Question

I need to write a binary tree to HDFS, which i will use to represent a desicion tree. But in order to do that i first need to create a BinaryTreeNode class, which will be the tree node. These are my class attributes:

private String name;
private String attribute;
private String attType;
private String condition;
private String lines;
private BinaryTreeNode leftChild;
private BinaryTreeNode rightChild;

So now i need to implement the write and readFields methods for reading and writing these nodes. These are what i have done:

public void write(DataOutput d) throws IOException 
{
    d.writeUTF(name);
    d.writeUTF(attribute);
    d.writeUTF(attType);
    d.writeUTF(condition);
    d.writeUTF(lines);
    //SOMETHIN FOR LEFT AND RIGHT CHILD
}

public void readFields(DataInput di) throws IOException 
{
    name=di.readUTF();
    attribute=di.readUTF();
    attType=di.readUTF();
    condition=di.readUTF();
    lines=di.readUTF();
    //SOMETHIN FOR LEFT AND RIGHT CHILD
}

BinaryTreeNode read(DataInput in) throws IOException
{
     BinaryTreeNode ob = new BinaryTreeNode();
     ob.readFields(in);
     return ob;
}

What i cant think of is how to write and read my 2 child nodes. Note that the tree is going to be build recursively and every node will have 0-2 childs. So my later purpose is to have a BinaryTree class that will have the attribute BinaryTreeNode root. Thanks

Answer 1

Ok i found the solution. I dont know if it is efficient enough but it works. Here is what i did. These are in my class definition:

    public void write(DataOutput d) throws IOException
    {
      d.writeUTF(name);
      d.writeUTF(attribute);
      d.writeUTF(attType);
      d.writeUTF(condition);
      d.writeUTF(lines);
      ObjectWritable left=new ObjectWritable(BinaryTreeNode.class,leftChild);
      left.write(d);
      ObjectWritable right=new ObjectWritable(BinaryTreeNode.class,rightChild);
      right.write(d);
    }

    public void readFields(DataInput di) throws IOException
    {
      name=di.readUTF();
      attribute=di.readUTF();
      attType=di.readUTF();
      condition=di.readUTF();
      lines=di.readUTF();
      leftChild=(BinaryTreeNode) ObjectWritable.readObject(di, new Configuration());
      rightChild=(BinaryTreeNode) ObjectWritable.readObject(di, new Configuration());

    }

And this is how i use them:

For writing:

      BinaryTreeNode bla=new BinaryTreeNode();
      //set the attributes
      ObjectWritable obj=new ObjectWritable(BinaryTreeNode.class,bla);
      obj.write(dos);

For reading:

      BinaryTreeNode bla=new BinaryTreeNode();
      bla= (BinaryTreeNode) ObjectWritable.readObject(in, conf);

This works fine. Now i am able to create my decision tree and store it just by storing my root. 😀