You can call an instance method using the . special form:

(.trim " hello ")
;;=> "hello"

You can call instance methods with arguments like this:

(.substring "hello" 0 2)
;;=> "he"

You can call an instance field using the .- syntax:

(def p (java.awt.Point. 0 1))
(.-x p)
;;=> 0
(.-y p)
;;=> 1

You can create instance of objects in one of two ways:

(java.awt.Point. 0 1)
;;=> => #object[java.awt.Point 0x3776d535 "java.awt.Point[x=0,y=1]"]

Or

(new java.awt.Point 0 1)
;;=> => #object[java.awt.Point 0x3776d535 "java.awt.Point[x=0,y=1]"]

You can call static methods like this:

(System/currentTimeMillis)
;;=> 1469493415265

Or pass in arguments, like this:

(System/setProperty "foo" "42")
;;=> nil
(System/getProperty "foo")
;;=> "42"

You can call a Clojure function from Java code by looking up the function and invoking it:

IFn times = Clojure.var("clojure.core", "*");
times.invoke(2, 2);

This looks up the * function from the clojure.core namespace and invokes it with the arguments 2 & 2.