Is there a way to overload the Map's toString() function?

Question

Hello everyone. I was wondering if its possible to overload the toString() function of the Map datastructure supplied by java.
I looked it up and it says:
public class HashMapextends AbstractMapSo I looked up AbstractMap and it says:
public abstract class AbstractMapextends Objectimplements MapAnd I found a toString() function in the AbstractMap saying:
toString
public String toString() Returns a string representation of this map. The string representation consists of a list of key-value mappings in the order returned by the map's entrySet view's iterator, enclosed in braces ("{}"). Adjacent mappings are separated by the characters ", " (comma and space). Each key-value mapping is rendered as the key followed by an equals sign ("=") followed by the associated value. Keys and values are converted to strings as by String.valueOf(Object). This implementation creates an empty string buffer, appends a left brace, and iterates over the map's entrySet view, appending the string representation of each map.entry in turn. After appending each entry except the last, the string ", " is appended. Finally a right brace is appended. A string is obtained from the stringbuffer, and returned.
here: http://java.sun.com/j2se/1.3/docs/a...ml#toString()So I did the following:
package parse;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.io.*;
//this class should store all the Messages or "Events" and
//you can access them based on their EntityID key.
public class MessageDB extends HashMap
     //database to hold the information
     //     holds the Alerts/messages
     public static Map<Integer, List<String>> AlertMap;
     //Constructor
     MessageDB()
          AlertMap = new HashMap<Integer, List<String>>();
     public String toString()
          return ( )//not sure what to put here...
     //print, outputs the contents of the hashMap
     public static void print()
        //want to print out the Key and all the Messages
          //associated with that key
          Set keys = AlertMap.keySet();          // The set of keys in the map.
          Iterator keyIter = keys.iterator();
          System.out.println("The map contains the following associations:");
          while (keyIter.hasNext()) {
                Object key = keyIter.next();  // Get the next key.
                Object value = AlertMap.get(key);  // Get the value for that key.
                System.out.println( "EntityID: " + key + "\n"
                              + "Message: " + value + "\n" );
     //overloaded print to print to user screen.
     public static void print(PrintWriter out)
          //want to print out the Key and all the Messages
          //associated with that key
          Set keys = AlertMap.keySet();          // The set of keys in the map.
          Iterator keyIter = keys.iterator();
          out.println("The map contains the following associations:");
          out.flush();
          while (keyIter.hasNext()) {
                Object key = keyIter.next();  // Get the next key.
                Object value = AlertMap.get(key);  // Get the value for that key.
               //out.flush();
               /* out.println( "   (" + key + "," + value + ")" );
               out.flush();*/
               // out.println("------------------\n");
               out.println("EntityID: " + key + "\n"
                              + "Message: " + value + "\n");
               //out.println("------------------\n");
               out.flush();
     void add(Message msg)
          //getting the position of the List by EntityID if avaiable
          List<String> Alert = AlertMap.get(msg.entityID);
          //checking to see if there is a unique Key already in the Map.
          if (Alert == null)
               //if there isnt a key in the map, add a new key, and a new List mapping
               //to the key EntityID;
                 Alert = new ArrayList<String>();
                 AlertMap.put(msg.entityID, Alert);
                 Alert.add(msg.message);
          else
          //adding message to List
               Alert.add(msg.message);
}Right now the output is like this:
The map contains the following associations:
EntityID: 99999
                  Message: [a test ]
EntityID: 800
                Message: [this is a test , again a test ]
EntityID: 801
                Message: [again a test ]
EntityID: 111
                Message: [a test again yes , a test once again ]You see how its automatically doing [message1, message2,...,message x]
By me calling this line of code:
out.println("EntityID: " + key + "\n"
                              + "Message: " + value + "\n");Because I found out it implicity calls the toString() method when concatinating it to a string.
What I would like it to display would be:
EntityID: 800
Message:
This is a test
This is a test again
any ideas would be great!
Or is there a way to just iterate over the messages, rather than doing it my way?
Message was edited by:
lokie

Answer 1

Hi,
When you do a String concatenation in java, calls are implicitly done to #toString().
Means, you are doing this:
while (keyIter.hasNext()) {
                Object key = keyIter.next();  /
                Object value = AlertMap.get(key); 
                System.out.println( "EntityID: " + key.toString()+ "\n"
                              + "Message: " + value.toString() + "\n" );
}The "[ [/b]... [b], ... , ... ]" you can see in your output is created by the implicit call to List#toString() method.
You can Override it in an anonymous class by doing:
if (Alert == null)
               //if there isnt a key in the map, add a new key, and a new List mapping
               //to the key EntityID;
                 Alert = new ArrayList<String>(){
                          @Override
                           public String toString(){
                            StringBuilder sb = new StringBuilder();
                   for(String s: this){
                    sb.append(s);
                    sb.append("\n");
                           return sb.toString();
                         };PS:
toString() is usually used for debbuging only...