trigger updateRelatedLeadOrOpportunity on Task (after insert,after update) {
List<Id> OpportunityIds = new List<Id>();
List<Opportunity> OpportunityList = new List<Opportunity>();
List<Id> LeadIds = new List<Id>();
List<Lead> LeadList = new List<Lead>();
List<Task> taskToProcess = new List<Task>();

    for(Task t :trigger.new)
        {
        //This will make sure that we only process the task who's subject is added/updated
        if(t.subject != null && (trigger.isInsert || trigger.newMap.get(t.id).subject != trigger.oldMap.get(t.id).subject)){
            //You'll have WhoId populated when the Task related to a Lead/Contact(only) 
            //and whatId will be populated if the task is realted to Account/Opportunity/Custom Objects...
            if(t.whatId !=null)
            {
                Schema.SObjectType tType= t.whatid.getSObjectType();
                if(tType == Opportunity.Schema.SObjectType)
                {
                    OpportunityIds.add(t.Whatid);
                    taskToProcess.add(t);
                }
                
            }

            if(t.whoId != null){

                Schema.SObjectType tType= t.whoId.getSObjectType();
                if(tType == Lead.Schema.SObjectType)
                {
                    LeadIds.add(t.whoId);
                    taskToProcess.add(t);
                }
            }
        
        
        }   
    }
    
    //Querying the related Opportunity as well as Lead based on whatid on Task
    Map<Id,Opportunity> OpportunityMap =  new Map<Id,Opportunity>([select id,Last_Activity_Subject__C from Opportunity where id in:OpportunityIds]);
    Map<Id,Lead> LeadMap =  new Map<Id,Lead>([select id,Last_Activity_Subject__C from Lead where id in:LeadIds]);
    }
    //Now we have all the tasks that we need to process in this loop
    for(Task t :taskToProcess){
        if(t.whatid!=null)
        {
            Schema.SObjectType tType= t.whatId.getSObjectType();
            if(tType == Opportunity.Schema.SObjectType){
                Opportunity opp = OpportunityMap.get(t.whatId);
                opp.Last_Activity_Subject__C = t.subject;
                OpportunityList.add(opp);
            }
            

        }
        
        if(t.whoId!=null){
            Schema.SObjectType tType= t.whoId.getSObjectType();
            if(tType == Lead.Schema.SObjectType){
                Lead lead = LeadMap.get(t.whoId);
                lead.Last_Activity_Subject__C = t.subject;
                LeadList.add(lead);
            }

        }
    }
    // updating the Opportunity and lead
    if(!OpportunityList.isEmpty()) update OpportunityList;
    if(!LeadList.isEmpty()) update LeadList;
    
}

@istest 
public class TestupdateRelatedLeadOrOpportunity{

    private Static testmethod void TestLastActivitUpdateSubject(){
        
        //You'll need to populate data into all other required fields and with respect to any validation rules if you have any.
        Lead l = new Lead(Firstname = 'TestFName'. Lastname = 'TestLName');
        insert l;

        //You'll need to populate data into all other required fields and with respect to any validation rules if you have any.
        Account a = new Account(Name = 'Test Account');
        insert a;
        Opportunity o = new Opportunity(Name = 'Test Oppotunity'. AccountId = a.id);
        insert o;

        //These tasks will be related to Lead & Opportunity hence it will cause the trigger to execute and upadte the task Subject to Last_Activity_Subject__C field
        List<task> t = new List<task>{ new task(WhoID = l.id, Subject='Call', Status='Completed', Priority='Normal'), new task(WhatID = o.id, Subject='Email', Status='Completed', Priority='Normal')};
        insert t;
        Lead lead = [select Last_Activity_Subject__C from Lead where id = :l.id];
        Opportunity opp = [select id,Last_Activity_Subject__C from Opportunity where id = :o.id];
        System.assertEquals('Call', lead.Last_Activity_Subject__C);
        System.assertEquals('Email', opp.Last_Activity_Subject__C);
    }
}