricesByBarcode) {
			if (pricesByBarcode == null) {
				throw new RuntimeException("Invalid price list");
			}
			display = d;
			this.pricesByBarcode = pricesByBarcode;
		}

		public void onBarcode(String barcode) {
			if ("".equals(barcode)) {
				display.onEmptyBarcode();
				return;
			}

			if (!pricesByBarcode.containsKey(barcode)) {
				display.onInvalidBarcode(barcode);
				return;
			}
			display.showPrice(pricesByBarcode.get(barcode));
		}
	}

	public static class Display {
		private String text;

		public void setText(String text) {
			this.text = text;
		}

		public String getText() {
			return text;
		}
	}

	public static class POSDisplay extends Display {
		public void onEmptyBarcode() {
			setText("Scanning error: empty barcode");
		}

		public void onInvalidBarcode(String barcode) {
			setText("No product with barcode " + barcode);
		}

		public void showPrice(String price) {
			setText(price.equals("$0.00") ? "FREE" : price);
		}		
	}

	@SuppressWarnings("serial")
	@Before
	public void setUp() {
		sale = new Sale(display, new HashMap<String, String>() {
			{
				put("123", "$12.50");
				put("456", "$20.00");
				put("333", "$0.00");
			}
		});
	}

	@Test
	public void productFound() throws Exception {
		assertForBarcodeDisplayShows("123", "$12.50");
	}

	@Test
	public void anotherProductFound() throws Exception {
		assertForBarcodeDisplayShows("456", "$20.00");
	}

	@Test
	public void productNotFound() throws Exception {
		assertForBarcodeDisplayShows("999", "No product with barcode 999");
	}

	@Test
	public void emptyBarcode() throws Exception {
		assertForBarcodeDisplayShows("", "Scanning error: empty barcode");
	}

	@Test
	public void freeProductHasDistinctFormat() throws Exception {
		assertForBarcodeDisplayShows("333", "FREE");
	}

	@Test(expected=RuntimeException.class)
	public void nullProductListIsInvalid() {
		new Sale(display, null);
	}

	private void assertForBarcodeDisplayShows(String barcode, String message) {
		sale.onBarcode(barcode);
		assertEquals(message, display.getText());
	}
}

First, my own observations, and then my comments on your comments.

1. Why the inconsistency between lines 20 and 21? If you plan to write this.x = x; in constructors, then I recommend doing it uniformly.
2. I'd like the exception you throw on a null pricesByBarcode to describe the problem more precisely.
3. You have decided to design Sale to require non-null a pricesByBarcode even though there is a code path through onBarcode() that doesn't require pricesByBarcode. If you sometimes need one and sometimes don't, what might that say about the design?
4. I see that you've moved methods responsible for displaying something into POSDisplay, which I quite like.
5. I wonder why you named the methods in POSDisplay inconsistently. You have the imperative showPrice() but the event handling onInvalidBarcode() and onEmptyBarcode().
6. You named the method onInvalidBarcode(), but is a barcode not in our catalog truly invalid? What other method name would describe this special case more accurately?
7. Why have Display and POSDisplay as separate classes?
8. I see that formatting $0 as "FREE" now appears in the view, which I find quite sensible. Now look at the method showPrice(String price). Should we model a price as text?

Otherwise, I like what you've done, and especially assertForBarcodeDisplayShows().

Regarding whether to write tests directly for POSDisplay, I think you can survive without it for a moment.

I decided to make a small number of improvements at https://gist.github.com/731349 to show you an example of how I approach the code you're provided here. You can clone the entire repository to step through the commits and examine the differences.