public static Key of(String key, ApplicationId appId) {
        return new StringKey(key, appId);
    }
@Test
    public void stringKey() {
        Key stringKey1 = Key.of(KEY_1, NetTestTools.APP_ID);
        Key copyOfStringKey1 = Key.of(KEY_1, NetTestTools.APP_ID);
        Key stringKey2 = Key.of(KEY_2, NetTestTools.APP_ID);
        Key copyOfStringKey2 = Key.of(KEY_2, NetTestTools.APP_ID);
        Key stringKey3 = Key.of(KEY_3, NetTestTools.APP_ID);

        new EqualsTester()
                .addEqualityGroup(stringKey1, copyOfStringKey1)
                .addEqualityGroup(stringKey2, copyOfStringKey2)
                .addEqualityGroup(stringKey3)
                .testEquals();
    }