Returns the parent of this thread group.
<p>
First, if the parent is not {@code null}, the
{@code checkAccess} method of the parent thread group is
called with no arguments; this may result in a security exception.
@return the parent of this thread group. The top-level thread group
is the only thread group whose parent is {@code null}.
@throws SecurityException if the current thread cannot modify
this thread group.
@see java.lang.ThreadGroupEx#checkAccess()
@see java.lang.SecurityException
@see java.lang.RuntimePermission
Returns the parent of this thread group. <p> First, if the parent is not {@code null}, the {@code checkAccess} method of the parent thread group is called with no arguments; this may result in a security exception.
@return the parent of this thread group. The top-level thread group is the only thread group whose parent is {@code null}. @throws SecurityException if the current thread cannot modify this thread group. @see java.lang.ThreadGroupEx#checkAccess() @see java.lang.SecurityException @see java.lang.RuntimePermission