Map Reduce Programming: MultiMap overwrites existing key values - join

Question

I'm doing Join algorithm in MapReduce. In the Map phase, I made joinColumn as key and the tuple as value. In the reduce method, I have keys and values as (columnname, row). In the reduce phase, I need to separate the "row" into two based on which table they belong to.
I used MultiMap to do this. But the MultiMap is overwriting the existing value. To try to overcome this, I override "equals" and "hashcode" but this did not fix the problem.
public void reduce(Text key,Iterable<Text> values,Context context) throws IOException, InterruptedException{
Multimap<String,Table> entry=LinkedListMultimap.create();
for(Text val : values){
String[] row=val.toString().split(",");
Table t = new Table();
t.setTablename(row[0]);
t.setColumns(val);
entry.put(row[0],t);
}
for (String k: entry.keySet()){
System.out.println("Key : "+k);
Collection<Table> rows=entry.get(k);
Iterator<Table> i=rows.iterator();
while(i.hasNext()){
Table t=i.next();
System.out.println(t.getColumns());
}
}
public class Table {
private String tablename;
private Text columns;
public String getTablename() {
return tablename;
}
public void setTablename(String tablename) {
this.tablename = tablename;
}
public Text getColumns() {
return columns;
}
public void setColumns(Text columns) {
this.columns = columns;
}
#Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((columns == null) ? 0 : columns.hashCode());
result = prime * result
+ ((tablename == null) ? 0 : tablename.hashCode());
return result;
}
#Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
Table other = (Table) obj;
if (columns == null) {
if (other.columns != null)
return false;
} else if (!columns.equals(other.columns))
return false;
if (tablename == null) {
if (other.tablename != null)
return false;
} else if (!tablename.equals(other.tablename))
return false;
return true;
}
}
I'm getting the following output:
Key : S
R, 2, Don, Larson, Newark, 555-3221
R, 2, Don, Larson, Newark, 555-3221
Key : R
R, 2, Don, Larson, Newark, 555-3221
Key : S
R, 3, Sal, Maglite, Nutley, 555-6905
R, 3, Sal, Maglite, Nutley, 555-6905
Key : R
R, 3, Sal, Maglite, Nutley, 555-6905
Key : R
S, 4, 22000, 7000, part1
Key : S
S, 4, 22000, 7000, part1
It is overriding the existing values. Can anyone help me to sort out this problem?

Answer 1

Your problem is that the object returned by iterating over values is reused by the iterator. Instead of just assigning the value in setColumns(), you need to copy it. Something like:
public void setColumns(Text columns) {
this.columns = new Text(columns.toString());
}