Pass input date values to controller

Question

I am unable to pass date values to constructor :Attempt to deference null object: Vf page:

 <apex:pageBlockSectionItem > 
     <apex:outputLabel value="From Date" />
     <apex:actionRegion > 
        **<apex:inputField value="{!opp.From_Date__c}">** 
            <apex:actionSupport event="onchange"
                reRender="ajax" />
          </apex:inputField> 
     </apex:actionRegion>
 </apex:pageBlockSectionItem>

Controller:

 public Opportunity opp{get;set;}//property
 public controller_VfProdPage(ApexPages.StandardController controller) 
 {
    opp = new Opportunity(); 
    startMonth =  opp.From_Date__c.month();
    numMonth= new list<integer>();
    Integer monthBtwn = opp.From_Date__c.monthsBetween(opp.To_Date__c);    
}

Answer 1

<apex:pageBlockSectionItem > <apex:outputLabel value="From Date" />
     <apex:actionRegion > **<apex:inputField value="{!opp.From_Date__c}">** 
      <apex:actionSupport event="onchange" action="{!priValue}" 
                            rerender="none"
      reRender="ajax" /></apex:inputField> </apex:actionRegion>
     </apex:pageBlockSectionItem> 

Controller:

    public controller_VfProdPage(ApexPages.StandardController controller) 
         {
         opp = new Opportunity(); 
        startMonth =  opp.From_Date__c.month()!=null?opp.From_Date__c.month():Date.today();  //add default value
  numMonth= new list<integer>();
  Integer monthBtwn = opp.From_Date__c!=null && opp.To_Date__c!=null?opp.From_Date__c.monthsBetween(opp.To_Date__c): 0;

        }

    public void priValue() {
         system.debug('----------------->'+opp.From_Date__c);
       startMonth =  opp.From_Date__c.month();
       numMonth= new list<integer>();
       Integer monthBtwn = opp.From_Date__c.monthsBetween(opp.To_Date__c);
    }

in your constructor opp.From_Date__c is null and you are trying to use it. First give it any value. Same with the opp.From_Date__c.monthsBetween(opp.To_Date__c); validate that these are not null.

Answer 2

When you are calling opp.From_Date__c.month(); you are calling .month() on null so you need to first check for null value in following way

public controller_VfProdPage(ApexPages.StandardController controller) 
 {
 opp = new Opportunity(); 
 startMonth =  opp.From_Date__c.month()!=null?opp.From_Date__c.month():null;
  numMonth= new list<integer>();
  Integer monthBtwn = opp.From_Date__c!=null && opp.To_Date__c!=null?opp.From_Date__c.monthsBetween(opp.To_Date__c): 0;

}