Multiplying Values of Two HashMaps

Question

If I have two HashMap s, hm1 and hm2 , how can I iterate through the two and multiply the two values together at each point in the two HashMap s and sum the total? They are both ordered identically, so I don't need to worry about the keys, just the values.

The data is in the form

hm1 = 
(A, 3)
(B, 4)
(C, 7)

hm2 =
(A, 4)
(B, 6)
(C, 3)

then I want to do something like this but obviously this code is incorrect because I'm only iterating through hm1.

double sum = 0.00;
for (Map.Entry<String, Double> hm : hm1.entrySet()) {
    sum += hm1.getValue() * hm2.getValue();
}

So I would basically loop through and do:

1st iteration: Sum = 0 + 3*4
2nd Iteration: Sum = 12 + 4*6
3rd iteration: Sum = 36 + 7*3
Exit Loop:
Sum = 57

Thanks for any help you can give me.

Answer 1

Given that the two HashMaps have equal amount of entries, and that all the entries have the same corresponding key, then a solid way to aquire the sum would be like this:

double sum = 0;

/* iterate through all the keys in hm1 */
/* order doesn't matter */
for (String key : hm1.keySet()) {
    double value1 = hm1.get(key);
    double value2 = hm2.get(key);
    sum += value1 * value2;
}

Answer 2

You can use the key from your iteration over the first map to get the value for the second one:

double sum = 0.00;
for (Map.Entry<String, Double> hm : hm1.entrySet()) {
   double hm2Value = hm2.get(hm.getKey());
   sum += hm.getValue() * hm2Value;
}

Note that this only works if both maps have the same keys. If keys are missing in either, then you have to think about how to deal with that.

Answer 3

Java Version 8, you can use Stream API:

    Map<String, Double> hm1 = new HashMap<String, Double>() {{
        put("A", 3.0);
        put("B", 4.0);
        put("C", 7.0);
    }};

    Map<String, Double> hm2 = new HashMap<String, Double>() {{
        put("A", 4.0);
        put("B", 6.0);
        put("C", 3.0);
    }};

    double sum = hm1.entrySet()
        .stream()
        .filter(entry -> hm2.containsKey(entry.getKey()))
        .map(entry -> entry.getValue() * hm2.get(entry.getKey()))
        .mapToDouble(value -> value)
        .sum();