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For network programming, C, C++, Java, C#, Python, etc are all good languages. You should also have skills in powershell, bash, cmd, tcl etc.
Historically: scripting and cmdline (bash, netsh, CLIs, batchjobs (powershell), awk/sed/perl, wsh, chaining streams together, etc). Articles + "quotes [emphasis mine]" 1. "The importance isn't in picking a programming language" [it's knowing the concepts] 2. "[A language] lets you focus on what’s valuable and important, not on the mundane." 3. "The sky is not falling. You don’t have to learn coding if you don’t want to but doing so will evolve your skillset and possibly accelerate your career." Knowing Python could give you some easy wins; it's C++ behind the scenes (extensible) and many things you work with arose from people in the C zone.  https://keepingitclassless.net/2013/10/im-a-networking-guy-and-im-here-to-talk-about-programming/  http://packetpushers.net/programming-101-for-network-engineers-why-bother/  http://lostintransit.se/2016/07/09/general-network-engineering-vs-coding/
Why did you tag java? Use better tags such as "networking"
https://www.sololearn.com/discuss/520852/?ref=app Hope you meant this