Jad decompile .class file issues - find some unknown characters in private variables

Question

When I used JAD to decompile .Class Files, the private variables show unknown characters in private variables.

My command as below:

jad.exe -r -ff -d classes -s java classes/**/*.class

I have tried other decompilers (CFR,Procyon,Fernflower) and all have similar issue. Have any better way to overwrite these unknown characters to other readability words ?