How to convert .jar or .class to .dex file?

Here is a solution that was helpful in my case...

Suppose .jar file sits in "c:\temp\in.jar". In command prompt window cd to ..\android-sdk\platform-tools.To get .apk execute:

dx --dex --output="c:\temp\app.apk" "c:\temp\in.jar"

To get .dex file execute:

dx --dex --output="c:\temp\dx.dex" "c:\temp\in.jar"

For mac, the dx command tool is stored in different directory as said in the top answer. Here is how you do it in mac:

   exampleName$ /Users/exampleName/Library/Android/sdk/build-tools/25.0.2/dx --dex --output="/Users/exampleName/Downloads/classes-dex2jar.jar" "/Users/exampleName/Downloads/name.dex"

More detailed explanation:

• The dx tool is stored in the android SDK, but in a different directory on macs, the path for a mac is /Users/exampleName/Library/Android/sdk/build-tools/25.0.2/
• The command is the same for windows and mac, the path is just different
• The command for both platforms is dx --dex--output="c:\temp\app.apk" "c:\temp\in.jar"
• The tool may take a few minutes to produce the dex file

If it's still not working for some reason, make sure you are following these requirements:

I know this is late, but I had to figure this out on my own after hours of trial and error—hope it helps some other mac user

To avoid downloading the entire Android Studio:You can go to command-tools download and extract the folder. Then go into it and find your sdkmanager tool. Then on your command-line type sdkmanager "build-tools;28.0.1". You can select which version of build-tools is appropriate depending on the java version you are working with. This will download it into the root directory of your command-line tools. Go into this folder and you should see the dx tool. Then you can type dx --dex --output="nameoffile.dex" nameoffile.jar.

A little extra note: If you are trying to convert a massive jar you might get a OUT OF MEMORY error. In this case you just pass the JVM the increase heap option dx -JXmx256m ....