IntStream reduce

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()).reduce(0,
            (hamcount, index) -> hamcount + (leftStrand.charAt(index) != rightStrand.charAt(index) ? 1 : 0));
    }

    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 reduce() method, whose accumulator is initialized to 0. For each value of index, the lambda function inside the reduce() method compares the values of the elements in the two strands at the index of index. Using a ternary operator, the reduce() method adds 1 to the accumulator for only those indexes whose elements differ between the two strands. Otherwise it adds 0. The difference variable is set to the value returned from the reduce() method, which is the value of its accumulator.

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