Test-First Challenge: Spreadsheet

This exercise offers you a chance to practice test-first through a series of challenges.

Each challenge provides a set of tests. Your job is to write code that supports the tests.

My goals are:

  1. To give people learning or practicing test-first a way to begin small-step incremental programming, without the pressure of having to invent tests.
  2. To find out how similar the designs will be.

The original form of the challenges were posted on two egroups: extremeprogramming (for regular readers there) and xplorations (for low volume). The tests are in Java, but should have an obvious translation to other languages.

Problem Domain

For the problem, I'll use a (super-) simple spreadsheet. My plan is to explore formulas and dependencies before dealing with any user interface.

What To Do

  • Implement code to meet the tests, one at a time. (No fair peeking ahead:)
  • You may feel the need to add more tests than I've given, either to explore an area more thoroughly, or to let yourself take smaller steps. This is a good impulse – add the tests you want.
  • Refactor after each test.
  • Reflect on the process
    • Are these the tests you'd have written?
    • Did you create the simplest code you could?
    • Did you spend much time debugging?
    • Any surprises in how your system is structured?
  • (Optional) After you've done a particular challenge, I'd love to hear from you. Email to William.Wake@acm.org. If you want to make life easy and interesting for me, make the message like this:
    Subject: CHALLENGE PART n
    Body: Brief description of the decisions you made ("I used a 
    Frobble to calculate interest, and I split Foos into Bars and Bazs.")
    Attachment: A Zip file of your source code, named TFn.zip.

    I'd be glad to hear your design decisions and comments even if you don't want to send your code. I won't re-post anything you send me without your permission.

Challenge, Part 1

Domain: A simple spreadsheet with columns A..Z, AA.., and rows 1..n.

Challenge Tests

public void testThatCellsAreEmptyByDefault() {
  Sheet sheet = new Sheet();
  assertEquals("", sheet.get("A1"));
  assertEquals("", sheet.get("ZX347"));

// Implement each test before going to the next one.

public void testThatTextCellsAreStored() {
  Sheet sheet = new Sheet();
  String theCell = "A21";

  sheet.put(theCell, "A string");
  assertEquals("A string", sheet.get(theCell));

  sheet.put(theCell, "A different string");
  assertEquals("A different string", sheet.get(theCell));

  sheet.put(theCell, "");
  assertEquals("", sheet.get(theCell));

// Implement each test before going to the next one; then refactor.

public void testThatManyCellsExist() {
  Sheet sheet = new Sheet();
  sheet.put("A1", "First");
  sheet.put("X27", "Second");
  sheet.put("ZX901", "Third");

  assertEquals("A1", "First", sheet.get("A1"));
  assertEquals("X27", "Second", sheet.get("X27"));
  assertEquals("ZX901", "Third", sheet.get("ZX901"));

  sheet.put("A1", "Fourth");
  assertEquals("A1 after", "Fourth", sheet.get("A1"));
  assertEquals("X27 same", "Second", sheet.get("X27"));
  assertEquals("ZX901 same", "Third", sheet.get("ZX901"));

// Implement each test before going to the next one.
// You can split this test case if it helps.

public void testThatNumericCellsAreIdentifiedAndStored() {
  Sheet sheet = new Sheet();
  String theCell = "A21";

  sheet.put(theCell, "X99"); // "Obvious" string
  assertEquals("X99", sheet.get(theCell));

  sheet.put(theCell, "14"); // "Obvious" number
  assertEquals("14", sheet.get(theCell));

  sheet.put(theCell, " 99 X"); // Whole string must be numeric
  assertEquals(" 99 X", sheet.get(theCell));

  sheet.put(theCell, " 1234 "); // Blanks ignored
  assertEquals("1234", sheet.get(theCell));

  sheet.put(theCell, " "); // Just a blank
  assertEquals(" ", sheet.get(theCell));

// Refactor before going to each succeeding test.

public void testThatWeHaveAccessToCellLiteralValuesForEditing() { 
  Sheet sheet = new Sheet();
  String theCell = "A21";

  sheet.put(theCell, "Some string"); 
  assertEquals("Some string", sheet.getLiteral(theCell));

  sheet.put(theCell, " 1234 "); 
  assertEquals(" 1234 ", sheet.getLiteral(theCell));

  sheet.put(theCell, "=7"); // Foreshadowing formulas:)
  assertEquals("=7", sheet.getLiteral(theCell));

// We'll talk about "get" and formulas next time.

