List activeCustomers = new ArrayList();
for (Customer customer : customers) {
if (customer.isActive()) {
activeCusomers.add(customer);
}
}

becomes:

List activeCustomers = new ArrayList(customers.size()) {{
for (Customer customer : customers)
if (customer.isActive())
add(customer);
}};

It is more of a taste issue — the code is semantically identical — but using the anonymous instance and {{ … }} is a visual pattern which make collection transformation look like its own idiom.

Cheers,
–binkley