Populating Map Using Loops

Many times I faced that I need a Value with its "Group of values".
Getting a unique value in Set is a idea, but what if you have a group of values associated with it.

So that time Map is the best way to settle it down.
we can take Map in many ways but here I am using
Map<String, List<String>>

I used this map allot, because populating this map with Key and Values is a great method.
Here am showing two methods same but in different ways.

Here is the first one.

1:  //Map of Opportunity  
2:    Map<Id, Set<Id>> opportunityMap = new Map<Id, Set<Id>>();  
3:    //Loop through the Opportunity records  
4:    for(Opportunity opp : opportunities) {  
5:      //Check if map key contains System Field  
6:      if(opportunityMap.containsKey(opp.Opportunity_Field__c)) {  
7:       //Get the Values of the Map and add Id to it.  
8:       opportunityMap.get(opp.Opportunity_Field__c).add(opp.Id);  
9:      }else {  
10:       //Creat a new Set at values and add Id to it.  
11:       opportunityMap.put(opp.Opportunity_Field__c, new Set<Id>{opp.Id});   
12:      }  
13:     }  
14:    }  

Here is the same but another way to write it

1:  //Loop through list  
2:  for(Opportunity opp : opportunities) {  
3:    //List of Opportunity  
4:    List<Opportunity> opps = new List<Opportunity>();  
5:   //Check for values of the Map  
6:   &#160 if(mapOpportuinties.containsKey(opp.Fishbowl_Customer_Number__c) == false) {  
7:    //Add to list  
8:    opps.add(opp);  
9:   &#160} else {  
10:     //Add to list  
11:     opps = mapOpportuinties.get(opp.Fishbowl_Customer_Number__c);  
12:     opps.add(opp);  
13:   }  
14:     //Populating map  
15:     mapOpportuinties.put(opp.Fishbowl_Customer_Number__c, opps);  
16:   }  

Using the above snippet every unique value will have a common group of values. its like

Object1 = value1, value2 value3
Object2 = value1, value2, value3
Object3 = value2, value3, value4
Object4 = value5, value6, value7

Cheers....!!!!!

Happy Coding.