Awesome! One question about the example you gave me.
When you say "clears the collection", the expected value should just be "B0,
B1" right? Because the collection gets cleared of the old contained value
A0...A2.
MyWritable foo = new MyWritable();
// foo contains "A0", "A1", "A2" in its namelist
foo.readFields(in);
// attempt to save the collection of Strings
Collection<String> myStrings = foo.getNameList();
// clears the collection and replaces it with "B0", "B1"
foo.readFields(in);
// Expected: "A0", "A1", "A2", "B0", "B1"
// Contains: "B0", "B1", "B0", "B1"
myStrings.addAll(foo.getNameList());
On Mon, Oct 20, 2008 at 7:23 PM, Chris Douglas <[EMAIL PROTECTED]>wrote:
>
> On Oct 20, 2008, at 6:43 PM, Yih Sun Khoo wrote:
>
> Thanks Chris and Joman for your detailed explanations.
>>
>> Would this be a good example of using a shallow copy? Also I'm trying to
>> wrap my head around why the shallow copy is needed. You mentioned it is to
>> eliminate any state from the values the list might have formerly
>> contained.
>> Could you give me an example of when the deep copy implementation would
>> have
>> been terrible?
>>
>
> MyWritable foo = new MyWritable();
>
> // foo contains "A0", "A1", "A2" in its namelist
> foo.readFields(in);
>
> // attempt to save the collection of Strings
> Collection<String> myStrings = foo.getNameList();
>
> // clears the collection and replaces it with "B0", "B1"
> foo.readFields(in);
>
> // Expected: "A0", "A1", "A2", "B0", "B1"
> // Contains: "B0", "B1", "B0", "B1"
> myStrings.addAll(foo.getNameList());
>
>
>
> @SuppressWarnings("unchecked")
>>
>> *public* ArrayList<String> getNameList() {
>>
>> *return* (ArrayList<String>) nameList.clone();
>>
>> }
>>
>
> You may not need the shallow copy, depending on how you're using the type.
> If your contract with the user is that getNameList() is invalid if
> readFields is called again, you can call that out in documentation. The more
> canonical pattern is to return an iterator, which will fail in this case
> (instead of silent, unexpected results).
>
> Also is it common to have instead used a one of the existing writables,
>> like
>> DoubleWritable, as part of my custom writable. I would imagine in the
>> readFields method I would call the DoubleWritable's readFields method.
>>
>
> There's usually no reason to allocate an object for a primitive type in a
> composite type. It doesn't buy you much, since you're already using
> DataInput and DataOutput interfaces, which define the wire format.
>
> What would go into (re)using a collection of Text instead of String? Could
>> I perhaps use an ArrayList<Text> and in the readFields and write methods
>> call each Text's readFields and write methods appropriately?
>>
>
> Depending on how your data looks, tracking the "valid" size of your
> collection inside your type may permit you to reuse Text objects instead of
> creating new, immutable Strings. Unless you start noticing it as a
> bottleneck, you're probably fine with Strings, though. -C
>
>
>
>> On Mon, Oct 20, 2008 at 11:38 AM, Chris Douglas <[EMAIL PROTECTED]
>> >wrote:
>>
>> TupleWritable is not a general-purpose type. It's used for map-side
>>> joins,
>>> where the arity of a tuple is fixed by construction. Its intent is a
>>> transient type with very, very specific applications in mind.
>>>
>>> It sounds like you don't need a general list type, as you don't need to
>>> worry about encoding the type of object your list contains. Writables are
>>> *not* supposed to read to the end of the stream they're given; they are
>>> to
>>> consume a full instance from the stream (i.e. it must consume all "its"
>>> bytes from a stream, even if it ultimately discards them). Given these
>>> constraints, Writable types of variable size almost always encode their
>>> length explicitly. As Joman mentioned, your constructor must initialize
>>> all
>>> its elements. Further, readFields must not retain any state from the
>>> value
>>> it formerly contained, so you need to clear the list before you add more
>>> values to it. This means your getNameList method will need to do a
>>> shallow
>>> copy of its elements if the caller stores a reference to the list.
