public static final String CLOSED_STATUS = 'Closed';
Case cId {get;set;}
Case stat {get;set;}
private ApexPages.StandardController stdController;

public String Status;
public String CaseNumber;

//constructor
public CaseClass(ApexPages.StandardController stdController){
    this.stdController = stdController;
    cId = (Case)stdController.getRecord();

}
public PageReference saveAndReturn()
{

Case upstat = [select Status from case where Id =: cId.Id];
upstat.Status = 'Closed';
update upstat;
CaseNumber='1000432';
    stdController.save();


return null;