APEX: Get a Set<Id> of Id's from a List<SObject> of Objects

set-of-ids-from-list-of-objects.cls

// Query list of Accounts
List<Account> accounts = [
    SELECT
        Id, Name
    FROM
        Account
];

// get Set of Ids of the Object
Set<Id> ids = new Set<Id>(new Map<Id, Account>(accounts).keySet());
System.debug('Account Ids as Set<Id>: ' + ids);