Software Design Principles II - Object-Oriented Programming and Data Structures | CS 2110, Exams of Computer Science

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CS 2110

Software Design Principles II

Juan Altmayer Pizzorno

port25.com

Overview

Last week:

Design Concepts & Principles

Refactoring

Today: Test-Driven Development

TDD + JUnit by Example

Test Driven Development

We’ll go about in small iterations

1. add a test

2. run all tests and watch the new one fail

3. make a small change

4. run all tests and see them all succeed

5. refactor (as needed)

We’ll use JUnit

JUnit

What do JUnit tests look like?

SmallSet.java package edu.cornell.cs.cs2110; public class SmallSet { ... } SmallSetTest.java package edu.cornell.cs.cs2110; import org.junit.Test; import static org.junit.Assert.;* public class SmallSetTest { @Test public void testFoo () { SmallSet s = new SmallSet(); ... assertTrue (...); } @Test public void testBar() { ... } }

A First Test

We pick a feature and test it:

This doesn’t compile: size() is undefined

But that’s all right: we’ve started designing the

interface by using it

SmallSet

class SmallSet {}

SmallSetTest

class SmallSetTest {

@Test public void testEmptySetSize() {

SmallSet s = new SmallSet();

assertEquals(0, s.size());

Red Bar

We need the test to fail, so we define size()

Running the test

yields a red bar

indicating failure:

We’ve tested the test, and it works!

SmallSet class SmallSet { public int size() { return 42; } }

Adding Items

To implement adding items, we first test for it:

• add()^ is undefined, so to run the test we define it:

SmallSetTest

class SmallSetTest {

@Test public void testEmptySetSize() ...

@Test public void testAddOne() {

SmallSet s = new SmallSet();

s.add(new Object());

assertEquals(1, s.size());

SmallSet

public int size() ...

public void add(Object o) {}

Adding Items

The test now fails as expected:

It seems obvious we need to count the number of

items:

And we get a green bar:

SmallSet

private int _size = 0;

public int size() {

return 0;

return _size;

public void add(Object o) {

++_size;

Remember that Item?...

We need to remember which items are in the set...

All tests pass, so we can refactor that loop...

SmallSet private int _size = 0; public static final int MAX = 10; private Object _items[] = new Object[MAX]; ... public void add(Object o) { for (int i=0; i < MAX; i++) { if (_items[i] == o) { return; } } _items[_size] = o; ++_size; }

Refactoring

(...loop) which doesn’t “speak to us” as it could...

All tests still pass, so we didn’t break it!

SmallSet (before) public void add(Object o) { for (int i=0; i < MAX; i++) { if (_items[i] == o) { return; } } _items[_size] = o; ++_size; } SmallSet (after) private boolean inSet(Object o) { for (int i=0; i < MAX; i++) { if (_items[i] == o) { return true; } } return false; } public void add(Object o) { if (!inSet(o)) { _items[_size] = o; ++_size; } }

Size Matters

We first have add() check the size,

... re-run the tests, check for green,

define our own exception...

... re-run the tests, check for green,

and...

SmallSet public void add(Object o) { if (!inSet(o) && _size < MAX ) { _items[_size] = o; ++_size; } } SmallSetFullException public class SmallSetFullException extends Error {}

Testing for Exceptions

... finally test for our exception:

The test fails as expected,

so now we fix it...

SmallSetTest @Test public void testAddTooMany() { SmallSet s = new SmallSet(); for (int i=0; i < SmallSet.MAX; i++) { s.add(new Object()); } try { s.add(new Object()); fail(“SmallSetFullException expected”); } catch (SmallSetFullException e) {} }

Review

Started with a “to do” list of tests / features

could have been expanded

as we thought of more tests / features

Added features in small iterations

“a feature without a test doesn’t exist”

(1) add test (2) make it pass (3) refactor

Fixing a Bug

What if after releasing we found a bug?