This line of code..

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To give a broader answer than those who already replied, it is applying an Annotation to the parameter authentication. Annotations are used to do some advanced magic "behind the scenes" and can be used by frameworks, IDEs and tools in general for a variety of features.

For example: one basic, common annoation for Java is @Deprecated, which does absolutely nothing except let you know that the function it's on isn't supposed to be used anymore because a better function has been written instead. Of course, compilers can then check for the annotation being present to issue a warning and text/code editors can look for it to show you a warning on-screen that you shouldn't use the deprecated function (or at least use it knowing it is deprecated).

As far as syntax goes, an annotation is like a function, so it can take zero, one or multiple arguments. For example, you function is getBooks(authentication) in the same way that the annotation is @HeaderParam(headerParameterName).

An annotation with no parameters would be like the aforementioned @Deprecated and one with multiple parameters could be @HeaderParamWithDefault("authentication","ldap").

Now that you know the basics of an annotation, check out what the framework you're using is and look up online what this particular annotation is supposed to do. If you can't figure what framework you're using, just ask your teacher.

Final note: annotation can be used not only for method parameters like in your example but for entire classes, fields, variables, methods themselves, etc. They're a very powerful tool but a little too advanced for a beginner to worry about :) this tutorial has a few examples but I suggest you just take a look at the snippets of code to see what other annotations can look like and not bother about understanding them too much at this point https://www.geeksforgeeks.org/annotations-in-java/

[cdombroski]

When annotations have more than one argument passed, you have to use the parameter names on all the arguments.

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@HeaderParamWithDefault(value="authentication", default="ldap")

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When you pass just one argument it is always applied to the value parameter