Some JAR files are executables and you can launch them if you have Java environment installed in your system. Some are however not and users may attempt a JAR to EXE conversion which is possible with conversion utilities like the jar2exe.
Found 2 different conversion types using the same filename extensions.
Conversion jar to exe type 1:
Convert from Compressed archive file package for Java classes and data to Program executable.
Multiplatform software - convert jar to exe on any platform
A JAR to EXE converter
File type (from-> to)
Convert from jar file format
Compressed archive file package for Java classes and data
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned jar to exe. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
Conversion jar to exe type 2:
Convert from Compressed archive file package for Java classes and data to Self-extracting compressed file archive.
Microsoft Windows software - convert jar to exe on Windows
Popular open source file archiver with a high compression ratio