Final answer:
The restriction on using the super reference in a constructor is that it must be the first statement in the constructor, ensuring that the superclass is initialized before the subclass.
Step-by-step explanation:
The restriction on using the super reference in a constructor is that it must be the first statement in the constructor. When a class inherits from a superclass, it can use the super reference to call the constructor of the superclass. However, if a constructor does not explicitly call a superclass constructor, the Java compiler automatically inserts a call to the no-argument constructor of the superclass. If the superclass does not have a no-argument constructor, you must explicitly call one of the other constructors in the superclass by using the super reference.
Furthermore, you cannot call an instance method, access an instance field, or access an instance of the class until after the superclass constructor has completed. This is to ensure that the superclass is properly initialized before the subclass begins its own initialization.