Nothing is tough until u know it. Dan's exam is very good and it lets u know where u r missing in ur goal. There are various questions which will give u a borader knowledge about the concepts. Questions regarding wrapper classes and collections API makes u learn the API in detail. For eg. i would like to refer to a question which i wasnt even aware of its result:
I was amazed by the answer. i didnt know about the internal working of stringbuffer class, it made me go thru the stringbuffer code.
So, i would suggest u go thru the dan exam, may be the questions are easier in the real exam than in Dan's but it will help u gain a better understanding of various topics.