Java Stack Class Cast Exception

I have to write a program that reads a postfix expression from the keyboard and store it in a stack. I keep getting a class cast exception at case "+" : and I can not figure it out. Is anyone able to help me?

    String option = (String)stack.pop();
    while( stack  != null )
    {
        switch( option )
        {
        case "+":

            int left = (Integer)stack.pop();
            int right = (Integer)stack.pop();
            int result = left + right;
            String temp = (String) stack.pop();
            stack.push(result);

            break;