The map collection does not have an add() instance method, instead, it has put(key, value). The first parameter accepts the key, the second accepts the value. The datatype of these parameters must match what has been defined in the map declaration.

Here we're mapping a string to a decimal:

Map<String, Decimal> currencyToExchangeRate = Map<String, Decimal>();
currencyToExchangeRate.put('USD', 1);
currencyToExchangeRate.put('EUR', 0.94);
currencyToExchangeRate.put('CAD', 1.38);
currencyToExchangeRate.put('GBP', 0.84;

Here we're mapping a string to an integer:

Map<String, Integer> nameToAge = Map<String, Integer>();
nameToAge.put('Cyrus', 33);
nameToAge.put('Frank', 66);
nameToAge.put('Layla', 28);
nameToAge.put('Ziba', 35);

// nameToAge.put('Ziba', '35'); Not allowed! The value must be an Integer

When adding a key/value pair, if the key already exists in the map, the old value will be replaced with the new value.

Map<String, Decimal> currencyToExchangeRate = Map<String, Decimal>();
currencyToExchangeRate.put('USD', 1);
currencyToExchangeRate.put('EUR', 0.94); 
currencyToExchangeRate.put('EUR', 0.96); // The previous value, 0.94 has been removed from the Map

If the key is a string, we'll need to be careful. The key is case-sensitive.

Map<String, Decimal> currencyToExchangeRate = Map<String, Decimal>();
currencyToExchangeRate.put('USD', 1);
currencyToExchangeRate.put('EUR', 0.94); 
currencyToExchangeRate.put('eur', 0.96); // The key and value pairing above is not replaced, eur is considered a different key than EUR