String accountHref = account.getHref();

String value = account.getCustomData().get("foo");  
System.out.println(value); //prints "bar"

account.getCustomData().put("foo", "whatever");
account.save(); //persists change back to the server

//lookup the account:
account = client.getResource(href, Account.class);
value = account.getCustomData().get("foo");
System.out.println(value); // prints "bar", but SHOULD print "whatever"