Iterate over the keys instead of the values:

for (Id id : mailContainer.keySet())
{
    System.debug(id);
    System.debug(mailContainer.get(id));
}

You may use a Map for this:

public static String manageFilters(Map<String, Object> lines){
    System.debug('### lines : ' + lines);
    System.debug('### lines.values() : ' + lines.values());

    for(String key : lines.keySet()){
        System.debug('### lines.get(key) : ' + lines.get(key));
        System.debug('### >>> ' + lines.get(key));
        Map<Object, Object> test = (Map<Object, Object>)lines.get(key);
        System.debug('### test ' + test);
        System.debug('### test ' + test.get('ObjectName'));
    }

    return null;
}

Using account id as example is terrible idea because they'll be unique -> your lists would always have size 0. But yes, you should be able to do something like that.

Your stuff doesn't even compile, you can't use "=" in for(Account acc = [SELECT...]).

Let's say you have accounts and some of them have duplicate names. This would be close to what you need:

Map<string,List<Account>> accountMap = new Map<string,List<Account>>();
for(Account acc : [SELECT Name, Description FROM Account LIMIT 100]){
    if(accountMap.containsKey(acc.Name)){
        accountMap.get(acc.Name).add(acc);
    } else {
        accountMap.put(acc.Id, new List<Account>{acc});
    }
}

and then yes, if you'd have the key you can access accountMap.get('Acme Inc.')[0].Id

You are creating the maps like these:

Mapfield1.put(abRecs.id,abRecs.field1__c);

Mapfield2 .put(abRecs.id,abRecs.field2__c );

And later using this Mapfield2.get(books.field2Notes__c)value for comparison.

I think books.field2Notes__c which is using as key is not same key as abRecs.id. Thats why it is returning null.

If you want to fetch them all simultaneously, you have to do it outside the loop:

Map<String,String> aMap = new Map<String,String>();
Set<String> mapKeys = aMap.keySet();
List<String> mapValues = aMap.values();

Source: https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_methods_system_map.htm

If you absolutely need to all the keys inside a for loop (I don't know why... it sounds like bad practice to reference keys other than the one you're currently iterating), you can reference the mapKeys variable inside the for loop.

Or, just iterate and reference them one at a time as @MSCB described above

You are correct - when you add a record to a collection, that simply passes a reference to the existing record. This means there would never be more than one instance of the record, regardless of how many collections you add it to.

This isn't explicitly documented in the Apex developer's guide, but it explained in the following developerforce blog post:

http://blogs.developerforce.com/developer-relations/2012/05/passing-parameters-by-reference-and-by-value-in-apex.html

This works in the same way as Java, and there are a number of posts explaining pass by reference versus value in Java - here's one example:

https://stackoverflow.com/questions/40480/is-java-pass-by-reference/7034719#7034719

The only two things I can think of here are:

• It's a case issue. To verify, convert your keys to uppercase:

theMap.put(stringKey.toUpperCase(), theAccountList);

• By using Id as a variable name (itself a type), you are getting strange results

As others have already mentioned here, please post the actual code segment so we can help further or close this issue.

Thanks