java: java.util.Hashmap doesn't work for a custom object

kpachopoulos · 04-15-2007, 12:54 PM

Hi,
i have created a class named ID. I am trying to test it on a Hashmap, but the the "containsKey" and "get" operations do not work. Every operation of the main method returns as if the ID objects have not been inserted. Ideas?

Code:

package swarm.sys.id;


import java.util.*;
import java.io.*;

import swarm.sys.common.*;
import swarm.sys.interfaces.*;

public class ID implements IDInterface, Cloneable, Serializable
{
    String type;
    int body;
    String id;
    private static int idCounter=0;

    public ID(String type, int body)
    { 
      this.type=type;
      this.body=body;
      id=type+Ids.padWithZeroes(body);
    }


    public String getType()
    {
      return type;
    }//getType

    public String getBody()
    {
      return Ids.padWithZeroes(body);
    }//getBody

    public int getIntBody()
    {
      return body;
    }//getIntBody


    public boolean equals(Object o)
    {        
      if (!(o instanceof ID))
      {
        return false;
      }
      else
      {
          ID id=(ID)o;    

          return (type.equals(id.getType()) &&  (body==id.getIntBody()));         
      }            
    }//equals

    
    /* factory method, which produces new IDs and afterwards 
     * increments the body part
     */
    public static ID newID(String type)
    {
        return new ID(type,idCounter++);
    }//newID
    

    //implements deep cloning
    public Object clone()
    {
        return new ID(new String(this.type),this.body);
    }//clone


    public String toString()
    {
      return id.trim();
    }//toString

    
    public static void main(String[] args)
    {
        Hashtable<IDInterface,String> table=new Hashtable<IDInterface,String>();
        table.put(new ID("PEER_",1),"x1");
        table.put(new ID("PEER_",2),"x1");
        table.put(new ID("PEER_",3),"x2");
        table.put(new ID("PEER_",4),"x3");
        table.put(new ID("PEER_",5),"x3");
        
        ID id1=new ID("PEER_",1);
        ID id14=new ID("PEER_",14);        
        
                
        System.out.println("table.containsKey(new ID(PEER_,1)): "+table.containsKey(id1)); //should be true
        System.out.println("table.containsKey(new ID(PEER_,14)): "+table.containsKey(id14)); //should be false
        System.out.println("table.get(new ID(PEER_,1))"+table.get(id1)); //should be x1
    }

kpachopoulos · 04-15-2007, 01:48 PM

The problem would be solved if the ids that we search for and the ids used as keys inside the hashtable were references to each other and not different objects, ie:

Code:

    public static void main(String[] args)
    {
        ID id1=new ID("PEER_",1);
        ID id14=new ID("PEER_",14);        
        
        Hashtable<IDInterface,String> table=new Hashtable<IDInterface,String>();
        //table.put(new ID("PEER_",1),"x1");
        table.put(id1,"x1");
        table.put(new ID("PEER_",2),"x1");
        table.put(new ID("PEER_",3),"x2");
        table.put(new ID("PEER_",4),"x3");
        table.put(new ID("PEER_",5),"x3");
        

                
        System.out.println("table.containsKey(id1): "+table.containsKey(id1)); //should be true
        System.out.println("table.containsKey(new ID(PEER_,14)): "+table.containsKey(id14)); //should be false
        System.out.println("table.get(new ID(PEER_,1))"+table.get(id1)); //should be x1
    }

I think i have to implement an equals method somewhere. I have implemented it inside the ID class, but the Hashtable obviously doesn't use it... Can somebody explain how should i do it?

taylor_venable · 04-15-2007, 03:25 PM

You need to implement the hashCode :: int method in your class. Hashes are computed based on this value, not the value of equals :: bool.