Invariant means that neither subtype nor supertype can be accepted, the type has to be exactly that which is declared.

no subtypes hierarchy

Attributes
stringshared actual String string

A developer-friendly string representing the instance. Concatenates the name of the concrete class of the instance with the hash of the instance. Subclasses are encouraged to refine this implementation to produce a more meaningful representation.

Inherited Attributes
Attributes inherited from: Object
Inherited Methods
Methods inherited from: Object