Challenge, Part 2

What To Do

  • Do part 1 if you haven't yet.
  • Implement code for each test (one at a time), then refactor to simplify your code.
  • (Optional) Email me with results and feedback. (William.Wake@acm.org)

Challenge Tests

// Implement code for previous test before moving to next one.

public void testFormulaSpec() {
  Sheet sheet = new Sheet();
  sheet.put("B1", " =7"); // note leading space
  assertEquals("Not a formula", " =7", sheet.get("B1"));
  assertEquals("Unchanged", " =7", sheet.getLiteral("B1"));

// Next - start on parsing expressions

public void testConstantFormula() {
  Sheet sheet = new Sheet();
  sheet.put("A1", "=7");
  assertEquals("Formula", "=7", sheet.getLiteral("A1"));
  assertEquals("Value", "7", sheet.get("A1"));

// More formula tests. You may feel the need to make up 
// additional intermediate test cases to drive your code
// better. (For example, you might want to test "2*3" 

// before "2*3*4".) That's fine, go ahead and create them.
// Just keep moving one test at a time.

// We're doing expressions; you may need to do a spike
// (investigation) if you're not familiar with parsing.
// For background, look up "recursive descent" or
// "operator precedence". (Other techniques can work as well.)

// Order of tests - I'm familiar enough with parsing to think
// it's probably easiest to do them in this order (highest
// precedence to lowest). For extra credit, you might redo 
// this part of the exercise with the tests in a different order 
// to see what difference it makes.

public void testParentheses() {
  Sheet sheet = new Sheet();
  sheet.put("A1", "=(7)");
  assertEquals("Parends", "7", sheet.get("A1"));

public void testDeepParentheses() {
  Sheet sheet = new Sheet();
  sheet.put("A1", "=((((10))))");
  assertEquals("Parends", "10", sheet.get("A1"));

public void testMultiply() {
  Sheet sheet = new Sheet();
  sheet.put("A1", "=2*3*4");
  assertEquals("Times", "24", sheet.get("A1"));

public void testAdd() {
  Sheet sheet = new Sheet();
  sheet.put("A1", "=71+2+3");
  assertEquals("Add", "76", sheet.get("A1"));

public void testPrecedence() {
  Sheet sheet = new Sheet();
  sheet.put("A1", "=7+2*3");
  assertEquals("Precedence", "13", sheet.get("A1"));

public void testFullExpression() {
  Sheet sheet = new Sheet();
  sheet.put("A1", "=7*(2+3)*((((2+1))))");
  assertEquals("Expr", "105", sheet.get("A1"));

// Add any test cases you feel are missing based on 
// where your code is now.

// Then try your hand at a few test cases: Add "-" and "/"

// with normal precedence. 

// Next, error handling.

public void testSimpleFormulaError() {
  Sheet sheet = new Sheet();
  sheet.put("A1", "=7*");
  assertEquals("Error", "#Error", sheet.get("A1"));

public void testParenthesisError() {
  Sheet sheet = new Sheet();
  sheet.put("A1", "=(((((7))");
  assertEquals("Error", "#Error", sheet.get("A1")); 

// Add any more error cases you need. Numeric errors (e.g.,
// divide by 0) can return #Error too.

// Take a deep breath and refactor. This was a big jump.
// Next time we'll tackle formulas involving cells.

Challenge, Part 3

We're going to add dependencies now. This is one of those things that makes a spreadsheet a spreadsheet.

