Apex Map Tips

 //covert list to Map

List<Opportunity> opplst =new List<Opportunity>();

Map<Id, Opportunity> historyMap = new Map<Id, Opportunity>(opplst);

// Add SObject List to Map

Account[] accts=[SELECT Id,Name FROM Account LIMIT 10];

Map<Id, Account> m = new Map<Id, Account>();

// Add all the records to the map.

m.putAll(accts);

// Aggregate Query Result into a Map

Map<Id,AggregateResult> results = new Map<id,AggregateResult>(

[SELECT AccountId Id, COUNT(Id) ContactCount FROM Contact 

WHERE AccountId != NULL GROUP BY AccountId]);