//返回 a b c
Set<String> realmNames = princialCollection.getRealmNames();
System.out.println(realmNames);
//因为MyRealm1和MyRealm2返回的凭据都是zhang，所以排重了
Set<Object> principals = princialCollection.asSet(); //asList和asSet的结果一样
System.out.println(principals);
//根据Realm名字获取
Collection<User> users = princialCollection.fromRealm("c");
System.out.println(users);