How to show custom message when reloading the page

Question

I have a custom VF page. When the page is refreshed, i need to display custom message in page "There is no record to save". when i am clicking save button message will be displaying. But i need without clicking the save button i want to show custom message. Can anyone help? Thanks

Page:

<apex:page standardController="Account" extensions="myaccountcontroller">
<apex:form >
<apex:pagemessages ></apex:pagemessages>
    <apex:commandButton value="Save" action="{!Save}"/>
</apex:form>
</apex:page>

Controller:

public class myaccountcontroller
{
  public string accId {get;set;}
  public String finalamount {get;set;}

  public myaccountcontroller(ApexPages.StandardController sc)
  {
    accId = ApexPages.CurrentPage().getParameters().get('Id');

    doCalculation();
  }

  public void doCalculation()
  {
    Account AccObj = [SELECT Id,Name,Amount__c,Total_Amount__c FROM Account WHERE Id=:accId ];
    system.debug('TotalAmount ###:'+AccObj.Total_Amount__c );
   try{
        finalamount  = String.ValueOf(AccObj.Amount__c/0);
        system.debug('finalamount###:'+finalamount);
     }catch(Exception e)
     {
        if(e.getMessage() == 'System.NullPointerException: Attempt to de-reference a null object')
        {

          ApexPages.addmessage(new ApexPages.message(ApexPages.severity.ERROR,'Unite Price should not be empty'));
        }
     }
  }


}

Answer 1

You can use apex:pageMessage to display message on page load and then on button click you can hide this message.

<apex:page standardController="Account" extensions="myaccountcontroller">
<apex:form >
<apex:pageMessage summary="There is no record to save." severity="info" strength="3" rendered="{!messageflag}"/>

and in controller

public class myaccountcontroller
{
  public string accId {get;set;}
  public String finalamount {get;set;}
  public boolean messageflag {get;set;}  //declare the variable.

  public myaccountcontroller(ApexPages.StandardController sc)
  {
    accId = ApexPages.CurrentPage().getParameters().get('Id');
     messageflag  = true;
    doCalculation();
  }

  public void doCalculation()
  {
        messageflag  = false;