Directories may or may not have their own entries in ZIPs

Today I found that one of my Gradle plugins choked on a JAR file that should have been perfectly normal. My plugin tried to extract the JAR by creating a file on disk for each entry, which ended up including directories. That was surely a bug, but it was strange that only some JAR files triggered it. Specifically, JARs created by a Gradle Java project jar task caused the problem, but those created by an Android createFullJarVariant task did not.

Apparently it is optional for ZIP (JAR) "entries" to be created for directories, even if there are files in those subdirectories. I examined example JARs in PowerShell with the ZipArchive class, finding that only the crash-causing JAR had directories in its Entries collection.