What can be done if the Java jar file could not be launched?

As the error message - e if the Java jar file could not be launched says, you can check the Console (/Applications/Utilities/Console) for error logs. You could also open a Terminal window and run the jar from the command line, which should cause any errors to output into the terminal. If your jar is in your downloads folder you would run:

  java -jar ~/Downloads/algs4.jar

Once you have the error, if you can't figure it out, update your question and we can try to provide more information.