Design Patters in Java core library

GoF's Design Pattern implementations found in both the Java SE and Java EE API's.
more: http://stackoverflow.com/questions/1673841/examples-of-gof-design-patterns-in-javas-core-libraries

Design patterns in one line: http://www.dofactory.com/net/design-patterns

Design patterns explained with Java classes
http://www.tutorialspoint.com/design_pattern/

Java Collections Cheat Sheet

Source: https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjgFLPosNUOWvHe2Ah2DMKkQJrVGThQgpdPixuChwCAtaCS9i50Ipy7gklwR3Eima3xe-gdsplRdGFY0zZ8tlboe8ZgFnQaSiZvGgcywojwQ1wdaAf8g2smM6XkNVffoa8oqSvXG-ECrFg/s1600/Java+collection+cheat+sheet.PNG

HashMap internal implementation in Java

Below link provides good explanation about this topic.

http://javarevisited.blogspot.in/2011/02/how-hashmap-works-in-java.html

http://javahungry.blogspot.com/2013/08/hashing-how-hash-map-works-in-java-or.html

HashSet internal implementation in Java

HashMap.put method will return
 
  oldValue: if key is already present, then it will overwrite the value and returns old value
  null: if key is not present, then it will add this key,value pair and then returns null.

In case of HashSet, internally it has a hashmap. When we call add method it will call 'put' method of the hashmap and if put method returns null means new entry is added into hashmap and hashset

more at: http://javahungry.blogspot.com/2013/08/how-sets-are-implemented-internally-in.html

Java purpose of AtomicInteger

We can use AtomicInteger to avoid count++ issue in mutli-threading environment.

AtomicInteger provides incrementAndGet() and decrementAndGet() methods which are thread safe.

With use of AtomicInteger, we can solve this problem without using synchronized keyword

more: https://docs.oracle.com/javase/tutorial/essential/concurrency/atomicvars.html

          http://javarevisited.blogspot.in/2012/01/how-to-write-thread-safe-code-in-java.html

Websites to learn Algorithms, Data Structures and Java

For learning algorithms and data structures, please follow below links.

1. http://www.geeksforgeeks.org/data-structures/

2.  http://www.geeksforgeeks.org/fundamentals-of-algorithms/

3. http://www.careercup.com/

4. http://javarevisited.blogspot.in/

  Will update more.