how to write trigger using after insert

Question

I tried this code but I am getting error anyone please help on this:

trigger fillValue on Employee__c (after insert, after update) {
    map<decimal,contact> mapcon = new map<decimal,contact>();
    list<employee__C> ee = new list<employee__c>();
    list<contact> con = [select id,test__c from contact where test__C != null];

    if(trigger.isinsert){
        for(employee__c emp : trigger.new){
            for(contact c :con){
                if(emp.test__C == c.test__c){
                    mapcon.put(c.test__c,c);
                }
            }
        }

        for(employee__c eee : trigger.new){
            if(mapcon.size()>0){
                eee.contact__C = mapcon.get(eee.test__C).id;
            }
            ee.add(eee);
        }
        insert ee;
    }
}

Error:

Invalid Data. Review all error messages below to correct your data. Apex trigger fillValue caused an unexpected exception, contact your administrator: fillValue: execution of AfterInsert caused by: System.FinalException: Record is read-only: Trigger.fillValue: line 14, column 1

Answer 1

You can not update/insert same records which are part of trigger variable Trigger.New in after trigger

You must use before trigger if you want to make changes in same object records on which trigger is written.

trigger fillValue on Employee__c (before insert, before update) {
    map<decimal,contact> mapcon = new map<decimal,contact>();
    list<employee__C> ee = new list<employee__c>();
    list<contact> con = [select id,test__c from contact where test__C != null];

    if(trigger.isinsert){
        for(employee__c emp : trigger.new){
            for(contact c :con){
                if(emp.test__C == c.test__c){
                    mapcon.put(c.test__c,c);
                }
            }
        }

        for(employee__c eee : trigger.new){
            if(mapcon.size()>0){
                eee.contact__C = mapcon.get(eee.test__C).id;
            }
            ee.add(eee);
        }       }
}

Answer 2

In after insert you are trying to insert record it will surely give you error. So use @Ashwani's answer and remove the insert ee; line from your code. It will solve your problem.

if(mapcon.size()>0 && mapcon.containsKey(eee.test__C)){
       eee.contact__C = mapcon.get(eee.test__C).id;
}