remove duplicates from Map [closed]

Question

I have a list which contains duplicate entries.

List<Account> aList1=new List<Account>([select Id,Name from account where Name LIKE '%Falcon%']);
Map<String,Account> aMap=new Map<String,Account>();
for(Account a:aList1)
{
    aMap.put(a.Name,a); // remove duplicate if any
}

Now I have created a trigger on account object that prevents duplicates.

Now I want to compare all values in aMap with all records in aList2. If there are any duplicates they are removed.

List<Account> aList2=[select ID,Name from Account ORDER BY Name];
for(Account a:aList2)
{
   if(aMap.containsKey(a.Name))
       aMap.remove(a.Name);  removes duplicates from aList1
}

Database.Insert(aMap.values(),false);

But after the for Loop, aMap is empty.Please let me know.

Answer 1

Here, aMap and aList1 are subsets of aList2. You are looping through aList2 and removing all the elements in aMap.

The following statement will be true always

if(aMap.containsKey(a.Name))

And the respective map element will be removed here:

aMap.remove(a.Name);

At the end of looping, aMap will be empty.

Update :

Scenario: aList1 and aMap are in source org where as aList2 is in destination org.

Querying for all accounts in destination organization is very inefficient. You'll run into all sorts of limits (SOQL rows, Apex CPU limit etc.,). In destination org, this would be my query:

List<Account> aList2=[select ID,Name from Account WHERE Name In :aMap.keySet()];

This will give you the already existing accounts that you are trying to insert. Loop through aList2, and remove duplicate elements from aMap.

Answer 2

Maps work on a Key/Value pairs concept where the Key is always unique. If you your Map already contains a record with a duplicate Key, when you add the duplicate the original will be replaced automatically.

If all you care about is having unique Names and not the order in which they are added/removed all you would need to do is loop over your records and add them to the Map. Then you could get the values of the Map to obtain your unique records.

List<Account> aList1=new List<Account>([select Id,Name from account where Name LIKE '%Falcon%']);
Map<String,Account> aMap=new Map<String,Account>();
for(Account a:aList1)
{
    aMap.put(a.Name,a); //<-- At this point uniqueness is guaranteed.
}

List<Contact> uniqueContacts = aMap.values()// <-- Get unique Contact records.