>>>
>>> This should work:
>>>
>>> public void readFields(DataInput in) throws IOException {
>>> nameList.clear();
>>> score = in.readDouble();
>>> final int len = WritableUtils.readVInt(in);
>>> for (int i = 0; i < len; ++i) {
>>> nameList.add(Text.readString(in));
>>> }
>>> }
>>>
>>> public void write(DataOutput out) throws IOException {
>>> out.writeDouble(score);
>>> WritableUtils.writeVInt(out, nameList.size());
>>> for (String name : nameList) {
>>> Text.writeString(out, name);
>>> }
>>> }
>>>
>>> You can improve your performance by (re)using a collection of Text
>>> instead
>>> of String (since the latter is immutable), but that requires more work.
>>> -C
>>>
>>>
>>> On Oct 19, 2008, at 3:39 PM, Yih Sun Khoo wrote:
>>>
>>> I think when it comes to the TupleWritable being part of a custm
>>> writable,
>>>
>>>> you cannot just say tupleWritable.readFields(in) and
>>>> tupleWritable.write(out)
>>>>
>>>> I might be wrong. Has anyone successfully implemented a TupleWritable
>>>> with
>>>> ,say, a DoubleWritable in a custom writable?
>>>>
>>>> On Sun, Oct 19, 2008 at 3:33 AM, Joman Chu <[EMAIL PROTECTED]> wrote:
>>>>
>>>> hrm, try implementing the read(DataInput in) method, as well as a
>>>>
>>>>> blank constructor MyWritable() that fills dummy values into your
>>>>> instance variables. For example this should be all you need for
>>>>> read(DataInput in),
>>>>>
>>>>> public static MyWritable read(DataInput in) throws IOException {
>>>>> MyWritable w = new MyWritable();
>>>>> w.readFields(in);
>>>>> return w;
>>>>> }
>>>>>
>>>>> EDIT: I was able to sort of replicate your error. In my constructor, i
>>>>> had my instance variables assigned to null. Make sure you assign them
>>>>> to new instances of whatever Writable you are using.
>>>>>
>>>>>
>>>>> Joman Chu
>>>>> http://www.notatypewriter.com/
>>>>> AIM: ARcanUSNUMquam
>>>>>
>>>>>
>>>>>
>>>>> On Sun, Oct 19, 2008 at 5:10 AM, Yih Sun Khoo <[EMAIL PROTECTED]>
>>>>> wrote:
>>>>>
>>>>> Joman to add a little bit more to one of my previous mails about the
>>>>>> readFields methods
>>>>>>
>>>>>> Have you ever had something like this?
>>>>>>
>>>>>> public class MyWritable implements Writable {
>>>>>> private DoubleWritable doubleWritable;
>>>>>> private TupleWritable tupleWritable;
>>>>>>
>>>>>> public void readFields(DataInput in) throws IOException {
>>>>>> doubleWritable.readFields(in);
>>>>>> tupleWritable.readFields(in);
>>>>>> }
>>>>>>
>>>>>> public void write(DataOutput out) throws IOException {
>>>>>> doubleWritable.write(out);
>>>>>> tupleWritable.write(out);
>>>>>> }
>>>>>>
>>>>>>
>>>>>> }
>>>>>>
>>>>>> On Sun, Oct 19, 2008 at 1:59 AM, Joman Chu <[EMAIL PROTECTED]>
>>>>>> wrote:
>>>>>>
>>>>>> I've never used TupleWritable, so hopefully somebody else can help you
>>>>>>
>>>>>>> with that.
>>>>>>> Joman Chu
>>>>>>> http://www.notatypewriter.com/
>>>>>>> AIM: ARcanUSNUMquam
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> On Sun, Oct 19, 2008 at 4:40 AM, Yih Sun Khoo <[EMAIL PROTECTED]>
>>>>>>> wrote:
>>>>>>>
>>>>>>> Also, I've noticed TupleWritable to be quite useful.
>>>>>>>> What are good techniques for using TupleWritable in a mapping phase
>>>>>>>>
>>>>>>>> for a
>>>>>>>
>>>>>>
>>>>> "list of Text" when you do not know the size of that "list" ahead of
>>>>>>
>>>>>>>
>>>>>>>> time
>>>>>>>
>>>>>>
>>>>>
>>>>>> Say I had a custom writable which implemented TupleWritable and the
>>>>>>>>
>>>>>>>> custom
>>>>>>>
>>>>>>> writable contained a setter method
>>>>>>>> mycustomwritable.setTupleWritable( ... )
>>>>>>>>
>>>>>>>> Where the ellipsis is, there lies the TupleWritable. However I'm
>>>>>>>>
>>>>>>>> wondering
>>>>>>>
>>>>>>> since TupleWritable can be constructed using
>>>>>>>>
>>>>>>>> TupleWritable(Writable[]),
>>>>>>>
>>>>>>
>>>>> how
>>>>>>
>>>>>>>
>>>>>>> do I dynamically resize the Writable[] and add Text elements to it
>>>>>>>>
>>>>>>>> when I
>>>>>>>
>>>>>>
>>>>> don't know the size of the Writable[] very well. Does this make
>>>>>>
>>>>>>>
>>>>>>>> sense?
