Need help designing some JUnit tests

Rhino <>
Thu, 20 May 2010 17:48:54 +0000 (UTC)
I'm getting more and more comfortable with JUnit after a long absence
from it but I'm still struggling with composing tests for some
situations. If anyone can help with any of these situations, I'd love to
hear your suggestions.


Scenario 1 - Moving Targets

What is the best way to test a method whose output is unpredictable
because that output is a moving target? For example, I have a method that
uses Locale.getAvailableLocales() to display all of the Locales on the
current JVM. It works fine but how do I write a JUnit test, given that an
upgrade to the JVM could introduce additional Locales? I'd have the same
kind of problem if a method was returning a list of area codes or cell
phone providers. There must be some standard way of writing a JUnit test
for that but I'm drawing a blank.


Scenario 2 - Constructors

Given a hypothetical class named Foo where the constructor is:

public Foo () {
 //anything from no code at all to umpteen lines

is this adequate as a JUnit test?

public void testFoo() {

Foo myFoo = new Foo();
if (!myFoo instanceof Foo) fail ("Failed to instantiate Foo");

If not, what would be a better test?

Scenario 3 - getInstance()

Given a hypothetical class named Fuzz where the constructors and
getInstance() methods are:

private Fuzz() {
// do something

private Fuzz(Locale locale) {
// do something
// initialize instance variable for locale

public getInstance() {
   return Foo();

public getInstance(Locale locale) {

  return Foo(locale);

Is it necessary to write JUnit tests for the constructors, given that
they are private?

Would the following be adequate as JUnit tests for the getInstance()

public void testGetInstance() {

  Fuzz fuzz = Fuzz.getInstance();
  if (!fuzz instanceof Fuzz) fail("Failed to instantiate Fuzz");

public void testGetInstancelocale() {

  Fuzz fuzz = Fuzz.getInstance(new Locale("FR", "fr"));
  if (!fuzz instanceof Fuzz) fail("Failed to instantiate Fuzz"):

If not, what tests would be better?

Lastly - and thanks for bearing with me this far - is it normal practice
to write JUnit tests for all methods that are in parent classes of the
class being tested? For example, while writing tests for one of my
classes, the wizard in Eclipse asked if I wanted to produce test methods
to test methods of Object like equals() and wait(). Should I be writing
tests for those methods as well as for those in my own class? I'm
inclined to assume that the methods in Object, for example, have already
been thoroughly tested and further testing by me seems redundant. Even
parent classes that I myself wrote would presumably have had their own
sets of JUnit Tests. Then again, the Eclipse developers presumably put
that functionality in there for a reason so I'm left unclear about
whether I should be trying to write tests for them too.

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