How To Populate A Map With List or Set (Where Collections As a Value) in Salesforce

As a developer most of the times we will be working on collections.One of the most common used collection in salesforce is maps.

Creating a simple maps(value as a string,integer,object etc..) is very easy

• Map<String,String>
• Map<String,Integer>
• Map<key,objects>

The real picture comes into scenario when we want to build some complex maps(where value of a map it self again a collection like list,set etc) 

• Map<String,list<Account>>
• Map<integer,set<integer>>
• Map<AccountId,list<Conatct>>

Most common mistakes that we do while building this complex maps are

• Old value gets overrides by new values.
• When your in loop you map always ends up with just holding the last record in list/set.
• Didn't checking whether value is already is exist or not.

Scenario:

Let take a take I want to create a map which will takes case status as key and it's value is all the cases comes under the same status.

Map<String,List<Case>> mapStatusToCases = new Map<String,List<Case>>()

for( Case cs: [SELECT id,Status,CaseNumber FROM Case limit 20])
{
 //Check whether with same status any record is already is added to map or not
 
 if(mapStatusToCases.containsKey(cs.Status)) 
 {
    // If you directly put this case into the map it will overrides all the existing cases,
   // which is not correct.So get all the existing records and store it list/set.
  List<Case> existingCase = mapStatusToCases.get(cs.Status);
  //add this new record to existing list  
  existingCase.add(cs);
  // now put the same status into the map with all latest case records(existing+new),
  // because key is same so it will overrides existing values with this latest list
  mapStatusToCases.put(cs.Status,existingCase); 
 }
 
 else // this executes if the status is not there in map(it means coming first time)
 {
     //prepare variable of map value type
  List<case> lstNewCases = new List<Case>();
  // adding a new record
  lstNewCases.add(cs); 
  // add the new status as a key and case as a value into map
  mapStatusToCases.put(cs.Status,lstNewCases); 
 }
}

System.debug('All Key Set...'+mapStatusToCases.keySet());
System.debug('All Values in Map...'+mapStatusToCases.values());
System.debug('Complete Map Details...'+mapStatusToCases);

For better understanding in above I have written each statement in a separate line. The same code can also be written in an optimized way as shown below.Both are valid and same.

Map<String,List<Case>> mapStatusToCases = new Map<String,List<Case>>()

for( Case cs: [SELECT id,Status,CaseNumber FROM Case limit 20])
{
 //Check whether with same status any record is already is added to map or not
 
 if(mapStatusToCases.containsKey(cs.Status)) 
 {
  //this statement mapStatusToCases.get(cs.Status) will returns the list
  //So,I'm adding new record to existing list directly
  mapStatusToCases.get(cs.Status).add(cs); 
 }
 
 else // this executes if the status is not there in map(it means coming first time)
 {
     //Delcared a new list and directly adding a new record to that
  mapStatusToCases.put(cs.Status,new List<Case>{cs}); 
 }
}

System.debug('All Key Set...'+mapStatusToCases.keySet());
System.debug('All Values in Map...'+mapStatusToCases.values());
System.debug('Complete Map Details...'+mapStatusToCases);

OutPut:

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