Calculate weighted average with Java 8 streams

Question

How do I go about calculating weighted mean of a Map where the Integer value is the weight for the Double value to be averaged. eg: Map h

Answer 1

You can use this procedure to calculate the weighted average of a map. Note that the key of the map entry should contain the value and the value of the map entry should contain the weight.

     /**
     * Calculates the weighted average of a map.
     *
     * @throws ArithmeticException If divide by zero happens
     * @param map A map of values and weights
     * @return The weighted average of the map
     */
    static Double calculateWeightedAverage(Map map) throws ArithmeticException {
        double num = 0;
        double denom = 0;
        for (Map.Entry entry : map.entrySet()) {
            num += entry.getKey() * entry.getValue();
            denom += entry.getValue();
        }

        return num / denom;
    }

You can look at its unit test to see a usecase.

     /**
     * Tests our method to calculate the weighted average.
     */
    @Test
    public void testAveragingWeighted() {
        Map map = new HashMap<>();
        map.put(0.7, 100);
        map.put(0.5, 200);
        Double weightedAverage = calculateWeightedAverage(map);
        Assert.assertTrue(weightedAverage.equals(0.5666666666666667));
    }

You need these imports for the unit tests:

import org.junit.Assert;
import org.junit.Test;

You need these imports for the code:

import java.util.HashMap;
import java.util.Map;

I hope it helps.