Working Effectively with Legacy Code

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Ch. 2: Working with feedback

A test is not a unit test if:

1. It talks to a database.
2. It communicates across a network.
3. It touches the file system.
4. You have to do special things to your environment to run it.

Ch. 3: Sensing and separation

Fake objects

Sale sale = new Sale();
sale.scan(barcode); // scan() calls Display.showLine(item)

public interface Display
{
    void showLine(String line);
}

public class Sale
{
    private Display display;

    public Sale(Display display) {
        this.display = display;
    }

    public void scan(String barcode) {
        // item = get_item(barcode);
        String lineItem = item.name() + " " + item.price();
        display.showLine(lineItem);
    }
}

public class FakeDisplay implements Display
{
    private String lastLine = "";
    public void showLine(String line){
        this.lastLine = line;
    }
    public String getLastLine(){
        return lastLine;
    }
}

// test
public class SaleTest extends TestCase
{
    public void testDisplayAnItem() {
        FakeDisplay display = new FakeDisplay();
        Sale sale = new Sale(display);
        sale.scan("1");
        assertEquals("xxx: $1.00", display.getLastLine());
    }
}

// test with Mock
public class SaleTest extends TestCase
{
    public void testDisplayAnItem() {
        MockDisplay display = new MockDisplay();
        display.setExpectation("showLine", "xxx: $1.00")
        Sale sale = new Sale(display);
        sale.scan("1");
        display.verify();
    }
}

A seam is a place where you can alter behavior in your program without editing in that place.