6 Classes and Objects
a collection of fields;
a collection of methods;
initial value expressions for the fields; and
initialization variables that are bound to initialization arguments.
An object is a collection of bindings for fields that are instantiated according to a class description.
The object system allows a program to define a new class (a derived class) in terms of an existing class (the superclass) using inheritance, overriding, and augmenting:
inheritance: An object of a derived class supports methods and instantiates fields declared by the derived class’s superclass, as well as methods and fields declared in the derived class expression.
overriding: Some methods declared in a superclass can be replaced in the derived class. References to the overridden method in the superclass use the implementation in the derived class.
augmenting: Some methods declared in a superclass can be merely extended in the derived class. The superclass method specifically delegates to the augmenting method in the derived class.
An interface is a collection of method names to be implemented by a class, combined with a derivation requirement. A class implements an interface when it
declares (or inherits) a public method for each variable in the interface;
is derived from the class required by the interface, if any; and
specifically declares its intention to implement the interface.
A class can implement any number of interfaces. A derived class automatically implements any interface that its superclass implements. Each class also implements an implicitly-defined interface that is associated with the class. The implicitly-defined interface contains all of the class’s public method names, and it requires that all other implementations of the interface are derived from the class.
A new interface can extend one or more interfaces with additional method names; each class that implements the extended interface also implements the original interfaces. The derivation requirements of the original interface must be consistent, and the extended interface inherits the most specific derivation requirement from the original interfaces.
Classes, objects, and interfaces are all values. However, a class or interface is not an object (i.e., there are no “meta-classes” or “meta-interfaces”).