Iterating through hashmap keys and comparing

Question

I want to iterate through each key of a hashmap and compare it to each key below it (so I don't compare keys twice). I want to see which slopes are perpendicular to each other. If the product of the slopes is -1, they are and I will do something with the arraylists with those keys.

So far I have built the map using:

Map<Double, List<Double>> map = new HashMap<Double, List<Double>>();

for(int i = 0; i < n; i++) {
    for(int j = i + 1; j < n; j++) {
        if(line[4][i] == line[4][j]) {
            double distance = 7.0;   //distance();

            List<Double> array = (map.containsKey(line[4][i])) ? map.get(line[4][i]):new ArrayList<Double>();
            array.add(distance);
            map.put(line[4][i], array);

            // System.out.println(Arrays.asList(array));
            // System.out.println(distance);
        }
    }
}

And attempt to iterate through the map with:

Iterator<Entry<Double, List<Double>>> i = map.entrySet().iterator();
while(i.hasNext()) {
    Entry next = i.next();
    i.remove();
    for(Entry e : map.entrySet()) {
        System.out.println(next + " " + e);

        //ERROR: The method parseDouble(String) in the type Double is not applicable for the arguments (Object)
        if((Double.parseDouble(next.getKey()) * (Double.parseDouble(e.getKey()))) == -1) 
            System.out.println("pair"); //do something 
    }
}

But I can't parse an object to a double, and I'm lost as to how to continue. Any help is appreciated

Answer 1

to your question, use type qualifiers for the next Entry:

Map.Entry<Double, List<Double>> next = i.next();

and here too:

Map.Entry<Double, List<Double>> e : map.entrySet()

then the compiler will know that getKey() will return Doubles, and allow to multiply them with each other.