Question

JCIP defines Guarded objects as:

Guarded. A guarded object can be accessed only with a specific lock held. Guarded objects include those that are encapsulated within
other thread-safe objects and published objects that are known to be guarded by a specific lock.

Which can be cited as an example of such an object inside core Java ?

Answer 1

What comes to my mind immediately is the list held by the SynchronizedList instance returned by Collections.synchronizedList(). Here’s a part of its source code:

static class SynchronizedList<E>
    extends SynchronizedCollection<E>
    implements List<E> {

    final List<E> list;

    public boolean equals(Object o) {
        synchronized (mutex) {return list.equals(o);}
    }
    public int hashCode() {
        synchronized (mutex) {return list.hashCode();}
    }

    public ListIterator<E> listIterator() {
        return list.listIterator(); // Must be manually synched by user
    }

The list object is not thread safe, but is guarded by the mutex. The object returned by listIterator() should be manually guarded by the same mutex (which is the synchronized list itself)