And you can wonder what the difference is. But think about it for a second, then do this exercise.

Pretty obvious when you see them all together.

But for the exam, you should have them memorized so that if next Tuesday the guy down the hall asks, “What’s that response method that lets me add a value to an existing header?” you can, without the slightest pause, say “It’s addHeader, and it takes two Strings for the name and value.” Just like that.

Both setHeader() and addHeader() will add a header and value to the response if the header (the first argument to the method)is not already in the response. The difference between set and add shows up when the header is there. In that case:

setHeader() overwrites the existing value

addHeader() adds an additional value

When you call setContentType(“text/html”), you’re setting a header just as if you said:

setHeader(“content-type”, “text/html”);

So what’s the difference? No difference... assuming you type the “content-type” header correctly. The setHeader() method won’t complain if you misspell the header names—it just thinks you’re adding a new kind of header. But something else will fail later, because now you haven’t properly set the content type of the response!

Make it Stick

There was a response from the node with headers and quite a payload. Not one header I tell you had more than one value for setHeader() was used in the code.

(as opposed to addHeader(), get it?)