Re: Using my own classes as keys and values

[Sagar Naik]

Hey Steve,
WritableComparable should be implemented by classes meant as Keys
Writable should be implemented by classes meant as Values
The class as Keys need comparing ability because they undergo sorting
The keys undergo sorting , hence they need a comparison


Steve Schlosser wrote:

Hello all

Is there a best practice for using my own classes as keys and values?

My first attempt at doing this was successful - I built a
BigIntegerWritable class using IntWritable as a template.  It was easy
because BigInteger has methods converting to and from byte arrays,
which I could then write into the DataOutput or read from the
DataInput.

It seems like I should be able to use object serialization to write
to/read from the DataOutput/Input objects and make my own classes
implement the Writable interface.  It seems like I should be able to
do something like this:

import java.io.*;

import org.apache.hadoop.io.*;

public class Sample implements Writable {

    Address address;
    SampleValue value;         // sampled value at this point

    public Sample(Address a, SampleValue v) {
        address = a;
        value = v;
    }

    public SampleValue getValue()  { return value;}
    public Address getAddress() { return address; }

    public String toString () {
        return (address.toString() + " " + value.toString());
    }

[...]

    public void readFields(DataInput in) throws IOException {
        ObjectInputStream oin = new ObjectInputStream((DataInputBuffer)in);

        try {
            address = (Address)oin.readObject();
            value = (SampleValue)oin.readObject();
        } catch (ClassNotFoundException e) {
            throw new IOException(e.toString());
        }

    }

    public void write(DataOutput out) throws IOException {
        ObjectOutputStream oout = new ObjectOutputStream((DataOutputBuffer)out);

        oout.writeObject(address);
        oout.writeObject(value);
    }
}

This code compiles, but throws exceptions at runtime, complaining that
WritableComparator can not access a member of class Sample with
modifiers "".  Can someone tell me what this exception is talking
about?

Do I need to implement a WritableComparator for each class that I want
to implement Writable?

Thanks again for the help.

-steve

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