Clojure “dumb” simple lazy example

user=> (def x (lazy-seq (cons (do (println "evaluated") (+ 1 2)) (list 1 2 3))))
#'user/x
user=> (def y (cons (do (println "evaluated") (+ 1 2)) (list 1 2 3)))
evaluated
#'user/y

Clojure immutable collections for Java env

(Include clojure.jar in your lib path)

List list = new PersistentList("t1");
try {
	list.add("t2");
} catch (UnsupportedOperationException e) {
	System.out.println("It seems that this operation is not supported.");
}
System.out.println("List size: " + list.size());

Map map = new PersistentArrayMap(new Object[]{"key1", "value1"});
try {
	map.put("key2", "value2");
} catch (UnsupportedOperationException e) {
	System.out.println("It seems that this operation is not supported.");
}
System.out.println("Map size: " + map.size());

This can be used as an alternative for
java.util.Collections.unmodifiableList(…)

Lazy clojure

Now, to demonstrate lazy vs. non-lazy behavior in clojure:

(defn create-lazy [i] 
    (if (zero? i) 
    ()
    (lazy-seq (cons nil (create-lazy (dec i))))))

(defn create-non-lazy [i] 
    (if (zero? i) 
    () 
    (cons nil (create-non-lazy (dec i)))))

(create-lazy 1000000)
(try (create-non-lazy 1000000)
    (catch java.lang.StackOverflowError e
        (println "StackOverflow error")))

Of course, in real life situations, easier methods can be used, like

user=> (replicate 7 nil)
(nil nil nil nil nil nil nil)
user=>