How to show custom object fields in standard visual force page

Question

i am trying to add one button in lead detail page layout.on clicking on this button i want show a visual force page showing custom object details.my custom object is tractor__c.so far i am trying this code

<apex:page docType="html-5.0" standardController="Lead" extensions="LeadTractorExtension">
<style>
</style>
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css"/>
  <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
  <script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js"></script>



    <div class="row">
        <div class="col-sm-12">
            <div class="form-group">
            <h3>Search for Tractor</h3>
            </div>
            <div class="col-sm-12 form-group">
                <apex:form >
                <apex:pageBlock >



    <div class="col-sm-3">
        <label>Tractor Make</label>

        <apex:inputField value="{!tractor .Manufacturer__c}"/>
     </div> 
     <div class="col-sm-3">
        <label>Tractor Model</label>

         <apex:inputField value="{!tractor.Model_Name__c }"/>
     </div>
     <div class="col-sm-3">
        <label>Deck Size</label>

         <apex:inputField value="{!tractor.Deck__c }"/>
    </div>
    <div class="col-sm-3">
        <label>Rake Model</label>

         <apex:inputField value="{!tractor.Rake_Model__c }"/>
    </div>


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

        </div>  

            <div class="row">
                <div class="col-sm-offset-4 ">
            <button>Check For Availebility</button>
            </div>
            </div>
    </div>
  </div>

and for my extension class is like

public class LeadTractorExtension {
public  Tractor__c tractor{get;set;}



public LeadTractorExtension(ApexPages.StandardController stdController) {
    this.tractor = (Tractor__c)stdController.getRecord();
   getTractorDetails(); 
}
public Tractor__c getTractorDetails(){
    return tractor;
}

} i am getting error like Invalid conversion from runtime type Lead to Tractor__c .if i change standard controller name to tractor__c .page will work .but this page wont list in lead page layout.In Lead object TractorInfo__c is the lookup field.can any one help to solve this issue

Answer 1

standardController="Lead" means that what you get from stdController.getRecord() will also be a Lead. You cannot cast one type of sObject into another type, which is what causes the runtime error you experienced.

You probably meant to create a new record:

public Tractor__c tractor { get; set; }
// we use this in case we need the lead record later.
private ApexPages.StandardController stdController;

public LeadTractorExtension(ApexPages.StandardController stdController) {
  tractor = new Tractor__c();
  this.stdController = stdController;
}

It's also possible you meant to query a record or something else, but there's not enough detail in your question to know for sure.