Declaring a map will look really similar to declaring a list or set:

Map<String, String> userIdToUserName; 

Here we've declared a variable named userIdToUserName and assigned the type Map<String, String>. We'll need to initialize this variable so it's not null:

Map<String, String> userIdToUserName = new Map<String, String>(); 

On the right-hand side of the equals sign, the new operator creates a Map<String, String>. Our variable is no longer null! The () is a constructor, so we can keep this empty or optionally pass in another map to initialize:

Map<String, String> userIdToUserName = new Map<String, String>();  
Map<String, String> userIdToUserNameDuplicate = new Map<String, String>(userIdToUserName);  

As you saw in the last lesson, a map can be initialized directly with key and value pairs using the key => value structure and the => operator.

Map<String, String> userIdToUserName = new Map<String, String>{'Ux001' => 'Codey'}; 

We're mapping the string key Ux001 to the string value Codey. Let's use a comma-separated list to initialize the map with more pairs:

Map<String, String> userIdToUserName = new Map<String, String> {
     'Ux001' => 'Codey', 
     'Ux002' => 'Flo', 
     'Ux003' => 'Ruth', 
     'Ux004' => 'Astro'
}; 

// Updated the spacing here to improve legibility 

The keys of a map must be unique, but the values don't have to be:

Map<String, Decimal> currencyToExchangeRate = new Map<String, Decimal> {
     'USD' => 1,
     'EUR' => 0.95,
     'GBP' => 0.95
}; 

// EUR and GBP can both exist in the map even though they have the same values