function Collection()
{
	var collection= {};
	var order = [];
		
	this.add = function( property, value )
	{
		if( !this.exists(property) )
		{
			collection[property] = value;
			order.push(property);
		}
	}
	
	this.remove = function( property )
	{
		collection[property] = null;
		var ii = order.length;
		while( ii-- > 0 )
		{
			if( order[ii] == property )
			{
				order[ii] = null;
				break;
			}
		}
	}
	
	this.toString = function()
	{
		var output = [];
		for( var ii=0;ii < order.length; ++ii )
		{
			if( order[ii] != null )			
			{
				output.push( "[" + order[ii] + "=" + collection[order[ii]] + "]" );
			}
		}
		return output;
	}
	
	this.getKeys = function( )
	{
		var keys = [];
		for( var ii=0;ii < order.length; ++ii )		
		{
			if( order[ii] != null )
			{
				keys.push( order[ii] );
			}
		}
		return keys;
	}
	
	this.update = function(property, value)
	{
		if( value != null )
		{
			collection[property] = value;
		}
		var ii= order.length;
		while( ii-- > 0 )		
		{
			if( order[ii] == property )
			{
				order[ii] = null;
				order.push( property );
				break;
			}
		}
	}
	
	this.exists = function(property)
	{
		return collection[property] != null;
	}
}