一个小问题，这个warning的怎么解决？

[alcla]

javac -Xlint *.java
waring:[unchecked] unchecked conversion
found PQcomparator
require:java.utill.Comparator<? super node>
bestSucc = new PriorityQueue<node>(1,new PQComparator());
1 warning

my PQComparator class is:
import java.util.Comparator;
class PQComparator implements Comparator{
node t1;
node t2;
public int compare(Object a,Object b){
t1= (node)a;
t2= (node)b;
return t1.getNum()-t2.getNum();
}
}