I have written methods for pathogen shedding and accumulation in a grid.
I have created 3 different ArrayLists: one to hold the x coord, one the hold the y coord and one to hold the deposit time (the pathogen has a halflife, I need to know deposit time).
I use the following :

public void depositPathogen()

if(this.infected==true)

if(RandomHelper.nextDouble() < infectionRate)
xDepositList.add(infectedX); yDepositList.add(infectedY);
timeDepositList.add(myTick);


public void checkForPathogenPickup()
if(xDepositList.contains(myX) && yDepositList.contains(myY)){
for (int i=0; i<xDepositList.size(); i++){
if (xDepositList.get(i)== myX && yDepositList.get(i)==myY) {
double myDepositTime = timeDepositList.get(i);
myPathogenInfectivity = myPathogeInfectivity + pathogHalfLife(myDepositTime,myTick)

the methods work but since my lists get really large, it seriously increases computing time. Any ideas of how this can be done better?
Thanks,