System Design
1. singleton pattern
Singleton design pattern in java
http://howtodoinjava.com/2012/10/22/singleton-design-pattern-in-java/
**a thorough explanation of singleton pattern**
2. factory pattern
Factory design pattern in java
http://howtodoinjava.com/2012/10/23/implementing-factory-design-pattern-in-java/
3. abstract factory pattern
Abstract factory pattern in java
http://howtodoinjava.com/2012/10/29/abstract-factory-pattern-in-java/
4. prototype pattern
Prototype design pattern in java
http://howtodoinjava.com/2013/01/04/prototype-design-pattern-in-java/