>>>>>>>
>>>>>>
>>>>>
>>>>>>
>>>>>>>> On Sun, Oct 19, 2008 at 1:32 AM, Yih Sun Khoo <[EMAIL PROTECTED]>
>>>>>>>>
>>>>>>>> wrote:
>>>>>>>
>>>>>>
>>>>>
>>>>>> Let's say in the reduce phase your value happens to hold an
>>>>>>>>
>>>>>>>>> ArrayListWritable
>>>>>>>>> In this example, value is of type ArrayListWritable
>>>>>>>>> Maybe I've not thought about this or done this before, but how does
>>>>>>>>>
>>>>>>>>> one
>>>>>>>>
>>>>>>>
>>>>> "read data in from the DataInput stream" in the reduce phase so that
>>>>>>
>>>>>>>
>>>>>>>>> the
>>>>>>>>
>>>>>>>
>>>>> ArrayListWritable which is a value already passed to the reducer can
>>>>>>
>>>>>>>
>>>>>>>>> be
>>>>>>>>
>>>>>>>
>>>>> used
>>>>>>
>>>>>>>
>>>>>>> as ArrayListWritable
>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> On Sun, Oct 19, 2008 at 1:25 AM, Joman Chu <[EMAIL PROTECTED]>
>>>>>>>>>
>>>>>>>>> wrote:
>>>>>>>>
>>>>>>>
>>>>>
>>>>>> Since the ArrayListWritable extends ArrayList, you have access to
>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> all
>>>>>>>>>
>>>>>>>>
>>>>> the ArrayList methods as well. Once you read data in from the
>>>>>>
>>>>>>> DataInput stream, you should be able to use ArrayListWritable just
>>>>>>>>>> like a regular ArrayList.
>>>>>>>>>> Joman Chu
>>>>>>>>>> http://www.notatypewriter.com/
>>>>>>>>>> AIM: ARcanUSNUMquam
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> On Sun, Oct 19, 2008 at 4:16 AM, Yih Sun Khoo <[EMAIL PROTECTED]>
>>>>>>>>>>
>>>>>>>>>> wrote:
>>>>>>>>>
>>>>>>>>
>>>>>>> Hmm, what method from ArrayListWritable allows you to access the
>>>>>>>>
>>>>>>>>>
>>>>>>>>>>> different
>>>>>>>>>>
>>>>>>>>>> elements of the ArrayList? Would it be readFields? for example,
>>>>>>>>>>>
>>>>>>>>>>> in
>>>>>>>>>>
>>>>>>>>>
>>>>> a
>>>>>>
>>>>>>>
>>>>>>> reduce phase, if I needed to know the size of the array list, it
>>>>>>>>
>>>>>>>>>
>>>>>>>>>>> would
>>>>>>>>>>
>>>>>>>>>
>>>>>>> be
>>>>>>>>
>>>>>>>>>
>>>>>>>>>> easy if i were dealing with an arraylist because i could just say
>>>>>>>>>>> arraylist.size. How would i accomplish that with the writable
>>>>>>>>>>>
>>>>>>>>>>> counterpart?
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>> On Sun, Oct 19, 2008 at 1:04 AM, Joman Chu <[EMAIL PROTECTED]>
>>>>>>>>>>>
>>>>>>>>>>> wrote:
>>>>>>>>>>
>>>>>>>>>
>>>>>>>
>>>>>>>> Hi,
>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> For the ArrayList object, try taking a look at the
>>>>>>>>>>>> implementation
>>>>>>>>>>>>
>>>>>>>>>>>> of
>>>>>>>>>>>
>>>>>>>>>>
>>>>> ArrayListWritable by Jimmy Lin at UMD here:
>>>>>>
>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>
>>>>> https://subversion.umiacs.umd.edu/umd-hadoop/core/trunk/src/edu/umd/cloud9/io/ArrayListWritable.java
>>>>>
>>>>>
>>>>>> But basically in the readFields methods, I prefer using each
>>>>>>>>>>>>
>>>>>>>>>>>> Writable
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>> object's readFields method to read the data in. For example, for
>>>>>>>>
>>>>>>>>>
>>>>>>>>>>>> your
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>> double variable, I would use a DoubleWritable object and in the
>>>>>>>>
>>>>>>>>> MyWritable.readFields(DataInput in), I would use
>>>>>>>>>>>> nameofdoublewritable.readFields(in). For the
>>>>>>>>>>>> MyWritable.write(DataOutput out) method, I would use
>>>>>>>>>>>> nameofdoublewritable.write(out).
