Java and JVM Interoperability

public static Integer asInt (String s) {
  try {
    return Integer.parseInt(s);
  } catch (NumberFormatException e) {
    e.printStackTrace();
    return null;
  } finally {
    System.out.println("Attempted to parse as integer: " + s);
  }
}

(defn as-int
  [s]
  (try
   (Integer/parseInt s)
   (catch NumberFormatException e
     (.printStackTrace e))
   (finally
    (println "Attempted to parse as integer: " s))))

(throw (IllegalStateException. "I don't know what to do!"))
;>> IllegalStateException I don't know what to do!

public static int parseInt(String s) throws NumberFormatException
{
   ...
}

(defn length-of
  [^String text]
  (.length text))

(ns clj-prog.java-interop)

(defn accepts-anything-hint
  [^java.util.List x]
  x)

(defn accepts-anything-no-hint
  [x]
  x)

import clojure.lang.AFunction;

public final class java_interop$accepts_anything_hint extends AFunction {
    public java_interop$accepts_anything_hint() {
    }

    public static Object invokeStatic(Object x) {
        Object var10000 = x;
        x = null;
        return var10000;
    }

    public Object invoke(Object var1) {
        Object var10000 = var1;
        var1 = null;
        return invokeStatic(var10000);
    }
}

import clojure.lang.AFunction;

public final class java_interop$accepts_anything_no_hint extends AFunction {
    public java_interop$accepts_anything_no_hint() {
    }

    public static Object invokeStatic(Object x) {
        Object var10000 = x;
        x = null;
        return var10000;
    }

    public Object invoke(Object var1) {
        Object var10000 = var1;
        var1 = null;
        return invokeStatic(var10000);
    }
}

(defn length-of-hint
  [^String text]
  (.length text))

(defn length-of-no-hint
  [text]
  (.length text))

import clojure.lang.AFunction;

public final class java_interop$length_of_hint extends AFunction {
    public java_interop$length_of_hint() {
    }

    public static Object invokeStatic(Object text) {
        Object var10000 = text;
        text = null;
        return Integer.valueOf(((String)var10000).length()); (1)
    }

    public Object invoke(Object var1) {
        Object var10000 = var1;
        var1 = null;
        return invokeStatic(var10000);
    }
}

import clojure.lang.AFunction;
import clojure.lang.Reflector;

public final class java_interop$length_of_no_hint extends AFunction {
    public java_interop$length_of_no_hint() {
    }

    public static Object invokeStatic(Object text) {
        Object var10000 = text;
        text = null;
        return Reflector.invokeNoArgInstanceMember(var10000, "length", false); (1)
    }

    public Object invoke(Object var1) {
        Object var10000 = var1;
        var1 = null;
        return invokeStatic(var10000);
    }
}

(defn capitalize
  [s]
  (-> s
      (.charAt 0)
      Character/toUpperCase
      (str (.substring s 1))))

(time (doseq [s (repeat 100000 "foo")]
        (capitalize s)))
;>> "Elapsed time: 5040.218 msecs"

(defn fast-capitalize
  [^String s]
  (-> s
      (.charAt 0)
      Character/toUpperCase
      (str (.substring s 1))))

(time (doseq [s (repeat 100000 "foo")]
        (fast-capitalize s)))
;>> "Elapsed time: 154.889 msecs"

(set! *warn-on-reflection* true)

(defn capitalize
  [s]
  (-> s
      (.charAt 0)
      Character/toUpperCase
      (str (.substring s 1))))
;>> Reflection warning, NO_SOURCE_PATH:27 - call to charAt can't be resolved.
;>> Reflection warning, NO_SOURCE_PATH:29 - call to toUpperCase can't be resolved.
;>> Reflection warning, NO_SOURCE_PATH:29 - call to substring can't be resolved.

(defn split-name
  [user]
  (zipmap [:first :last]
          (.split ^String (:name user) " ")))

(defn file-extension
  ^String [^java.io.File f]   ; (1)
  (-> (re-seq #"\.(.+)" (.getName f))
      first
      second))

(.toUpperCase (file-extension (java.io.File. "image.png")))

(def a "image.png")

(java.io.File. a)
;>> Reflection warning, NO_SOURCE_PATH:1 - call to java.io.File ctor can't be resolved.

(def ^String a "image.png")

(java.io.File. a)
;=> #<File image.png>