Salesforce - Display data from Custom object to PageBlockTable using Apex

Here i have provided code sample to understand, how to display data from custom objects to PageBlockTable. I have chosen pageBlocktable because it has predefined style. Don't want to use any css styles for that.

Apex Design Code:

 <apex:page controller="ViewUserController">  
   <apex:form >  
     <apex:sectionHeader subtitle="All Users" title="Users" />  
     <apex:pageBlock >  
       <apex:pageBlockTable value="{!allusers}" var="a">  
         <apex:column headervalue="Name" value="{!a.Name__c}" />  
         <apex:column headervalue="Age" value="{!a.Age__c}" />  
         <apex:column headervalue="Class" value="{!a.Class__c}" />  
       </apex:pageBlockTable>  
     </apex:pageBlock>  
   </apex:form>  
 </apex:page>  

Apex Controller Code:

 
 public class ViewUserController  
 {  
     public List<User__c> allusers;    
     public List<User__c> getAllUsers()  
     {  
         if(allusers==null)  
         {  
             allusers=[SELECT Name__c,Age__c,Class__c from User__c];  
         }  

         return allusers;  
     }  
 }