SellOneItemTest.java

import static org.junit.Assert.*;

import java.util.*;

import org.junit.*;


public class SellOneItemTest {
    public static class Sale {
		private Display display;
        private PriceList priceList;

		public Sale(Display display, Map<String, String> pricesByBarcode) {
            this.display = display;
            this.priceList = new PriceList(pricesByBarcode);
        }

        public void onBarcode(String barcode) {
            display.formatAndDisplay(textForBarcode(barcode));
        }

		private String textForBarcode(String barcode) {
			if (barcode.isEmpty())
                return "Scanning error: empty barcode";
            
            else if (priceList.doesNotInclude(barcode))
            	return "No product with barcode " + barcode;
			
			else
            	return priceList.priceFor(barcode);
		}
    }
    
    public static class PriceList {
		public Map<String, String> pricesByBarcode;

		public PriceList(Map<String, String> pricesByBarcode) {
			this.pricesByBarcode = pricesByBarcode;
		}

		public boolean doesNotInclude(String barcode) {
			return !pricesByBarcode.containsKey(barcode);
		}

		public String priceFor(String barcode) {
			return pricesByBarcode.get(barcode);
		}
	}

    public static class Display {
        private String text;

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

		private String format(String text) {
			if (isZeroDollars(text)) {
        		return "FREE";
        	} else {
        		return text;
        	}
		}

		private boolean isZeroDollars(String price) {
			return "$0.00".equals(price);
		}

        public String getText() {
            return text;
        }
    }
	
	private static Map<String, String> products() {
		return new HashMap<String, String>() {
            {
                put("123", "$12.50");
                put("456", "$20.00");
            }
        };
	}
    
    @Test
    public void productFound() throws Exception {
    	assertEquals("$12.50", textDisplayedFor("123"));
    }

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

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

    @Test
    public void emptyBarcode() throws Exception {
    	assertEquals("Scanning error: empty barcode", textDisplayedFor(""));
    }
    
    @Test
    public void noProductsToFind() throws Exception {
		assertEquals("Scanning error: empty barcode", textDisplayedFor("", null));
    }
    
    @Test
    public void productWithPriceZeroDisplayedAsFree() throws Exception {
    	Map<String, String> aFreeProduct = new HashMap<String, String>() {
    		{
    			put("789", "$0.00");
    		}
    	};

        assertEquals("FREE", textDisplayedFor("789", aFreeProduct));
    }
    
    @Test
    public void zeroDisplayedAsFree() throws Exception {
    	Display display = new Display();
    	display.formatAndDisplay("$0.00");
    	
    	assertEquals("FREE", display.getText());
    }
    
	private static String textDisplayedFor(String barcode) {
		return textDisplayedFor(barcode, products());
	}

	private static String textDisplayedFor(String barcode, Map<String, String> pricesByBarcode) {
		Display display = new Display();
		Sale sale = new Sale(display, pricesByBarcode);

		sale.onBarcode(barcode);

        return display.getText();
	}
}

You're welcome, Louis. I'm glad I could help. Please write a little about your experience, and enjoy the practice. Take care.