MY TEST CLASS :

@isTest
public class Test_ContractoverlapDateHandler{

    public static testMethod void TestContractdate(){
      Profile pf = [Select Id from Profile where Name = 'System Administrator'];
        User u = new User();
        u.FirstName = 'Test';
        u.LastName = 'User';
        u.Email = 'testuser@test123456789.com';
        u.CompanyName = 'test.com';
        u.Title = 'Test User';
        u.Username = 'testuser@test123456789.com';
        u.Alias = 'testuser';
        u.CommunityNickname = 'Test User';
        u.TimeZoneSidKey = 'Australia/Sydney';
        u.LocaleSidKey = 'en_AU';
        u.EmailEncodingKey = 'ISO-8859-1';
        u.ProfileId = pf.Id;
        u.LanguageLocaleKey = 'en_US';
        insert u;
        
          system.runAs(u){ 
          
          Trigger_Status__c ts=new  Trigger_Status__c();
          ts.Name='ContractTrigger';
          ts.Active__c=true;
          insert ts;
           String prospectRecordTypeId = Schema.SObjectType.Account.getRecordTypeInfosByName().get('Prospect Account').getRecordTypeId();

          
                     Account acc = new Account(Name = 'Sample', Active__c = true, Aquire__c = true, Type = 'Prospect Account', 
                                      RecordTypeId = prospectRecordTypeId, Migrated__c = false, 
                                      Estimated_Total_Air_Travel_Spend__c = 0, Manual_Revenue_Update__c = false,
                                      Agency__c = 'N', Dealing_Flag__c = 'N', Aquire_Override__c = 'N' ,Contract_End_Date__c = Date.Today()
                                      );
                                    
                                    
                    insert acc;
                
                    Contact con = new Contact(FirstName = 'Sample', LastName = acc.Name, AccountId = acc.Id);
                
                    insert con;
                
                
                    Opportunity opp = new Opportunity(  Name = 'Opp'+acc.Name , AccountId = acc.Id,
                                              Amount = 500000, Category__c = 'Corporate Airfares', CloseDate = Date.Today(),
                                              StageName = 'Qualify');
                                              
                    insert opp;
                
                    Opportunity opp1 = new Opportunity(  Name = 'Opp1'+acc.Name , AccountId = acc.Id,
                                              Amount = 600000, Category__c = 'Corporate Airfares', CloseDate = Date.Today(),
                                              StageName = 'Qualify');
                                              
                    insert opp1;
                
               
                    Boolean b=true;
                    Proposal__c prop = new Proposal__c(Name = 'Proposal '+opp.Name , Account__c = opp.AccountId, Opportunity__c = opp.Id,
                                              Active__c = true, Type__c = 'Corporate Airfares', International_Annual_Share__c = 70,
                                              Domestic_Annual_Share__c = 90, Frequent_Flyer_Status_Upgrade__c = 'No',
                                              MCA_Routes_Annual_Share__c = 70, Valid_From_Date__c = Date.Today(),
                                              Qantas_Annual_Expenditure__c = 500000, Qantas_Club_Discount__c = 'No',
                                              NAM_Approved__c = false, NAM_Rejected__c = false, Pricing_Approved__c = false, Pricing_Rejected__c = false,
                                              DO_Approved__c = false, DO_Rejected__c = false, TeamLead_Approved__c = false, TeamLead_Rejected__c = false,
                                              Approval_Required_Others__c = false,Status__c='Accepted by Customer',Proposal_End_Date__c=date.today().addmonths(12),Proposal_Start_Date__c=date.today()
                                              );
                    insert prop;
                    
                    Proposal__c prop1 = new Proposal__c(Name = 'Proposal '+opp1.Name , Account__c = opp1.AccountId, Opportunity__c = opp1.Id,
                                              Active__c = true, Type__c = 'Corporate Airfares', International_Annual_Share__c = 70,
                                              Domestic_Annual_Share__c = 90, Frequent_Flyer_Status_Upgrade__c = 'No',
                                              MCA_Routes_Annual_Share__c = 70, Valid_From_Date__c = Date.Today(),
                                              Qantas_Annual_Expenditure__c = 500000, Qantas_Club_Discount__c = 'No',
                                              NAM_Approved__c = false, NAM_Rejected__c = false, Pricing_Approved__c = false, Pricing_Rejected__c = false,
                                              DO_Approved__c = false, DO_Rejected__c = false, TeamLead_Approved__c = false, TeamLead_Rejected__c = false,
                                              Approval_Required_Others__c = false,Status__c='Accepted by Customer',Proposal_End_Date__c=date.today().addmonths(24),Proposal_Start_Date__c=date.today());                
            
                    insert prop1;   
                 
                            
                    Contract__c contr = new Contract__c(Name = 'Contract '+opp.Name , Account__c = prop.Account__c, Opportunity__c = prop.Opportunity__c, Proposal__c = prop.Id,
                                              Active__c = true, Type__c = 'Corporate Airfares', International_Annual_Share__c = 70,
                                              Domestic_Annual_Share__c = 90, Frequent_Flyer_Status_Upgrade__c = 'No', Contracted__c = true,
                                              MCA_Routes_Annual_Share__c = 70, Contract_Start_Date__c = Date.Today(), Contract_End_Date__c = Date.Today().addmonths(12),
                                              Qantas_Annual_Expenditure__c = 500000, Qantas_Club_Discount__c = 'No', Status__c = 'Signature Required by Customer'                                           
                                              );
                                              
