#{ Java: Colecciones, listas, mapas, etc. }

http://www.codercorp.com/blog/java/why-concurrenthashmap-is-better-than-hashtable-and-just-as-good-hashmap.html
http://javarevisited.blogspot.com/2010/10/difference-between-hashmap-and.html
http://stackoverflow.com/questions/40471/differences-between-hashmap-and-hashtable
http://stackoverflow.com/questions/1361784/hashmap-and-hashtable-in-multithreaded-environment
http://www.coderanch.com/t/374477/java/java/Books-java-collections-framework

#{ Java: Listas y colecciones }

HashSet: Colección que no permite valores repetidos, si se intenta añadir uno repetido el método "add" retorna falso:

HashSet hs = new HashSet();  
hs.add(1);  
System.out.println(hs.add(3)); //retorna true , añadido éxitosamente. 
System.out.println(hs.add(3)); //retorna false, no se añadió el objeto.  
hs.add(1);//

http://www.coderanch.com/t/503961/java/java/Adding-duplicate-values-HashSet

#{ Articulos de Java y Collections }

http://www.reloco.com.ar/prog/java/collections.html
http://www.reloco.com.ar/prog/java/