How Java Hadoop Mapper can send multiple values

Question

My mapper needs to send the following tuples:

And I want to send to reducer the custID as a key, and as value th

Answer 1

The simplest I can think of is just to merge them into a single string:

output.collect(custID, prodID + "," + rate);

Then, split if back up on the reducers.

If you post a little more code from your mapper maybe we could give a better example.

UPDATE: That said, you asked for the best way. The most correct way is probably to create a separate class grouping prodID and rate together and send that.

Answer 2

The best way is to write CustomWritables

This is for double value. You can change that to Text or String

import java.io.DataInput;
import java.io.DataOutput;
import java.io.IOException;
import org.apache.hadoop.io.Writable;


/**
 * @author Unmesha SreeVeni U.B
 *
 */
public class TwovalueWritable implements Writable {
    private double first;
    private double second;

    public  TwovalueWritable() {
        set(first, second);
    }
    public  TwovalueWritable(double first, double second) {
        set(first, second);
    }
    public void set(double first, double second) {
        this.first = first;
        this.second = second;
    }
    public double getFirst() {
        return first;
    }
    public double getSecond() {
        return second;
    }
    @Override
    public void write(DataOutput out) throws IOException {
        out.writeDouble(first);
        out.writeDouble(second);
    }
    @Override
    public void readFields(DataInput in) throws IOException {
        first = in.readDouble();
        second = in.readDouble();
    }

    /* (non-Javadoc)
     * @see java.lang.Object#hashCode()
     */
    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        long temp;
        temp = Double.doubleToLongBits(first);
        result = prime * result + (int) (temp ^ (temp >>> 32));
        temp = Double.doubleToLongBits(second);
        result = prime * result + (int) (temp ^ (temp >>> 32));
        return result;
    }
    /* (non-Javadoc)
     * @see java.lang.Object#equals(java.lang.Object)
     */
    @Override
    public boolean equals(Object obj) {
        if (this == obj) {
            return true;
        }
        if (obj == null) {
            return false;
        }
        if (!(obj instanceof TwovalueWritable)) {
            return false;
        }
        TwovalueWritable other = (TwovalueWritable) obj;
        if (Double.doubleToLongBits(first) != Double
                .doubleToLongBits(other.first)) {
            return false;
        }
        if (Double.doubleToLongBits(second) != Double
                .doubleToLongBits(other.second)) {
            return false;
        }
        return true;
    }
    @Override
    public String toString() {
        return first + "," + second;
    }
}

And from mapper you can just emit it as

context.write(key,new TwovalueWritable(prodID,rate));

Hope this helps.