<apex:page controller="calc" >
  <apex:form >
  <apex:pageBlock >
  <apex:inputText maxlength="10" value="{!result}" size="12"/>
  <apex:panelGrid title="Calculator" cellpadding="2" columns="4" border="5" bgcolor="gold">
  <apex:commandButton value="9" action="{!nine}" />
  <apex:commandButton value="8" action="{!eight}"/>
  <apex:commandButton value="7" action="{!seven}"/>
  <apex:commandButton value="/" action="{!divide}"/>
  <apex:commandButton value="6" action="{!six}"/>
  <apex:commandButton value="5" action="{!five}"/>
  <apex:commandButton value="4" action="{!four}"/>
  <apex:commandButton value="*" action="{!multiply}"/>
   <apex:commandButton value="3" action="{!three}"/>
   <apex:commandButton value="2" action="{!two}"/>
   <apex:commandButton value="1" action="{!one}"/>
   <apex:commandButton value="+" action="{!add}"/>
   <apex:commandButton value="0" action="{!zero}"/>
   <apex:commandButton value="."/>
   <apex:commandButton value="-" action="{!minus}"/>
   <apex:commandButton value="=" action="{!equal}"/>
  </apex:panelGrid>
</apex:pageBlock>
</apex:form> 
</apex:page>






public with sharing class calc {

    public PageReference equal() {
            result=string.valueOf(num);
            return null;


    }


    public String result { get; set; }

    integer num=0;
   

    public PageReference minus() {
            num-=integer.valueOf(result);      
            result=null;
            return null;

    }


    public PageReference zero() {
            if(integer.valueOf(result)==num){
            result = '0';
            return null;
            }
        else{
            result+= '0';
            return null;
            }

    }


    public PageReference add() {
       
       num+=integer.valueOf(result);
            result=null;
            return null;

    }


    public PageReference one() {
            if(integer.valueOf(result)==num){
            result = '1';
            return null;
            }
        else{
            result+='1';
            return null;
            }

    }


    public PageReference two() {
        if(integer.valueOf(result)==num){
            result = '2';
            return null;
            }
        else{
            result+='2';
            return null;
            }
    }


    public PageReference three() {
         if(integer.valueOf(result)==num){
            result = '3';
            return null;
            }
        else{
            result+='3';
            return null;
            }
    }


    public PageReference multiply() {
           num=num*integer.valueOf(result);
            result=null;
            return null;

    }


    public PageReference four() {
         if(integer.valueOf(result)==num){
            result = '4';
            return null;
            }
        else{
            result+='4';
            return null;
            }
    }


    public PageReference five() {
         if(integer.valueOf(result)==num){
            result = '5';
            return null;
            }
        else{
            result+='5';
            return null;
            }
    }


    public PageReference six() {
         if(integer.valueOf(result)==num){
            result = '6';
            return null;
            }
        else{
            result+='6';
            return null;
            }
    }


    public PageReference divide() {
           num=num/integer.valueOf(result);
            result=null;
            return null;

    }


    public PageReference seven() {
        if(integer.valueOf(result)==num){
            result = '7';
            return null;
            }
        else{
            result+='7';
            return null;
            }
    }


    public PageReference eight() {
         if(integer.valueOf(result)==num){
            result = '8';
            return null;
            }
        else{
            result+='8';
            return null;
            }
    }


    public PageReference nine() {
         if(integer.valueOf(result)==num){
            result = '9';
            return null;
            }
        else{
            result+='9';
            return null;
            }
    }

}