I have a map which has more then one entry for each key value, how can I iterate through just those records for a particular key value. I don't want to do the following which iterates through the whole set, but just want to iterate through those for a particular key value, struggling to find the right syntax - probably missing something straightforward!

Map<id,List<ERP_Customer__c>> acctErpMap = new Map<id,List<ERP_Customer__c>>();

// code to put values in map to go here ... 

for (id key : acctErpMap.keyset())  
    System.debug(logginglevel.INFO, 'PersonAccount DEBUG: MAP key = ' + key);  
    System.debug(logginglevel.INFO, 'PersonAccount DEBUG: MAP value = ' + acctErpMap.get(key));         

Following the help given by those who responded to the call I re-coded as per following:
1. A map of lists has been created.
2. The id of the map has therefore been able to be kept unique
3. The code now posted below shows:
(a) Setting up and populating the map.
(b) Reading back through the map.

    Note: ERP_Customer__c is a child of the Account object.
    Create a map, acctErpMap, of lists as follows:
Map<id,List<ERP_Customer__c>> acctErpMap = new Map<id,List<ERP_Customer__c>>();
// Create an empty ERP list for each account record. 
For (Account acc : accList) 
    acctErpMap.put(acc.id, new List<ERP_Customer__c>());

//    Populate ERP_Customer__c list for each account being processed                 
for (ERP_Customer__c ERPChild : ERPFullList)

// Loop through accounts.
For (Account acc : accList)
    // Logic removed for purpose of post.
    // ...

    // Loop through ERP_Customer__c for this Account
    for (ERP_Customer__c ERPChild : acctErpMap.get(acc.id))
        // Logic removed for purpose of post...
        // ...
  • Do you know that particular key which you need? Will that always be constant? – Reshma Oct 5 '16 at 12:30
  • Yes I do know the key I need. Essentially I have an outer loop (looping around a list), each iteration of this loop provides me with a key which I want to use to access records in the map. Hope that makes sense. – Richard Penwell Oct 5 '16 at 12:51
  • I'm not sure I understand your question fully, but are you looking for something like for(value val : acctErpMap.get(key).values()){ //do logic } What type of Map is it? Is it a Map<Id,List<SObject>>? – Todd Sprinkel Oct 5 '16 at 13:36
  • I've posted an answer which might help, but without an edit to your question, which includes the type of the map, as @ToddSprinkel mentions, we won't be able to help you much further. – battery.cord Oct 5 '16 at 13:39
  • Yes the map I think does need to be Map<Id,List<SObject>> - as I've now included in code snippet. In which case I guess I would then have unique values as keys to the map. So then I probably need the for loop mentioned by you @ToddSprinkel to access the data. I guess the other part of the puzzle was populating a map with a list. – Richard Penwell Oct 5 '16 at 14:34

You can only have one value per key in a map. Calling Map.put(...) on the same key will replace the existing value in the given key.

Try running the snippet below in a developer console to display this behavior.

Map<String, Boolean> mapAndValues = new Map<String, Boolean>();

// Assign multiple values to same key 
mapAndValues.put('test', true);
mapAndValues.put('test', false);

System.debug(mapAndValues.get('test')); // will return false

An alternative option to using put on the same key would be to use a list type as the value to the map.

Map<String, List<Boolean>> mapAndValues = new Map<String, List<Boolean>>();

// Assign empty list  
mapAndValues.put('test', new List<Boolean>());

// Modify list reference from get 

System.debug(mapAndValues.get('test')); // will return (true, false) 

To iterate over the records in your list as specified in your question, you'll need to create a loop which acts on the list provided from calling get on the key.

// First log id to be used to get other records 
System.debug(logginglevel.INFO, 'PersonAccount DEBUG: MAP key = ' + id);  

// Loop over list at specified key 
for (ERP_Customer__c erp:acctErpMap.get(id))  
    // Log each record 
    System.debug(logginglevel.INFO, 'PersonAccount DEBUG: MAP value = ' + erp.Id);         

To check if a key is in a map, you need to use the method containsKey(...), as opposed to the method contains.

someMap.containsKey(someKey); // returns either true or false 
  • 1
    Whilst I was helped by comments (rather than an answer) asking me for further information given by @ToddSprinkel, this answer was also useful in my working out what to do. As this is the only actual answer I will mark it up as solving the issue, but in fact both did - thanks... – Richard Penwell Oct 5 '16 at 15:39
  • Guess I'll have to open another one for the syntax error - a bit new to this, but cannot work it out!! – Richard Penwell Oct 5 '16 at 15:52
  • If you're having trouble getting adjusted, I'd recommend taking a day or two to try the Apex Developer Trailhead. It might help. – battery.cord Oct 5 '16 at 16:02
  • Figured it out and posted my final code now - I don't disagree with your suggestion regarding Trailhead though! Thanks. – Richard Penwell Oct 7 '16 at 11:44
  • Lifesaver. Great common sense answer to how to make a list from the map. – amanda fouts Jun 28 '17 at 16:24

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