Java master Banzen was reviewing the code of a web application when he came across a new HTTP filter authored by his apprentice Satou. The old master called her to his office.

“You have begun including an ‘ETag’ header with each page you send,” said Banzen to the nun. “Explain, for I have not heard of this header.”

“The ETag is a hash of the page contents,” said Satou with pride, glad to know something her master did not. “I calculate it just before I return the requested page to the browser.”

“What is its purpose?” asked Banzen.

“An efficiency hack,” replied Satou. “If the browser asks for that same page later, it can include the last ETag with its request, effectively saying: I still possess this content that you sent me earlier. If the server then determines that a new response would have the same ETag, it discards the response and answers only: Nothing has changed.”

“Ah,” said Banzen, squinting at Satou’s code. “I notice that you obtain your ‘hash of the page contents’ by loading them into a byte-array called contents and then invoking contents.hashCode(). Are you certain that this will be a good digest value?”

Satou thought a moment. “The hashCode() method returns a four-byte int, so the chances of collision should be about four billion to one. I believe these are acceptable odds.”

The Java master congratulated his apprentice on her reasoning and bade her wait a few minutes while he fetched a suitable reward. Banzen returned with a box of assorted chocolates.

“Take one,” he said.

The chocolates were indistinguishable, but they were laid out in neat rows and columns with a legend on the lid of the box. Satou found a square labeled Cherry and popped the corresponding chocolate in her mouth.

Her expression changed from delight to confusion to horror, but she was too late to stop herself from swallowing the confection.

“That... wasn’t cherry...” she said, her pale face growing much paler.

“Of course not!” laughed Banzen. “‘Cherry’ is only the name of the square it was in. The contents of the chocolate are anyone’s guess. Here,” he said, moving a different chocolate to the vacated square. “If you liked that, have another.”