>>>>>>>>>>>>
>>>>>>>>>>>> Have a good one,
>>>>>>>>>>>>
>>>>>>>>>>>> Joman Chu
>>>>>>>>>>>> http://www.notatypewriter.com/
>>>>>>>>>>>> AIM: ARcanUSNUMquam
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> On Sun, Oct 19, 2008 at 3:30 AM, Yih Sun Khoo <[EMAIL PROTECTED]
>>>>>>>>>>>> >
>>>>>>>>>>>>
>>>>>>>>>>>> wrote:
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> I don't quite know how to write the read and write functions,
>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>>> but
>>>>>>>>>>>>
>>>>>>>>>>>
>>>>> I
>>>>>>
>>>>>>>
>>>>>>> want
>>>>>>>>
>>>>>>>>>
>>>>>>>>>> to
>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> write my own writable, which should have a
>>>>>>>>>>>>>
>>>>>>>>>>>>> DoubleWritable/double
>>>>>>>>>>>>
>>>>>>>>>>>
>>>>> value
>>>>>>
>>>>>>>
>>>>>>>>>> followed by a list of Strings/Text. This Writable will be used
>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>>> as
>>>>>>>>>>>>
>>>>>>>>>>>
>>>>> a
>>>>>>
>>>>>>>
>>>>>>> value.
>>>>>>>>
>>>>>>>>>
>>>>>>>>>>>> Is the code below the best way to go about writing such a
>>>>>>>>>>>>>
>>>>>>>>>>>>> writable?
>>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>
>>>>>>>> import java.io.DataInput;
>>>>>>>>>>>>> import java.io.DataOutput;
>>>>>>>>>>>>> import java.io.EOFException;
>>>>>>>>>>>>> import java.io.IOException;
>>>>>>>>>>>>> import java.util.ArrayList;
>>>>>>>>>>>>>
>>>>>>>>>>>>> import org.apache.hadoop.io.Writable;
>>>>>>>>>>>>>
>>>>>>>>>>>>> public class MyWritable implements Writable {
>>>>>>>>>>>>> private double score;
>>>>>>>>>>>>> private ArrayList<String> nameList;
>>>>>>>>>>>>>
>>>>>>>>>>>>> public void setScore(double score) {
>>>>>>>>>>>>> this.score= score;
>>>>>>>>>>>>> }
>>>>>>>>>>>>>
>>>>>>>>>>>>> public void setNameList(ArrayList<String> nameList) {
>>>>>>>>>>>>> this.nameList= nameList;
>>>>>>>>>>>>> }
>>>>>>>>>>>>>
>>>>>>>>>>>>> public double getScore() {
>>>>>>>>>>>>> return score;
>>>>>>>>>>>>> }
>>>>>>>>>>>>>
>>>>>>>>>>>>> public ArrayList<String> getNameList() {
>>>>>>>>>>>>> return nameList;
>>>>>>>>>>>>> }
>>>>>>>>>>>>>
>>>>>>>>>>>>> public void readFields(DataInput in) throws IOException {
>>>>>>>>>>>>> score= in.readDouble();
>>>>>>>>>>>>> try {
>>>>>>>>>>>>> do {
>>>>>>>>>>>>> nameList.add(in.readUTF());
>>>>>>>>>>>>> } while (true);
>>>>>>>>>>>>> } catch (EOFException eofe) {
>>>>>>>>>>>>> // continue; done
>>>>>>>>>>>>> }
>>>>>>>>>>>>> }
>>>>>>>>>>>>>
>>>>>>>>>>>>> public void write(DataOutput out) throws IOException {
>>>>>>>>>>>>> out.writeDouble(score);
>>>>>>>>>>>>> for (String name: nameList) {
>>>>>>>>>>>>> out.writeUTF(name);
>>>>>>>>>>>>> }
>>>>>>>>>>>>> }
>>>>>>>>>>>>> }
>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>
>
