A collection of all principals associated with a corresponding {@link Subject Subject}. A <em>principal</em> is just a security term for an identifying attribute, such as a username or user id or social security number or anything else that can be considered an 'identifying' attribute for a {@code Subject}. <p/> A PrincipalCollection organizes its internal principals based on the {@code Realm} where they came from when the Subject was first created. To obtain the principal(s) for a specific Realm, see the {@link #fromRealm} method. You can also see which realms contributed to this collection via the {@link #getRealmNames() getRealmNames()} method.