Please check the in-line answers...
On Sat, Feb 23, 2013 at 6:22 PM, Sai Sai <saigr...@yahoo.in> wrote:
>
> Hello
>
> I have a question about how Mapreduce sorting works internally with
> multiple columns.
>
> Below r my classes using 2 columns in an input file given below.
>
> 1st question: About the method hashCode, we r adding a "31 + ", i am
> wondering why is this required. what does 31 refer to.
>
This is how usually hashcode is calculated for any String instance
(s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1]) where n stands for length of
the String. Since in your case you only have 2 chars then it will be a *
31^0 + b * 31^1.
>
> 2nd question: what if my input file has 3 columns instead of 2 how would
> you write a compare method and was wondering if anyone can map this to a
> real world scenario it will be really helpful.
>
you will extend the same approach for the third column,
public int compareTo(WordPairCountKey o) {
int diff = word1.compareTo(o.word1);
if (diff == 0) {
diff = word2.compareTo(o.word2);
if(diff==0){
diff = word3.compareTo(o.word3);
}
}
return diff;
}
>
>
> @Override
> public int compareTo(WordPairCountKey o) {
> int diff = word1.compareTo(o.word1);
> if (diff == 0) {
> diff = word2.compareTo(o.word2);
> }
> return diff;
> }
>
> @Override
> public int hashCode() {
> return word1.hashCode() + 31 * word2.hashCode();
> }
>
> ******************************
>
> Here is my input file wordpair.txt
>
> ******************************
>
> a b
> a c
> a b
> a d
> b d
> e f
> b d
> e f
> b d
>
> **********************************
>
> Here is my WordPairObject:
>
> *********************************
>
> public class WordPairCountKey implements
> WritableComparable<WordPairCountKey> {
>
> private String word1;
> private String word2;
>
> @Override
> public int compareTo(WordPairCountKey o) {
> int diff = word1.compareTo(o.word1);
> if (diff == 0) {
> diff = word2.compareTo(o.word2);
> }
> return diff;
> }
>
> @Override
> public int hashCode() {
> return word1.hashCode() + 31 * word2.hashCode();
> }
>
>
> public String getWord1() {
> return word1;
> }
>
> public void setWord1(String word1) {
> this.word1 = word1;
> }
>
> public String getWord2() {
> return word2;
> }
>
> public void setWord2(String word2) {
> this.word2 = word2;
> }
>
> @Override
> public void readFields(DataInput in) throws IOException {
> word1 = in.readUTF();
> word2 = in.readUTF();
> }
>
> @Override
> public void write(DataOutput out) throws IOException {
> out.writeUTF(word1);
> out.writeUTF(word2);
> }
>
>
> @Override
> public String toString() {
> return "[word1=" + word1 + ", word2=" + word2 + "]";
> }
>
> }
>
> ******************************
>
> Any help will be really appreciated.
> Thanks
> Sai
>
