Apex Coding on Custom Lookup Controller.....

public class MyCustomLookupController
{  
  public CandidateOpportunityAssociation__c coa {get;set;}
  public String cid;
  public string test{get; set;}
  public List<CandidateOpportunityAssociation__c> SelectedFields {get; set;}
  string oppid;
  public MyCustomLookupController ()
  {
     try
     {
        this.cid= System.CurrentPageReference().getParameters().get('Candidate__c.ID');
       
        if(cid==null)
        {
          cid = System.CurrentPageReference().getParameters().get('CF00N400000022LF9');
        }
        SelectedFields = new List<CandidateOpportunityAssociation__c>();
        coa = new CandidateOpportunityAssociation__c();
        coa.CandidateId__c= cid;
        string a ='select Manager_Email_ID__c,CandidateId__c from CandidateOpportunityAssociation__c where CandidateOpportunityAssociation__c.CandidateId__c= \'' + cid+ '\'';
        //string a = 'SELECT Manager_Email__c from User where id = \'' + cid + '\'';
        System.Debug('a1 :' + a);
        SelectedFields = Database.Query(a);
     
       }
         catch(system.QueryException e)
         {
     
         }
   
   }

    public PageReference save()
    {
     
        system.debug('cid:'+coa.CandidateId__c);
        system.debug('oid:'+coa.OpportunityId__c);
        if(coa.CandidateId__c == Null)
        {
          coa.CandidateId__c = 'a0kS0000000e1Tv';
          //for(Candidate__c c : [select Candidate__c.ID from Candidate__c LIMIT 1])
          //{
          // system.debug('customid:'+c);
          // if(c!=null)
          // {
          //  coa.CandidateId__c = c.ID;
          // }
          // }
        }
        if(coa.OpportunityId__c==null)      
        {
           coa.OpportunityId__c = '006S0000005wBIy';
         // for(Opportunity op: [select Opportunity.ID from Opportunity LIMIT 1])
         //{
          //system.debug('customOpid:'+op);
          // if(op!=null)
          // {
           //  coa.OpportunityId__c = op.ID;
         //  }
         //}
        }
        insert coa;
        system.debug('coa:'+coa);
        //ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.Info, aId));
        //return new PageReference('https://cs1.salesforce.com/a09/o/'); //Page.https://ap1.salesforce.com/a0990000006CG0R; //'https://ap1.salesforce.com/a0990000006CG0R';
        //PageReference acctPage = new ApexPages.StandardController(coa).view();
       
        //PageReference acctPage = new PageReference('https://cs1.salesforce.com/a0k/o');//https://cs1.salesforce.com/a0k/o
        //coaid = [select Id from CandidateOpportunityAssociation__c where  ];
        PageReference acctPage = new PageReference('https://cs1.salesforce.com/' + coa.id);
        System.Debug('test123');
        acctPage.setRedirect(true);
        return acctPage;
            }
   
    public PageReference saveNew()
    {
       
        insert coa;
        PageReference acctPage = ApexPages.currentPage(); //new PageReference('https://c.ap1.visual.force.com/apex/MyCustomLookupController');
        acctPage.setRedirect(true);
        return acctPage;  
    }

    public PageReference cancel() {      
        //PageReference pageRef =  new PageReference('https://cs1.salesforce.com/a0k/o'); //('https://ap1.salesforce.com/' + aId);// a0A90000005fa5N');
        ////PageReference pageRef = ApexPages.currentPage();
        //pageRef.setRedirect(true);
        //return null; //pageRef ;  
         PageReference pg=new PageReference('https://cs1.salesforce.com/a0k/o');
       
           pg.setRedirect(true);
     
         
       
        return pg;
        }
}