source: sample/hadoop-0.17/tw/org/nchc/tuple/HashMapWritable.java

package tw.org.nchc.tuple;

import java.io.DataInput;
import java.io.DataOutput;
import java.io.IOException;
import java.util.Map;
import java.util.Set;
import java.util.HashMap;


import org.apache.hadoop.io.Writable;

public class HashMapWritable<K extends Writable, V extends Writable> extends HashMap<K, V> implements
        Writable {

  /**
     * 
     */
    private static final long serialVersionUID = 1L;

    /**
   * Creates a HashMapWritable object.
   */
  public HashMapWritable() {
    super();
  }
  
  /**
   * Creates a HashMapWritable object from a regular HashMap.
   */
  public HashMapWritable(HashMap<K, V> map) {
    super(map);
  }

  /**
   * Deserializes the array.
   * 
   * @param in
   *            source for raw byte representation
   */
  public void readFields(DataInput in) throws IOException {
    
    this.clear();

    int numEntries = in.readInt();
    if(numEntries==0) return;
    
    String keyClassName = in.readUTF();
    String valueClassName = in.readUTF();
    
    K objK;
    V objV;
    try {
      Class keyClass = Class.forName(keyClassName);
      Class valueClass = Class.forName(valueClassName);
      for (int i = 0; i < numEntries; i++) {
        objK = (K) keyClass.newInstance();
        objK.readFields(in);
        objV = (V) valueClass.newInstance();
        objV.readFields(in);
        put(objK, objV);
      }

    } catch (ClassNotFoundException e) {
      e.printStackTrace();
    } catch (IllegalAccessException e) {
      e.printStackTrace();
    } catch (InstantiationException e) {
      e.printStackTrace();
    }
    
  }

  /**
   * Serializes this array.
   * 
   * @param out
   *            where to write the raw byte representation
   */
  public void write(DataOutput out) throws IOException {
    // Write out the number of entries in the map
      out.writeInt(size());
      if(size()==0) return;
      
      // Write out the class names for keys and values
      // assuming that data is homogeneuos (i.e., all entries have same types)
      Set<Map.Entry<K, V>> entries = entrySet();
      Map.Entry<K, V> first = entries.iterator().next();
      K objK = first.getKey();
      V objV = first.getValue();
      out.writeUTF(objK.getClass().getCanonicalName());
      out.writeUTF(objV.getClass().getCanonicalName());

      // Then write out each key/value pair
      for (Map.Entry<K, V> e: entrySet()) {
        e.getKey().write(out);
        e.getValue().write(out);
      }
  }

}