Question

fringe = new PriorityQueue<Node>(10,new Comparator<Node>(){
            @Override
            public int compare(Node node1,Node node2)
            {
                if (f(node1)>f(node2))
                    return 1;
                else
                    return -1;
            }
        });

I declared a PQ to store some nodes and i want to store the nodes in non-decreasing order according to the f value.function f(Node node) is to calculate the f value for the node. so i override the comparator but right now i found that some nodes with larger f value are placed before the ones with smaller f value in the queue, i checked all over but still cannot find what goes wrong, i assume maybe it is the PQ declaration’s problem. Anyone can help me? Thanks in advance!

Answer 1

As stated by @izomorphius, PriorityQueue does not guarantee full ordering, only that the head is always minimal.

If you want full order – you might want to chose one of these possiblities:

1. Use a TreeSet – but note it will not allow duplicates. Also, as stated by @JoonasPulakka, here you will probably want to override equals() and hashCode() as well.
2. Use a List – populate it [unordered] and then use Collections.sort(List,Comparator) to sort it according to your comparator
3. use an array [Node[]], populate it [unordered] and then use Arrays.sort() to sort it according to your comparator.

EDIT:

Your editted Comparator does not enforce full ordering, and thus the result of using it is undefined:

let a,b be two Nodes such that f(a) == f(b)

compare(a,b) == compare(b,a) == -1

But the javadocs states:

The implementor must ensure that sgn(compare(x, y)) == -sgn(compare(y,
x)) for all x and y.

Thus, the result of using this [the editted] comparator is undefined.

EDIT2:

Your comments suggest you are looking for a second criteria – of “adding time”. You can add another field long timestamp to your Node object, and in your Comparator return a result based on this field if and only if f(node1) == f(node2). This will guarantee consistency, and will achieve the wanted feature.

Note: this field will be initialized once [and only once!] when you add an element to the queue for the first time [or when the object is created, if it’s an option].