True randomization in java

Question

Hi,
I need to split a file of 20290 lines into two files randomly ; each random split has to produce two subfiles, one representing 80%, the other 20% of the original 20290.
I need to produce 1000 different 80-20 splits.
I woudl like to write a class A that:
-- puts the input file into an ArrayList.
-- generats randomly a sequence of numbers between 0 and 20289.
-- prints out the file in that sequence splitted into 80-20%.
Then I would like to write a class B that:
-- calls class A 1000 times to produce 1000 different 80-20 splits
I see that there is a java.util.Random class, but I have never used it.
Could someone show me:
-- how do I make sure that the numbers generated are between 0 and the size of the input file minus one( assuming I put it all into an arraylist, for example, i.e. 20289) ?
-- how do I make sure that for each 80-20 split I seed Random() so that I get a different sequence of numbers ?
Thank you very much,
Grazia

Answer 1

Look at java.util.Collections.shuffle to randomly reorder your ArrayList.
If you have to do the randomization yourself (not using 'shuffle'), use the same java.util.Random instance over and over. Then you won't need to be sure you get 1000 different seeds.