IntStream map sum

import java.util.stream.IntStream;

public class Hamming {

    final private int difference;

    public Hamming(String leftStrand, String rightStrand) {
        if (leftStrand.length() != rightStrand.length()) {
            throw new IllegalArgumentException("strands must be of equal length");
        }
        difference = IntStream
            .range(0, leftStrand.length())
            .map(i -> leftStrand.charAt(i) != rightStrand.charAt(i) ? 1 : 0)
            .sum();
    }

    public int getHammingDistance() {
        return difference;
    }
}

This approach starts by importing java.util.stream.IntStream. It then defines a private final int variable to keep track of the difference between the strands.

After the validation code, the real work is done by the IntStream. It uses its range() method to iterate from 0 up to but not including the length of the left strand. Each number is passed into the map() method. For each value of i, the lambda function inside the map() method compares the values of the elements in the two strands at the index of i. Using a ternary operator, the map() method converts the index to 1 for only those indexes whose elements differ between the two strands. Otherwise it converts the index value to 0. The difference variable is set to the sum() of the converted indexes.

The getHammingDistance() method returns the difference variable, which is set to the correct value after the IntStream is done.