Problem with NumberFormatException in calculator

Hi I am doing a calculator program in lwuit like the one that v have in our windows calculator .........................I have some problems in my code......... I have did the initial coding for forms buttons and have even made the operators -,+,*,/ work but the problem now I am facing is continues pressing of any operator I am not getting the result as u get in win calculator ,so results cum only if = button I have added a if condition for = wherein if it is pressed the value will be shown now I have added a or operator to it

if (operand.equals("=")|| (operand.equals(operation))) and while executing I get a java.lang.NumberFormatException but continuing the execption I am able get the result when any operator is pressed so can u help me in solving me this execption ..........................I have one more problem say if I calculate 1+1+1 the result that cums is 13 ie it takes the previous value entered also so for this also I need a solution if u have any suggestion please feel free and tell me I would be pleased................

:) Thank you