What To Do

  • Do parts 1 and 2 if you haven't yet.
  • Implement code for each test (one at a time), then refactor to simplify your code.
  • (Optional) Email me with results and feedback. (William.Wake@acm.org)

Challenge Tests

public void testThatCellReferenceWorks () {
  Sheet sheet = new Sheet();
  sheet.put("A1", "8");
  sheet.put("A2", "=A1");
  assertEquals("cell lookup", "8", sheet.get("A2"));

public void testThatCellChangesPropagate () {
  Sheet sheet = new Sheet();
  sheet.put("A1", "8");
  sheet.put("A2", "=A1");
  assertEquals("cell lookup", "8", sheet.get("A2"));

  sheet.put("A1", "9");
  assertEquals("cell change propagation", "9", sheet.get("A2"));

public void testThatFormulasKnowCellsAndRecalculate () {
  Sheet sheet = new Sheet();
  sheet.put("A1", "8");
  sheet.put("A2", "3");
  sheet.put("B1", "=A1*(A1-A2)+A2/3");
  assertEquals("calculation with cells", "41", sheet.get("B1"));

  sheet.put("A2", "6");
  assertEquals("re-calculation", "18", sheet.get("B1"));

public void testThatDeepPropagationWorks () {
  Sheet sheet = new Sheet();
  sheet.put("A1", "8");
  sheet.put("A2", "=A1");
  sheet.put("A3", "=A2");
  sheet.put("A4", "=A3");
  assertEquals("deep propagation", "8", sheet.get("A4"));

  sheet.put("A2", "6");
  assertEquals("deep re-calculation", "6", sheet.get("A4"));

// The following test is likely to pass already.
public void testThatFormulaWorksWithManyCells () {
  Sheet sheet = new Sheet();
  sheet.put("A1", "10");
  sheet.put("A2", "=A1+B1");
  sheet.put("A3", "=A2+B2");
  sheet.put("A4", "=A3");
  sheet.put("B1", "7");
  sheet.put("B2", "=A2");
  sheet.put("B3", "=A3-A2");
  sheet.put("B4", "=A4+B3");

  assertEquals("multiple expressions - A4", "34", sheet.get("A4"));
  assertEquals("multiple expressions - B4", "51", sheet.get("B4"));

// Refactor and get everything nice and clean.

// Next: (I almost made this a separate part, and when I
// originally did it, I did it in a different design session).
// So take a break if you need one.

// There's one big open issue for formulas: what about
// circular references? 

// I'll sketch some hints, but you should define your own tests
// that drive toward a solution compatible with your own 
// implementation.

public void testThatCircularReferenceDoesntCrash() {
  Sheet sheet = new Sheet();
  sheet.put("A1", "=A1");

// Just like errors return a special value, it might be nice
// if circular references did too. (See notes below).

public void testThatCircularReferencesAdmitIt () {
  Sheet sheet = new Sheet();
  sheet.put("A1", "=A1");
  assertEquals("Detect circularity", "#Circular", sheet.get("A1"));

// You might come up with some other approach that suits your 
// taste. We won't be exploring this corner of the solution 
// any further; you just want a scheme that blocks silly mistakes.
// Make sure you test deep circularities involving partially
// evaluated expressions.

// A hint: if you blindly evaluate an expression you have no
// control over how deep the expression can be, since
// circular references appear to be infinitely deep.

// Where are we? I intend to spend the next two parts hooking
// up a GUI. Then there will be an optional part that pushes 
// things in an unexpected direction just to get a sense
// of our software's robustness.

Part 4, Moving Toward a GUI

What To Do

  • Do parts 1 through 3 if you haven't yet.
  • Implement code for each test (one at a time), then refactor to simplify your code.
  • (Optional) Email me with results and feedback. (William.Wake@acm.org)

Challenge Tests

We'll start by using a feature of JUnit that we haven't used before: a setUp() method. To use this, declare your variables at the class level, and initialize them in the setUp() method. There's a corresponding tearDown() method that we won't need, but which can be useful for some tests. JUnit works like this: call setUp(), call one of your tests, call tearDown(), call setUp(), call your next test, call tearDown(), etc. So, each test can assume it's got things just as setUp() left them.

Quick Design

The easiest thing to use in the GUI will be a JTable, which provides the 2-d grid we expect to see. The best way to use a JTable is usually to give it a TableModel. The TableModel acts as a wrapper between the GUI-oriented JTable and your object.

