java.util.zip.DeflaterOutputStream will only call Deflater#end() if it uses a default Deflater instance (i.e. one it created itself). This means that users of the DeflaterOutputStream constructors taking a Deflater parameter have to call Deflater#end() explicitly if they want resources to be freed before garbage collection.

While this makes sense it should be mentioned in the JavaDoc for DeflaterOutputStream, probably in the documentation for the constructors taking a Deflater parameter.