There are 23 design patterns in java each addressing one use case and there is minor differences between them. You need to know all of them and more importantly when to use which. We commonly use Iterator interface to iterate over a collection which is infact one of the design pattern itself called Iterator design pattern. Other common design pattern that should be known are Singelton, Factory, abstract factory, Observer listener, flyweight, Mediator.
There is a videoCD from Design pattern which has all the design patterns explained very neatly. I personally reccommend that.
Thanks and Regards, Pradeep Kumar
SCJP 1.6, SCWCD 5.0