Either change the inner classes to be static, i.e.:
public class Exceptions {
public static class NoSpaceException extends RuntimeException {
...
}
public static class NotValidTypeException extends RuntimeException {
...
}
public static class NoItemException extends RuntimeException {
...
}
}
Or, if you don't want to change anything, you can create instances through an instance of Exceptions:
Exceptions exceptions = new Exceptions();
RuntimeException e = exceptions.new NoItemException();
The first approach is preferable.
One more note, consider extending classes that are more specific than RuntimeException (e.g. IllegalArgumentException, IllegalStateExecption, etc).