I am doing the implementation of the prim algorithm
s = q2.poll (). getVal ();
if(visited[s]==false) {
mst.add(q.poll());
}
else {
while(visited[s]==true) {
q.poll();
s = q2.poll().getVal();
}
And when executing I get a null pointer exception for when I assign a value to s in the while.
Because you have
q.poll();
s = q2.poll().getVal();
enclosed in a "while" asking for an index of the array "visited [s]" and within the same while changing "s" for another value, it is most likely that the value given by s = q2.poll().getVal(); is not an index valid for "visited [s]" and java this trying to access a "null" value, that is, it does not exist and therefore skip the exception. One of the possible solutions is to add the condition asking for "null" in the while in this way:
while(visited[s]==true && visited[s]!=null) {
q.poll();
s = q2.poll().getVal();
}
I hope it will be useful for you. Good luck. Greetings.