                                              
                    insert contr;
                            
                           
               
                    Contract__c contr1 = new Contract__c(Name = 'Contract '+opp1.Name , Account__c = prop1.Account__c, Opportunity__c = prop1.Opportunity__c, Proposal__c = prop1.Id,
                                              Active__c = true, Type__c = 'Corporate Airfares', International_Annual_Share__c = 70,
                                              Domestic_Annual_Share__c = 90, Frequent_Flyer_Status_Upgrade__c = 'No', Contracted__c = true,
                                              MCA_Routes_Annual_Share__c = 70, Contract_Start_Date__c = Date.Today(), Contract_End_Date__c = Date.Today().addmonths(24),
                                              Qantas_Annual_Expenditure__c = 500000, Qantas_Club_Discount__c = 'No', Status__c = 'Signature Required by Customer'                                         
                                              );
                                            
                                              
                    insert contr1;
                    
                    Contract__c contr2 = new Contract__c(Name = 'Contract '+opp1.Name , Account__c = prop1.Account__c, Opportunity__c = prop1.Opportunity__c, Proposal__c = prop1.Id,
                                              Active__c = true, Type__c = 'Corporate Airfares', International_Annual_Share__c = 70,
                                              Domestic_Annual_Share__c = 90, Frequent_Flyer_Status_Upgrade__c = 'No', Contracted__c = true,
                                              MCA_Routes_Annual_Share__c = 70, Contract_Start_Date__c = Date.Today(), Contract_End_Date__c = Date.Today().addmonths(24),
                                              Qantas_Annual_Expenditure__c = 500000, Qantas_Club_Discount__c = 'No', Status__c = 'Signature Required by Customer'                                       
                                              );
                                            
                                              
                    insert contr2;          
     
                }
              
        }     
             
 }

MY CONTROLLER:

public class overlapDateHandler{

    public void overlapDateMethod(List<Contract__c> contractList){
    if(checkRecursive.runOnce())
    {

//list<Contract__c> contractList=new list<Contract__c>();
List<Id> conId = new List<Id>();
List<Id> conAccId = new List<Id>();
contractList=Trigger.new;

     for(Contract__c conObj : contractList){
            
            if(conObj.Account__c  != null){
                
                conId.add(conObj.Id);
                conAccId.add(conObj.Account__c);
            }
        }
      list<Contract__c> contrList = new List<Contract__c>(); 
      map<id, list<Contract__c>> idVsContractmap = new map<id, list<Contract__c>>();  
      contrList = [Select name,Contract_Start_Date__c, Contract_End_Date__c, Account__c,Account__r.Id,Status__c From Contract__c where Status__c='Signed by Customer' and Active__c=true and Account__c=: conAccId];
        for(Contract__c controbj : contrList){
            
            if(idVsContractmap.containsKey(controbj.Account__r.Id)) {
                List<Contract__c> contrAccList = idVsContractmap.get(controbj.Account__r.Id);
                contrAccList.add(controbj);
                idVsContractmap.put(controbj.Account__c, contrAccList);
            } else {
                idVsContractmap.put(controbj.Account__c, new List<Contract__c> { controbj });
            }   
            
          }      
        system.debug('contractmap'+idVsContractmap);
       for(Contract__c conObj : contractList){
             
            if(conObj.Account__c != null){   
            system.debug('account'+idVsContractmap.containsKey(conObj.Account__c));
                if(idVsContractmap.containsKey(conObj.Account__c)){
                    Contract__c firstcontract=new Contract__c();
                    for(Contract__c contractObj : idVsContractmap.get(conObj.Account__c)){
                     //Logic Begins for manage start date and end date
                     
                     system.debug('firstcontract'+firstcontract);
                     if(firstcontract.id==null) {
                     firstcontract=contractObj;
                     system.debug('setsfirstcontract');
                     }
                     else{
                      system.debug('secondsection'+'secondsection');
                      system.debug('first contract '+firstcontract.name);
                      system.debug('second contract '+contractObj.name);
                      if(firstcontract.Contract_Start_Date__c<contractObj.Contract_Start_Date__c && firstcontract.Contract_End_Date__c<contractObj.Contract_End_Date__c ){
                          if(firstcontract.Contract_End_Date__c>=contractObj.Contract_Start_Date__c){
                          firstcontract.Contract_End_Date__c=contractObj.Contract_Start_Date__c.addDays(-1);
                          update firstcontract;//put the list and update
                          }                       
                        }  
                        else{
                        system.debug('elsesection');
                        if(contractObj.Contract_End_Date__c>=firstcontract.Contract_Start_Date__c){
                          contractObj.Contract_End_Date__c=firstcontract.Contract_Start_Date__c.addDays(-1);
                          update contractObj;// put the list and update
                          }    
                        
                        }
                                               
                    }   
                   }
            }
        } 

}
}
    
    
    }
}