I'm trying to compare the output of running strings through GZIP on both Java, and the CLI gzip command. The outputs are not the same, and I've figure out why, but not sure how to get them to jive with each other.
I've read a number of questions on SO, have read the man pages for gzip, and the code for both GZIPOutputStream and DeflaterOutputStream. The default compression level for GZIPOutputStream (set through Deflator) is "-1", and there's little explanation as to what that means. Furthermore, gzip CLI only allows for values between 1 and 9, inclusive.
So is there a way I can change the compression settings in either Java or the gzip command to make them produce the same output?
No. Java is using the zlib deflator, which is derived from but not exactly the same as the older gzip command-line utility deflator. They will generally not produce the same output and there are no settings to coerce them to do so.
Compression level -1 requests the default compression level, which in the current zlib implementation is level 6.
I would have to ask why you care to get their outputs to be the same. All that matters is that the compression is lossless, i.e., that both the gzip and Java compressed streams produce the same original data when decompressed. There is no requirement that, for example, different versions of zlib produce the same output at the same compression level.