I had similar problems when I first started using Clojure. I had done some Java development years ago, but was still pretty unfamiliar with the libraries out there.
Intro
I find the easiest way to use Java is to not really use it. I think a book would be a little bit much to just get started using Java from Clojure. There isn't that much you really need to know, unless you really start getting down into the JVM/Java libraries. Let me explain.
Spend more time learning how to use Clojure inside and out, and become familiar with Clojure-Contrib. For instance, sqrt is in generic.math-functions in clojure.contrib.
Many of the things you'll need are in fact already in Clojure–but still plenty are not.
Become familiar with calling conventions and syntactic sugar in Clojure for using Java. e.g. Math/sqrt, as per your example, is calling the static method (which just a function, basically) sqrt from the class Math.
Anyway, here's a guide that should help you get started if you find yourself really needing to use Java. I'm going to assume you've done some imperative OO programming, but not much else. And even if you haven't, you should be okay.
Isaac's Clojurist's Guide to Java
Classes
A class is a bundle of methods (functions which act on the class) that
can also be a data type: e.g. to create a new class of the type Double : (Double. 1.2) which initializes the class Double (the period is the syntactic sugar for calling the class constructor methods, which initialize the class with the values you provide) with the value 1.2.
Now, look at the Double class in the Java 6 API:
Double
public Double(double value)
Constructs a newly allocated Double object that represents the
primitive double argument.
Parameters:
value - the value to be represented by the Double.
So you can see what happened there. You "built" a new Double with value 1.2, which is a double. A little confusing there, but really a Double is a class that represents a Double and can do things relating to doubles.
Static Methods
For instance, to parse a Double value out of a string, we can use the static method (meaning we don't need a particular instance of Double, we can just call it like we called sqrt) parseDouble(String s):
(Double/parseDouble "1.2") => 1.2
Not to tricky there.
Nonstatic Methods
Say we want to use a Java class that we initialized to something. Not too difficult:
(-> (String. "Hey there") ;; make a new String object
(.toUpperCase)) ;; pass it to .toUpperCase (look up -> to see what it does)
;; toUpperCase is a non-static method
=> "HEY THERE"
So now we've used a method which is not static, and which requires a real, live String object to deal with. Let's look at how the docs say it works:
toUpperCase
public String toUpperCase()
Converts all of the characters in this String to upper case using
the rules of the default locale. This method is equivalent to
toUpperCase(Locale.getDefault()).
Returns:
the String, converted to uppercase.
So here we have a method which returns a string (as shown by the "String" after the public in the definition, and takes no parameters. But wait! It does take a parameter. In Python, it'd be the implicit parameter self: this is called this in Java.
We could also use the method like this: (.toUpper (String. "Hey there")) and get the same result.
More on Methods
Since you deal with mutable data and classes in Java, you need to be able to apply functions to Classes (instances of Classes, really) and not expect a return value.
For instance, say we're dealing with a JFrame from the javax.swing library. We might need to do a number of things to it, not with it (you generally operate with values, not on them in functional languages). We can, like this:
(doto (JFrame. "My Frame!");; clever name
(.setContentPane ... here we'd add a JPanel or something to the JFrame)
(.pack) ;; this simply arranges the stuff in the frame–don't worry about it
(.setVisibleTrue)) ;; this makes the Frame visible
doto just passes its first argument to all the other functions you supply it, and passes it as the first argument to them. So here we're just doing a lot of things to the JFrame that don't return anything in particular. All these methods are listed as methods of the JFrame in the documentation (or its superclasses… don't worry about those yet).
Wrapping up
This should prepare you for now exploring the JavaDocs yourself. Here you'll find everything that is available to you in a standard Java 1.6 install. There will be new concepts, but a quick Google search should answer most of your questions, and you can always come back here with specific ones.
Be sure to look into the other important Clojure functions like proxy and reify as well as extend-type and its friends. I don't often use them, but when I need to, they can be invaluable. I still am understanding them myself, in fact.
There's a ton out there, but it's mostly a problem of volume rather than complexity. It's not a bad problem to have.
Additional reading: