Simple Caluclator coding for Beginers

Visual Force Page:

<apex:page controller="Calculator">
<apex:form >
 
    <apex:pageBlock title="Simple caluclator">
        <apex:pageBlockSection columns="1">
            <apex:pageBlockSectionItem >
Please Give First Number<apex:inputText value="{!number1}"/>
 </apex:pageBlockSectionItem>
 <apex:pageBlockSectionItem >
Please Give Second Number<apex:inputText value="{!number2}"/>
</apex:pageBlockSectionItem>
<apex:pageBlockSectionItem >
Please Provide The Operator <apex:inputText value="{!operator}"/>
</apex:pageBlockSectionItem>
</apex:pageBlockSection>
<apex:pageBlockButtons location="bottom">
<apex:commandButton value="ShowResult" action="{!calculation}" ></apex:commandbutton>
<apex:commandButton value="Refresh" action="{!Refresh}" rendered="{!flag}" ></apex:commandbutton>
 </apex:pageBlockButtons>

 <apex:pageBlockSection rendered="{!flag}">
 <apex:pageblockSectionItem >

 Result <apex:outputText value="{!result}"  >


 </apex:outputText>

 </apex:pageblockSectionItem>
  </apex:pageBlockSection>



 </apex:pageBlock>
</apex:form>
</apex:page>


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Controller
-------------------------------------------------------------------------------------------------------------------------


public class Calculator
{


 public boolean flag { get; set; }
public integer number1{get;set;}
public integer number2{get;set;}
public string operator{get;set;}
public double result{get;set;}


public Calculator()
{
flag=false;
}
public void calculation()
{
flag=true;
if(operator.contains('+'))
result=number1+number2;
if(operator.contains('-'))
result=number1-number2;
if(operator.contains('*'))
result=number1*number2;
if(operator.contains('%'))
result=number1/number2;
}
public void Refresh()
{
flag=false;
number1=null;
number2=null;
operator=null;


}

}