public class ReversePolishEvaluator implements ExpressionEvaluator
{
	
	public int evaluate(ExpressionTokenizer et) throws TokenException
	{
		int first = 0;
		int second = 0;

		if(et.nextTokenType() != ExpressionTokenizer.TokenType.OP)
		{
			
			// label...
			// call evaluate on label
			
			// grabs first integer
			first = et.nextInteger();

			
			if(et.nextTokenType() == ExpressionTokenizer.TokenType.OP)
				return first;
		
			
			while(et.hasNextToken())
			{		
				// gets results of computation
				second = evaluate(et);

				switch(et.nextOperator())
				{
					case '+':
						first += second;
						break;
	
					case '-':
						first -= second;
						break;
				
					case '*':
						first *= second;
						break;
				
					case '/':
						first /= second;
						break;
				}
				
				if(et.nextTokenType() == ExpressionTokenizer.TokenType.OP) return first;
			}
		}
		return first;
		
	}
	
}