Objects must implement java.lang.Comparable<T> (Exception: cannot be cast to java.lang.Comparable)
Ordered collection (java.lang.Comparable<T>)
null elements not allowed
Duplicates elements allowed

PriorityQueue<String> pq = new PriorityQueue<>();

// pq.add(null); // java.lang.NullPointerException
pq.add("c");
pq.add("bb");
pq.add("bb");
pq.add("aaa");
pq.add("aaa");
pq.add("aaa");

while (!pq.isEmpty())
    System.out.println(pq.remove());

// pq.forEach(System.out::println); // elements of the queue might be printed in a random order

Output:

aaa
aaa
aaa
bb
bb
c

PriorityQueue<String> pq = new PriorityQueue<>(Comparator.comparing(String::length));

// pq.add(null); // java.lang.NullPointerException
pq.add("c");
pq.add("bb");
pq.add("bb");
pq.add("aaa");
pq.add("aaa");
pq.add("aaa");

while (!pq.isEmpty())
    System.out.println(pq.remove());

// pq.forEach(System.out::println); // elements of the queue might be printed in a random order

Output:

c
bb
bb
aaa
aaa
aaa