The easiest way to create a TableModel is to subclass AbstractTableModel, so that's what we'll do. The TableModel is 0-based, so we'll use column 0 to hold the row's index, and we'll store spreadsheet row n in TableModel's row n-1.

Sheet sheet;
TableModel table;

public void setUp() {
  sheet = new Sheet();
  table = new SheetTableModel (sheet);        

// As usual, do one test at a time and refactor after each.

// For now, we're willing to hard-code a maximum spreadsheet size.
// A future story can deal with this.

int LAST_ROW_INDEX = 99;
public void testTableModelRequiredOverrides() {
  assertTrue (table.getColumnCount() > LAST_COLUMN_INDEX);
  assertTrue (table.getRowCount() > LAST_ROW_INDEX);
  assertEquals ("", table.getValueAt(10,10));

// Take a look at AbstractTableModel's documentation before doing this test.

public void testColumnNames() {
  assertEquals ("", table.getColumnName(0));
  assertEquals ("A", table.getColumnName(1));
  assertEquals ("Z", table.getColumnName(26));
  assertEquals ("AW", table.getColumnName(LAST_COLUMN_INDEX));

public void testThatColumn0ContainsIndex() {
  assertEquals ("1", table.getValueAt(0,0));
  assertEquals ("50", table.getValueAt(49, 0));
  assertEquals ("100", table.getValueAt(LAST_ROW_INDEX,0));

// Remember, one test at a time, followed by refactoring.

public void testThatMainColumnsHaveContents() {
  sheet.put ("A1", "upper left");
  assertEquals ("upper left", table.getValueAt(0,1));

  sheet.put ("A100", "lower left");
  assertEquals ("lower left", table.getValueAt(LAST_ROW_INDEX, 1));

  sheet.put ("AW1", "upper right");
  assertEquals ("upper right", table.getValueAt(0, LAST_COLUMN_INDEX));

  sheet.put ("AW100", "lower right");
  assertEquals ("lower right", table.getValueAt(LAST_ROW_INDEX, LAST_COLUMN_INDEX));

public void testThatStoresWorkThroughTableModel() {
  table.setValueAt("21", 0, 1);
  table.setValueAt("=A1", 1, 1);

  assertEquals("21", table.getValueAt(0,1));
  assertEquals("21", table.getValueAt(1,1));

  table.setValueAt("22", 0, 1);
  assertEquals("22", table.getValueAt(0,1));
  assertEquals("22", table.getValueAt(1,1));

// We've established that the table model can get and set values.
// But JTable uses an event notification mechanism to find out
// about the changes.

// To test this, we'll introduce a test helper class. It's a very
// simple listener, and will assure us that notifications are
// sent when changes are made.

// There's a couple of design decisions implicit here. One is that
// we won't attempt to be specific about which cells change; we'll
// just say that the table data has changed and let JTable refresh
// its view of whichever cells it wants. (Because of cell dependencies,
// changes in one cell could potentially no others, all others,
// or anything in between.) We might revisit this decision during
// performance tuning, and try to issue finer-grained notifications.

// The other decision is that we have no mechanism for our Sheet
// to tell the table model about changes. So changes will either need
// to come in through the table model, or we'll have to add some
// notification mechanism to Sheet. For now, just make changes through the table model.

   public class TestTableModelListener implements TableModelListener {
      public boolean wasNotified = false;
      public void tableChanged(TableModelEvent e) {wasNotified = true;}
public void testThatTableModelNotifies() {
  TestTableModelListener listener = new TestTableModelListener();
  table.addTableModelListener (listener);
  assertTrue (!listener.wasNotified);

  table.setValueAt("22", 0, 1);

  assertTrue (listener.wasNotified);

// Note the cast in our test here. Previous tests have been straight
// implementations of TableModel functions; now we're saying that 
// our model has some extra functions. We'll face a small tradeoff later
// when we want access to the feature: if we get the model back from JTable,
// we'll have to cast it; if we don't want to cast it we'll have to
// track it somewhere.

public void testThatSheetTableModelCanGetLiteral() {
  sheet.put("A1", "=7");
  String contents = ((SheetTableModel)table).getLiteralValueAt(0, 1);

  assertEquals("=7", contents);

// We've left isCellEditable() false, on the assumption that the way to edit
// the cell is to go to a textbox provided for that purpose (rather than
// in place).

