How to cover trigger.oldmap.get () in test class

Question

public void updateAccessLevelsOnUpdate(final List<Opportunity> newOppList, Map<Id, Opportunity> oldOppMap){
        if(Istrue == true){
        try{
            Map<Id, Id> opportunityAssignedToMap = new Map<Id, Id>();
            Set<String> uniqueOppUserCombination = new Set<String>();

            for(Opportunity opp :newOppList){
                **if(opp.Assigned_To__c != oldOppMap.get(opp.Id).Assigned_To__c && opp.Assigned_To__c != null){
                    opportunityAssignedToMap.put(opp.Id, opp.Assigned_To__c);**
                    uniqueOppUserCombination.add(opp.Id+'_'+opp.Assigned_To__c);
                }
            }

            List<OpportunityTeamMember> oppTeamMemberList = new List<OpportunityTeamMember>();
            oppTeamMemberList = getOpportunityTeamMember(opportunityAssignedToMap);

            if(oppTeamMemberList.size() > 0){
                Insert oppTeamMemberList;
            }    

            List<OpportunityShare> opportunityShareList = new List<OpportunityShare>();
            opportunityShareList = getOpportunityShareList(opportunityAssignedToMap, uniqueOppUserCombination);

            if(opportunityShareList.size() > 0){
                update opportunityShareList;
            }
        }
        catch(exception e){for(Opportunity opp :newOppList) opp.addError('Sorry- '+e.getMessage());}
    }
    } 

Test class

 @isTest(seeAllData=false)
    public Class OpportunityTriggerHandler_Test{



    static testMethod void updateAccessLevelsOnInsert_test()

  {

      Test.startTest();
      init();
      Opportunity opp= [select ID , Assigned_To__c,Name from  Opportunity where property__c =: newproperty.Id limit 1];       
      opp.Name = 'Suresh';
      update opp;      
      newopportunity.add(opp); 

      Map <id,Opportunity> oldOppMap= new map<id,Opportunity>();
      Opportunity OpportunityID= [select ID , Assigned_To__c from  Opportunity where property__c =: newproperty.Id limit 1];     
      oldOppMap.put(newUser.Id,OpportunityID);



      OpportunityTriggerHandler Opportunityobj =new OpportunityTriggerHandler();       
      Opportunityobj.updateAccessLevelsOnUpdate(newopportunity,oldOppMap); 

      Test.stopTest();


     } 
     }

Answer 1

To make this code enter it's body:

if(opp.Assigned_To__c != oldOppMap.get(opp.Id).Assigned_To__c && opp.Assigned_To__c != null) {
    ...
}

assuming your code is called from a trigger, you will need to modify the Assigned_To__c value of one or more of the Opportunities you create in your code.

Assuming the field isn't defaulted to the current User this should be enough:

  opp.Name = 'Suresh';
  opp.Assigned_To__c = UserInfo.getUserId();
  update opp;

If it is already set to the current User, you will need to create a new User in your test code and use the Id of that; Google for examples.