From my understanding, this is a single statement in Java:

while((x % p == 0l) && (x > p)) x /= p;

If that is the case, why is this a syntax error:

final AtomicLong subtotal = new AtomicLong(limit);

final long maxPrimeFactor = primeStream.takeWhile(p -> p * p <= limit)
                                       .filter(p -> subtotal.get() % p == 0)
                                       .peek(p -> while((subtotal.get() % p == 0) && (subtotal.get() > p)) subtotal.set(subtotal.get() / p)) // Syntax error here.
                                       .dropWhile(p -> p != subtotal.get())
                                       .findFirst()
                                       .getAsLong();

And this is not a syntax error.

final AtomicLong subtotal = new AtomicLong(limit);

final long maxPrimeFactor = primeStream.takeWhile(p -> p * p <= limit)
                                       .filter(p -> subtotal.get() % p == 0)
                                       .peek(p -> { while((subtotal.get() % p == 0) && (subtotal.get() > p)) subtotal.set(subtotal.get() / p)); } // No syntax error here.
                                       .dropWhile(p -> p != subtotal.get())
                                       .findFirst()
                                       .getAsLong();