Part 5, GUI

One of the first XPlorations articles I wrote was to address the question, "Can you do a GUI test-first?" My answer there was, "Yes, but…" and this exercise might lead you to the same opinion.

The goal is to make a simple GUI that looks sort of like this:

  | Label  |  Text entry   |  OK  |
  |                              ^|
  |        Grid (JTable)         ||
  |                              v|

The idea is that if you click in the grid, the label tells which cell you're in, and the text entry field contains the literal value of that cell. You can edit it, and when you click "OK", it puts the value back in the cell. Then the whole spreadsheet updates accordingly. With this, you should have a minimalist but working spreadsheet.

Quick Design

I'll put everything in a subclass of JFrame, using the Swing objects JLabel, JTextField, JButton, and JTable. In my frame, I'll have global variables for each of these components. (I won't bother exposing intermediate panels or other stuff not important outside the class.)

Most of the problem will be setup and hookup. I'll want to hook the table to the model, and set up listeners so that the label and textfield know when the selection has changed. I'll need a listener on the ok button to tell when to update the grid with the edited value. And all these objects have various configuration options to set up as well.

Timothy Wall has pointed out that there are tools designed to test GUIs: http://abbot.sf.net, http://jemmy.netbeans.org, and http://jfcunit.sf.net. You might want to give one of these a try and compare the result.

Challenge Tests

Here are my declarations and setUp routine for my test now:

Sheet sheet;
TableModel table;
SheetTableModel model;  // New for part 5
SheetFrame frame;       // New for part 5

public void setUp() {
  sheet = new Sheet();
  table = new SheetTableModel (sheet);  
  model = new SheetTableModel(sheet);
  frame = new SheetFrame(model);

// Here are the tests:

public void testThatFrameHasRightParts () {
  assertSame(model, frame.table.getModel());

public void testThatRowAndColumnSelectionAllowed() {

    public class TestSelectionListener implements ListSelectionListener {
        public boolean wasNotified = false;
        public TestSelectionListener() {}
        public void valueChanged(ListSelectionEvent e) {
            wasNotified = true;            
// I expect this test to pass; it verifies how I think listeners work.
// You might call it a spike and omit it.

public void testThatSelectionsNotifyListeners() {
    TestSelectionListener listener = new TestSelectionListener();

    assertTrue (!listener.wasNotified);  

    frame.table.changeSelection (3, 2, false, false);
    assertTrue (listener.wasNotified);

    listener.wasNotified = false;
    frame.table.changeSelection (1, 1, false, false);
    assertTrue (listener.wasNotified);

// If you need info on hooking up a selection listener, see 
// http://java.sun.com/docs/books/tutorial/uiswing/components/table.html#selection

public void testThatLabelIsUpdatedWhenSelectionChanges() {
    assertEquals("", frame.label.getText());
    frame.table.changeSelection (0, 1, false, false);
    assertEquals("A1", frame.label.getText());

    frame.table.changeSelection (10, 10, false, false);
    assertEquals("J11", frame.label.getText());

// You might add a main() routine to SheetFrame and
// see how the GUI is looking.

public void testThatEditorSeesLiteralValue() {
    model.setValueAt("=7", 1, 1);
    assertEquals("=7", frame.editor.getText());

// We would like to have a way to programmatically let the 
// text field click "Enter", but I don't see a mechanism.
// So we'll use the okButton instead.

public void testThatEditedValueGetsSaved() {
    model.setValueAt("=7", 1, 1);

    assertEquals("=8", frame.model.getLiteralValueAt(1,1));
    assertEquals("8", frame.model.getValueAt(1,1));

public void testThatValuePropagationWorks () {
  frame.model.setValueAt("7", 0,1);
  frame.model.setValueAt("=A1+2", 2,2);
  assertEquals("9", frame.model.getValueAt(2,2));
  assertEquals("=A1+2", frame.model.getLiteralValueAt(2,2));
  frame.model.setValueAt("10", 0,1);
  assertEquals("12", frame.model.getValueAt(2,2));

