It is not possible to create a thread without ThreadGroup.
For more details look here: https://www.eg.bucknell.edu/~mead/Java-tutorial/essential/threads/group.html
"If you create a new Thread without specifying its group in the
constructor, the runtime system automatically places the new thread in
the same group as the thread that created it (known as the current
thread group and the current thread, respectively). So, if you leave
the thread group unspecified when you create your thread, what group
contains your thread?
When a Java application first starts up, the Java runtime system
creates a ThreadGroup named main. Unless specified otherwise, all new
threads that you create become members of the main thread group."
And here: http://www.java2s.com/example/java-book/thread-group.html