Requirement for building intermediate API for Angular/java

I do not understand your requirement problems.

 Usually Angular does post using JSON format. C# Controller inside takey JSON converts it into Class structure, when sending back, C# converts result object into JSON again and Angular receives JSON answer.

 

So there is no need for you to convert in any way. The only thing you may have to do is build classes that match the data bein posted. You may also want to use the NuGet package Newtonsoft.JSON internally for converting JSON to object, but if you use .Net Core there won't be no need for that either.

 

So if I am not understanding you problem wrong - there actually is none ;)