I recently wrote a Slack application and restricted myself to using only the JDK. For some reason, I wanted to reminded myself of how awkward Java programming is for newbies.
No one chooses a Java implementation without the expectation of needing to include a shedload of external libraries. Luckily, Java has the most best of class libraries available, that are easily incorporated with Maven, and that have facilitated the great variety of successful applications being built and maintained every day. The JDK's standard libraries are, however, doddering and incomplete, especially as to building applications for the internet. Scripting languages like Python, PHP, and Ruby do have standard libraries that have evolved to incorporate the internet. And these languages are being successfully used by newbies . Nevertheless, I had my question to answer.
My application uses Slack's outgoing webhooks and so is a specialized HTTP server. The JDK does have an HTTP server, but it is in the com.sun namespace and does not implement the Servlet API. Implementing the Servlet API would give the newbie the experience of using a standard container and, moreover, allow his or her application to be deployed to any number of cloud providers. Implementing an HTTP server is not a quick task and so I choose to use the JDK's.
When Slack sends a webhook request the body content is url form encoded, eg "a=1&b=2&b=3". (You more often see this used in URL queries.) Apart from an early spat over delimiting with ampersands vs semicolons this encoding has been universally, consistently used for decades. Unfortunately, the JDK does not have a standard means to encode or decode this data. I implemented a decoder.
The response to the webhook is a JSON encoded message. JSON's adoption rate has been breakneck, but standard libraries have caught up. The Java Community Process (JCP) did define a JSON API several years ago, but an implementation is not in the JDK. While outputting JSON is straightforward, ensuring it is syntactically correct calls for having support. I implemented an encoder
Lastly, and most sadly, is the situation with fixed width hex values. Slack wants colors expressed in the RGB hex notation, ie #RRGGBB. JDK has a Color class, but it can't be used to create an RGB hex value. (Its Color.toString() method is practically useless.) To format the value you can use
String.format("%02x%02x%02x", c.getRed(), c.getGreen(), c.getBlue())
but, honestly, what newbe is going to know that or have any hope of Googling the answer?
Gumble over. Back to real world Java development.
 That academics use Java in introductory programming classes is appalling.
 It is not really "encoding" and "decoding," but marshalling and unmarshalling.