### Question:
ResourceConverterProviderImpl implements ResourceConverterProvider { protected String getCombinedConverterKey(String serverResourceType, String clientResourceType){ return serverResourceType + "<=>" + (clientResourceType != null ? clientResourceType.toLowerCase() : "null"); }  ToClientConverter<? super Resource, ? extends ClientResource, ToClientConversionOptions> getToClientConverter(String serverType); ToClientConverter<? super Resource, ? extends ClientResource, ToClientConversionOptions> getToClientConverter(String serverType, String clientType); ToServerConverter<? super ClientResource, ? extends Resource, ToServerConversionOptions> getToServerConverter(String serverType, String clientType); @Override ToClientConverter<? super Resource, ? extends ClientResource, ToClientConversionOptions> getToClientConverter(Resource serverObject); @Override ToServerConverter<? super ClientResource, ? extends Resource, ToServerConversionOptions> getToServerConverter(ClientResource clientObject); ToServerConverter<? super ClientResource, ? extends Resource, ToServerConversionOptions> getToServerConverter(String clientType); @Override Class<? extends ClientResource> getClientTypeClass(String clientType); @PostConstruct void initialize();  }### Answer:
@Test public void getCombinedConverterKey(){ assertEquals(provider.getCombinedConverterKey("serverType", "clientType"), "serverType<=>clienttype"); assertEquals(provider.getCombinedConverterKey(null, "clientType"), "null<=>clienttype"); assertEquals(provider.getCombinedConverterKey("serverType", null), "serverType<=>null"); assertEquals(provider.getCombinedConverterKey(null, null), "null<=>null"); }