We're going to go through these next few instance methods quickly. I have full faith in you!

When we add key/value pairs to a map, we're increasing the size of a map. We can determine the size of a map using the size() instance method:

Map<String, Decimal> currencyCodeToExchangeRate = new Map<String, Decimal> {
      'USD' => 1,
      'CAD' => 1.39,
      'MXN' => 18.49,
      'EUR' => 0.94,
      'GBP' => 0.83
};

Integer mapSize = currencyCodeToExchangeRate.size(); // returns 5

We can remove a key and value pair from a map using the remove(key) instance method. This method will remove the pairing and return the value that was removed:

Map<String, Decimal> currencyCodeToExchangeRate = new Map<String, Decimal> {
      'USD' => 1,
      'CAD' => 1.39,
      'MXN' => 18.49,
      'EUR' => 0.94,
      'GBP' => 0.83
};

Decimal valueRemoved = currencyCodeToExchangeRate.remove('EUR');  // Returns 0.94
Integer mapSize = currencyCodeToExchangeRate.size(); // returns 4

Decimal eurExchangeRate = currencyCodeToExchangeRate.get('EUR'); // returns NULL

valueRemoved = currencyCodeToExchangeRate.remove('EUR');  // Returns NULL because it's not in the Map anymore 

Let's keep this lightning round going. To check if a map is empty, we can use the isEmpty() instance method, which returns a boolean value:

Map<String, Decimal> currencyCodeToExchangeRate = new Map<String, Decimal>();

Boolean isMapEmpty = currencyCodeToExchangeRate.isEmpty(); // returns TRUE

currencyCodeToExchangeRate.put('USD', 1);

isMapEmpty = currencyCodeToExchangeRate.isEmpty(); // returns FALSE

Map<Integer, String> idToName; 
isMapEmpty = idToName.isEmpty(); // Runtime Error! System.NullPointerException: Attempt to de-reference a null object