for loop

public class Hamming {

    private int difference = 0;

    public Hamming(String leftStrand, String rightStrand) {
        if (leftStrand.length() != rightStrand.length()) {
            throw new IllegalArgumentException("strands must be of equal length");
        }
        for (int i = 0; i < leftStrand.length(); i++){
            if (leftStrand.charAt(i) != rightStrand.charAt(i)){
                difference++;
            }
        }
    }

    public int getHammingDistance() {
        return difference;
    }
}

This approach starts be defining a private int variable to keep track of the difference between the strands.

After the validation code, the real work is done by the for loop. It iterates from 0 up to but not including the length of the left strand. For each value of i, it compares the values of the elements in the two strands at the index of i. If the values are different, the difference variable is incremented.

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