// See discussion below on acceptance tests.
public void testAcceptanceTest1() {
    SheetTableModel model;
    SheetFrame frame;

    model = new SheetTableModel(new Sheet());
    frame = new SheetFrame(model);
    frame.table.changeSelection(0,1,false,false);   // A1

    frame.table.changeSelection(1,1,false,false);   // A2
    assertEquals("72", frame.table.getValueAt(1,1));
    frame.table.changeSelection(0,1,false,false);   // A1
    assertEquals("30", frame.table.getValueAt(1,1));


What I've found is that GUI testing works "ok" for some things but there tends not to be an easy way to do other things. For example, I didn't see an easy way to programmatically click "Enter" in the edited box. (I know you can set up a Robot and fool with events, but I wasn't willing to try that hard.) Another example is the difficulty of dealing with pop-up menus, tooltips, dialogs, etc.

The other problem I have is that I just don't think of everything important without actually seeing the GUI grow. So even if I use test-first, I'll mix it with running the application and seeing how it looks. (For example, we didn't address column widths, or seriously look at what happens when you select multiple cells, and my solution doesn't leave the grid element selected after an edit.)

I try to keep the GUI work limited to direct setup and hookup of objects. It can be very tempting to say "oh, this is GUI so I don't need to test it," but don't over-do that. Make sure that the GUI is not making application-level or algorithmic decisions.

I do sometimes end up creating programmatic acceptance tests, like the last test. I try to minimize my need for these because they're a fair bit of work to program, and the customer can't really look at them and verify them. In this case, the test checks the hookup, which is ok. But for other spreadsheet tests, I might automate something that reads an input file and a "golden" result, then compares to the actual contents of the sheet. In this way I could check formulas, circularity, etc. without having to hand-code so much.

Part 6, Bonus Round

This part is designed to let you assess the flexibility of your design. I'm just going to provide general directions; you can develop your own tests for any area you undertake.

"Obvious" extensions

There are a lot of places where we can expand on our spreadsheet. How much impact would each of these have? Do they affect the core of what you've done?

  • Allow spaces in formulas (if you don't already).
  • Support for floating point numbers.
  • More cell data types: date, money, etc. (Do you have different cell types yet?)
  • Range specifications and simple functions. For example, "=@sum(b1:c29)" should add up those cells. (Does this affect circularity detection?)
  • GUI improvements
    • Color index column a different column and right-justify it.
    • Select by row or column.
    • Column widths default to a useful size.
  • Input/output. Allow your spreadsheet to read and write "csv" (comma-separated value) files.
  • Add a graphing module.


If you haven't performance-tuned, your solution probably re-parses (or at least re-evaluates) a value every time it's referenced. Try a test like this:

A1 1
  A2 =A1+A1+A1+A1+A1+A1+A1+A1+A1
  A3 =A2+A2+A2+A2+A2+A2+A2+A2+A2
  A4 =A3+A3+A3+A3+A3+A3+A3+A3+A3
  A5 =A4+A4+A4+A4+A4+A4+A4+A4+A4
  A6 =A5+A5+A5+A5+A5+A5+A5+A5+A5
  A7 =A6+A6+A6+A6+A6+A6+A6+A6+A6
    etc. as needed

  Then change A1 to 2.

Get a profiler and see where the time is being spent. Can you speed up evaluation? How hard would it be to cache values during an update cycle (instead of re-calculating for each reference)? Would it make sense to have cells know which cells to notify when they changed? What other tests would help you understand the spreadsheet's performance?

Non-rectangular spreadsheet

The last couple challenges focused on using our Sheet as a traditional 2-d grid. But the idea of dependency and automatic updating is not really dependent on the shape.

