Tests written against an implementation rely on internals of the code which are not part of the interface. As a very simple example, say you have a module which takes input A and B, merges that data into a data structure C, then performs some calculations on it and gives you back D. If you were testing against the interface(black-box), you'd write tests which put in A and B and check whether you get the correct D out. If you were to write a test against the implementation(white-box), you'd also check whether C conforms to your expectations or whether the algorithm to turn C into D works a particular way. If later the module writer decides to change the implementation and do without the intermediate C (or change it in some way), the test breaks.

---

---

Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!

Titles consisting of a single word are discouraged, and in most cases are disallowed outright.

Read Where should I post X? if you're not absolutely sure you're posting in the right place.

Please read these before you post! —

• How do I compose an effective node title?
• How do I post a question effectively?
• Markup in the Monastery

Posts may use any of the Perl Monks Approved HTML tags:

a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr

You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)

	For:		Use:
	&		&
	<		&lt;
	>		&gt;
	[		&#91;
	]		&#93;

Link using PerlMonks shortcuts! What shortcuts can I use for linking?

See Writeup Formatting Tips and other pages linked from there for more info.