Hello all, 
Question: i want to create a custom field in lead object the lable would be Activity count (Number type & API Name: Activity_count__c).
(firstly i am dealing with the task only)For a Specific lead  Whenever task created with subject ='call' the count would be increase 0 or null to 1. if the subject is other then 'Call' the Activity count field Will remain same and it will not increase after the Activity count= 1 it will increase only once. Example= When Activity count= 1 and task created the  Activity count would be remain same as before.


Ans: I have Already implement this scenario. my trigger is working ( not working when the value of Activity count=null) . StilI have issue:
Why it is not working for null value that i have Discussed before?
The code coverage is still 35% after the creating test class I want atleast 75% 

​Trigger-
 

​trigger TaskUpdateLead on task (after delete, after insert,  after update) {
    
    Set<ID> LeadIds = new Set<ID>();
    String leadPrefix = Lead.SObjectType.getDescribe().getKeyPrefix();
    if(trigger.new!=null){
        for (Task t : Trigger.new) {
            if (t.WhoId!= null && string.valueof(t.WhoId).startsWith(leadPrefix) ) {
                if(!LeadIds.contains(t.WhoId)){
                    LeadIds.add(t.WhoId);
                }
            }
        }
    }
    
    if(trigger.old!=null){
        for (Task t2 : Trigger.old) {
            if (t2.WhoId!= null && string.valueof(t2.WhoId).startsWith(leadPrefix) )
            {
                if(!LeadIds.contains(t2.WhoId)){
                    LeadIds.add(t2.WhoId);
                }
            }
        }
    }
    if (LeadIds.size() > 0){
        List<Lead> leadsWithTasks = [select id,Activity_Count__c,(select id from Tasks where  Subject = 'Call'     LIMIT 1 ) from Lead where Id IN : Leadids   ];
        List<Lead> leadsUpdatable = new List<Lead>();
        for(Lead L : leadsWithTasks){
            L.Activity_Count__c = L.Tasks.size();
            leadsUpdatable.add(L);
        }
        if(leadsUpdatable.size()>0){
            
            update leadsUpdatable;
            
        }
        
    }
}

Test Class-
 

@istest
/** Test when subject is call**/
public Class TaskupdateLead
{
    Private static testMethod void TaskinsertLead()
    {
        Task t1 = new Task();
        t1.OwnerId = UserInfo.getUserId();
        t1.Subject = 'Call';
        t1.Status = 'Not Started';
        t1.Priority = 'Normal';
        insert t1;
    }
    private static testMethod void taskwidemail()
    {
        Task t2 = new Task();
        t2.OwnerId = UserInfo.getUserId();
        t2.Subject = 'Email';
        t2.Status = 'completed';
        t2.Priority = 'Normal';
        insert t2;
    } 
}

Hope some one will help me

Thanks And Regards
Bharat Sharma