How about a tree-based spreadsheet? Instead of A1, C32, etc., let the cells be named as words separated by dots: "Sales.Y2000.Q1" and so on. You might have formulas like this:

Sales =Sales.Y2000+Sales.Y2001
  Sales.Y2000 =Sales.Y2000.H1 + Sales.Y2000.H2
  Sales.Y2000.H1 =43000
  Sales.Y2000.H2 =54000

  Sales.Y2001 =Sales.Y2001.H1 + Sales.Y2001.H2
  Sales.Y2000.H1 =45000
  Sales.Y2000.H2 =74000

Updating should still work as before; if you increased Sales.Y2000.H2 by 6000, then Sales.Y2001 and Sales should increase by that same amount. You might like to display all this in some sort of tree control rather than a table.

  • How much of your GUI code could you retain? (If it's very much, it could be a sign that you're doing things not strongly related to the GUI; or perhaps your code is just so generalized it doesn't care about the shape.)
  • How big a change is this to your parser?
  • How much of the Sheet object could remain the same?
  • If you'd known you were moving to a tree in the first place, would you have built the same Sheet, or a different object?
  • If you did the performance tunings, did they make it easier or harder to make this change of direction?

Another approach might be to decouple the sheet from any pre-conceived shape. Perhaps you'd have a user interface that let people drag-and-drop formula pieces and connect them with arrows (like some programming environments or a data flow graph).

  • Would the Sheet object still be helpful?
  • Could you easily evolve from the Sheet to something more directly useful?
  • To the extent your design needed to change, would you feel test-first let you down?


I don't expect miracles out of test-first design, but I expect that I'll get a reasonably robust solution, that feels like it ended up a little different than it would have from an up-front design.

I hope the experience of "test-first without writing tests" has inspired you to try "test-first by writing tests".

Again, thanks to all participants. I've really enjoyed corresponding with people, reading code, and seeing the discussions about this exercise.

Closing Observations

Here's a few observations about the test-first challenge, from the author's point of view.

  • It feels "different" to do this as a remote customer/test-writer (compared to how it feels to do it as a programmer), but I hope it still gives a fair flavor of the process.
  • One set of tests definitely does not generate one solution. Some aspects have been similar between solutions, but others have varied a lot. The two biggest areas of variation: parsers and circularity. In parsers, I saw operator precedence, recursive descent, and several ad- hoc approaches. For circular references, I saw a counting approach and a couple variations of "track whether a reference is to a partially evaluated node".
  • Someone commented that "what's simplest depends on experience," and that's my experience too. For example, the ad-hoc parsers seem simple in one sense, but I have a lot more confidence in the correctness of the "standard" compiler-type solutions, and they didn't seem all that complex.
  • I find myself writing two types of tests. (I'm trying some new vocabulary here…) "Generative tests" are the ones that push my design forward, moving in broad strokes to a solution. "Elaborative tests" are the ones I write to really nail down a feature, sort of a theme and variations approach. The "challenge" tests tended toward the former. (For example, you can't look at my tests and tell where and whether spaces are acceptable in formulas, and you'd need more tests to really verify the expression processing.)
  • I hope you were able to create the extra tests needed to check your implementation.
  • The solution and the test influence each other. I believe if I'd been working towards an ad-hoc parser (rather than recursive descent, say), I'd have come up with different tests. I tend to do enough design that I can say "this could work", and then let the tests take it where they will. Like the old song, I "hold on loosely" to the design, and if the tests lead elsewhere, that's ok.
  • If you get the chance, it's really worth tackling the same problem two or three times. My test-first skills have improved with practice. Next time I try the spreadsheet, I might ignore the math part, and just have text cells and formulas with a "," operator that appends strings. Or try the same thing again, but have in mind "use operator precedence, and handle circularity by counting depth" instead of "recursive descent and partial evaluation check".

Participant Solutions

Several people who took the challenge in January 2002 have posted their solutions. I encourage that you not look at these before you attempt your own solution, and I encourage that you do look at them after you finish.

If you've posted your solution and want to share it, let me know and I'll add you to this list.

Resources and Related Articles

[Written January, 2002; fixed missing setup in part 3, February, 2003. Updated Sept. 04 to mention GUI test tools. Added James Cakalic's solution, August, '05.]

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