I have a map reduce program that takes input as firstname, lastname and mobile number. I want to group these 3 fields as a single key. For this, I have used WritableComparable interface for a class.

My code is:

private static class Multifield implements WritableComparable<Multifield> {

        Text firstname1;
        Text lastname1;
        Text mobile1;

        public Multifield(Text firstname1, Text lastname1,Text mobile1) {
            this.firstname1 = firstname1;
            this.lastname1 = lastname1;
            this.mobile1=mobile1;
        }
        public Multifield() {
            this.firstname1 = new Text();
            this.lastname1 = new Text();
            this.mobile1=new Text();

        }

        public void write(DataOutput out) throws IOException {
            this.firstname1.write(out);
            this.lastname1.write(out);
            this.mobile1.write(out);

        }

        public void readFields(DataInput in) throws IOException {

            this.firstname1.readFields(in);
            this.lastname1.readFields(in);
            this.mobile1.readFields(in);

        }

        public int compareTo(Multifield pop) {

            if(firstname1.equals(pop.firstname1))
            {
                return lastname1.compareTo(pop.lastname1);
            }
            else
            {
                return firstname1.compareTo(pop.firstname1);
            }
        }
        @Override
        public String toString() {
            return "Customer Details [firstname=" + firstname1 + ", lastname=" + lastname1
                    + ", mobile=" + mobile1 + "]";
        }

The above coding just compares firstname and lastname. So, I get the output only based on the first two fields.

How can I modify the compareTo() method to compare all three fields, so that I can group them together as a key?

The logic we use for comparing 2 fields is:

if(field1 = field2) then Result = 0;
if(field1 > field2) then Result = 1;
if(field1 < field2) then Result = -1;

For the sake of explanation, let's assume that, we are comparing 2 objects of same type, containing fields a, b and c.

Object 1 contains a1, b1 and c1 (instances of a, b and c)
Object 2 contains a2, b2 and c2 (instances of a, b and c)

Following are different cases, we need to handle:

a) if(a1 = a2, b1 = b2, c1 = c2) then Result = 0;  // (All equal)
b) if(a1 > a2)                   then Result = 1;  // (Since a1 > a2, we short circuit at a)
c) if(a1 < a2)                   then Result = -1; // (Since a1 < a2, we short circuit at a)
d) if(a1 = a2, b1 > b2)          then Result = 1;  // (Since b1 > b2, we short circuit at b)
e) if(a1 = a2, b1 < b2)          then Result = -1; // (Since b1 < b2, we short circuit at b)
f) if(a1 = a2, b1 = b2, c1 > c2) then Result = 1;  // (Since c1 > c2)
g) if(a1 = a2, b1 = b2, c1 < c2) then Result = -1; // (Since c1 < c2)

The code for this would be:

public int compareTo(Person otherPerson) {
    int cmp = firstName.compareTo(otherPerson.firstName);
    if(cmp != 0) return cmp;

    cmp = lastName.compareTo(otherPerson.lastName);
    if(cmp != 0) return cmp;

     return mobile.compareTo(otherPerson.mobile);
}

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