Undefined reference error while calling my own Java function from c++ for Android

Hi everybody

I need to call Java code from c++

I try to write my own function in GameActivity.java, do all other stuff from the instruction from here: Platform specific functionality in UE4 game? - Mobile - Epic Developer Community Forums

But I get the undefined reference linker error while calling it in line:

FJavaWrapper::CallVoidMethod(Env, FJavaWrapper::GameActivityThis, FJavaWrapper::AndroidThunkJava_MyFunc,Argument);

This line of code calling functions implemented by engine works well. Everything is implemented just like function AndroidThunkJava_LaunchURL

Looks like the AndroidJNI.h isn’t being loaded after changing, but if I write some syntax error code the compiler sees the errors.

If I do

jmethodID jmID = FJavaWrapper::AndroidThunkJava_MyFunc;
FJavaWrapper::CallVoidMethod(Env, FJavaWrapper::GameActivityThis, jmID, Argument);

I receive error:

MainFrameActions: Packaging (Android (ETC2)): UnrealBuildTool: C:\Program Files\Epic Games\4.8\Engine\Source\Runtime/Core/Public\GenericPlatform/GenericPlatformString.h:146: error: undefined reference to 'FJavaWrapper::AndroidThunkJava_MyFunc'

There’s the function

template <typename SourceEncoding, typename DestEncoding>
	static typename TEnableIf<
		// This overload should be called when the types are not compatible but the source is fixed-width, e.g. ANSICHAR->WIDECHAR.
		!TAreEncodingsCompatible<SourceEncoding, DestEncoding>::Value && TIsFixedWidthEncoding<SourceEncoding>::Value,
	>::Type Convert(DestEncoding* Dest, int32 DestSize, const SourceEncoding* Src, int32 SrcSize, DestEncoding BogusChar = (DestEncoding)'?')
		const SourceEncoding* InSrc         = Src;
		int32                 InSrcSize     = SrcSize;
		bool                  bInvalidChars = false;
		while (SrcSize)
			if (!DestSize)
				return NULL;

			SourceEncoding SrcCh = *Src++;
			if (CanConvertChar<DestEncoding>(SrcCh))
				*Dest++ = (DestEncoding)SrcCh;
				*Dest++       = BogusChar;
				bInvalidChars = true;

		if (bInvalidChars)
			LogBogusChars<DestEncoding>(InSrc, InSrcSize);

		return Dest;

And error is in line

*Dest++ = (DestEncoding)SrcCh;

What does this mean?