to stream or not to stream

gistfile1.java

// What I used to do in Java 6/7
Collection<String> accountIds = new ArrayList<>(); 
        for (GCAccount account : user.getGcAccounts()) {
            for (Account sf : account.getSfAccounts()) {
                accountIds.add(sf.getId());
            }
        }                        

// What I can do in Java 8
Collection<String> accountIds =
                user.getGcAccounts().stream()
                        .flatMap(gc -> gc.getSfAccounts().stream())
                        .map(Account::getId)
                        .collect(Collectors.toList());
                        
// What I expected I could do in Java 8                        
Collection<String> accountIds = 
                user.getGcAccounts().stream() // why not sequential() by default and offer parallel() on Collection ?
                        .flatMap(GcAccount::getSfAccounts)
                        .map(Account::getId)
                        .toList();
                        

                        
                        

In Groovy:

users.gcAccounts.sfAccounts.id.flatten()