I started learning Java a year back. Bought the book by K&B. Read the book randomly through the year, completed 7/10 chapters till November 2013, got sidetracked by my University exams and now I have three weeks and I feel I've forgotten everything.
How should I go about preparing now?
I want to give it by the second week of February and if possible get 90-100%. Am I too ambitious?
Well, it is possible, but it depends much more upon your proficiency with Java programming. If you are writing lot of Java code - day in, day out - then I don't think it will be difficult.
As Paul has mentioned, a mock exam will give you general idea about your preparation (and weak areas - on which you need to focus more).
I would recommend a plan as:
1) Start with a mock exam - identify your strengths and weaknesses
2) Actual preparation - make sure you are thorough with all required areas - theoretically (reading) and practically (writing code)
3) Close the study with few more mock exams - at this stage you should be getting good score on mock exam