The Collections object is not a Processing API object, but an underlying Java object, and is not available to all interpreters of Processing code (because not all interpreters are based on the JVM).
If you want to find the minimum value, it's three lines of code:
int minval = aaa.get(0);
for(int v: aaa) {
if(v < minval) { minval = v; }
}
Done, we have our minimum value. If we wrap this in a function, we can use it wherever we want:
int getMinValue(ArrayList<Integer> numberlist) {
int minval = numberlist.get(0);
for(int v: numberlist) {
if(v < minval) { minval = v; }
}
return minval;
}