Help: Problem With Seperate Arrays Overwriting Each Other!

Question

Hi,
I have this function that randomly allocates free time to coursework blocks. This method works because I have tested numerous times and it always prints of different allocations of time every time its called.
The problem that I have is that I have this method that called this method twice to store each different call within a separate ArrayList.
When I make the first call the block are assigned to the array fine. But when I called it a second time to store the next result within a separate Array the first Array result are overwritten . Any body have any idea why this is happening, I have asked numerous friends and even my supervisor and he can't find anything wrong with it. Any help would be very helpful...
Code:
public ArrayList<CourseworkTimeBlock> courseworkAllocation(ArrayList timetable, ArrayList<CourseworkTimeBlock> cCourseworkBlocks){
          // Calculates and stores the free "time blocks" that can be used
          // to allocate coursework.
          ArrayList<Integer> freeTimeBlocks = new ArrayList<Integer>();
          freeTimeBlocks = freeBlocks(timetable);
          Random random = new Random();
          ArrayList<CourseworkTimeBlock> courseworkTimeBlocks = new ArrayList<CourseworkTimeBlock>();
          for(int i = 0; i < cCourseworkBlocks.size(); i++){
               int allocateTo = -100;
               int randomPoint = 0;
               CourseworkTimeBlock courseworkBlock;
               courseworkBlock = cCourseworkBlocks.get(i);
               while(allocateTo < 0){
                    randomPoint = random.nextInt(freeTimeBlocks.size());
                    allocateTo = freeTimeBlocks.get(randomPoint);
               courseworkBlock.setBlockAllocated(allocateTo);
               courseworkTimeBlocks.add(courseworkBlock);
               freeTimeBlocks.set(randomPoint, -1 );
          for( int i = 0; i < courseworkTimeBlocks.size(); i++){
               System.out.println("CW: " + courseworkTimeBlocks.get(i).getBlockAllocated());
          return courseworkTimeBlocks;
     }These are the calls I make in a separate method:
ArrayList<CourseworkTimeBlock> courseworkTimetableOne = new ArrayList<CourseworkTimeBlock>();
ArrayList<CourseworkTimeBlock> courseworkTimetableTwo = new ArrayList<CourseworkTimeBlock>();
         courseworkTimetableOne = courseworkAllocation(fixedTimetable, courseworkBlocks);
         courseworkTimetableTwo = courseworkAllocation(fixedTimetable, courseworkBlocks);Edited by: ChrisUK on Mar 1, 2008 9:11 AM

Answer 1

ChrisUK wrote:
Thanks for the reply.
In terms of the:
courseworkTimetableOne = courseworkAllocation(fixedTimetable, courseworkBlocks);
courseworkTimetableTwo = courseworkAllocation(fixedTimetable, courseworkBlocks);They are both supposed to pass in the fixedTimetable object, the reason being is that this is a timetable and is passed through in order to find it "free time" blocks.
The courseworkTimetableOne and courseworkTimetableTwo are just different representations of how the coursework time blocks could be allocated to that given fixedtimetable. You're calling the same method with the same parameters, so both references are going to point to the same object. It's the same as if you just replaced the second line with courseworkTimetableTwo = courseworkTimetableOne.
You've got two references pointing to the same object.
The fact that you do this first:
ArrayList<CourseworkTimeBlock> courseworkTimetableOne = new ArrayList<CourseworkTimeBlock>();
ArrayList<CourseworkTimeBlock> courseworkTimetableTwo = new ArrayList<CourseworkTimeBlock>();means nothing. You create the two new ArrayLists, but then this
courseworkTimetableOne = courseworkAllocation(fixedTimetable, courseworkBlocks);
courseworkTimetableTwo = courseworkAllocation(fixedTimetable, courseworkBlocks);gets rid of them. The new ArrayLists you just created are lost.
Remember when you assign to a reference variable such as courseworkTimetableOne =, all that does is copy a reference so now courseworkTimetableOne is pointing to the same object that whatever is on the RHS is pointing to.
If you want two different lists, then either courseworkAllocation should make a copy of its internal list and return that, or else the caller should do it.
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