dll using dll decompiler tool
You need to use a PE (portable executable) viewer to look for entries in the export table. (Depends is one.)
Most .NET DLLs don't export functions that way. When they do, it's through a mechanism called Reverse P/Invoke, which isn't supported by most Microsoft .NET language compilers. The C++/CLI language was designed for this purpose.
You might find a shorter path to success with a Java-.NET bridge product. Or, a Java-COM bridge product if the .NET DLL exposes classes as COM objects. (Use OLE/COM Object Viewer to inspect a COM DLL.)
Also, be sure the DLL has the same bitness as your JVM process (e.g., java.exe or javaw.exe), unless you are using as an out-of-process COM object.