Object Oriented Languages

1. A new type of value - objects - encapsulate behavior (methods) and state (fields)
2. Classes group objects that have the same structure (methods and field list)
3. Inheritance allows new classes to be derived from existing ones (shared structure)
4. Polymorphism allows messages to be sent to objects of different classes

A Sample Object-Oriented Language

1. Class definitions: include fields and methods.
2. Classes have a constructor - used to initialize the state of objects upon creation.
3. Messages can be sent to objects - to invoke methods.
4. Single inheritance: classes extend a base-class. Each class knows its super-class. The class hierarchy is rooted in a single root class called "object".
5. Inheritance: when defining a sub-class, one can shadow the field of a parent class, or shadow the definition of a method of a parent class.
6. One can send messages to the super (parent) class of an object.

<program> ::= {<class-decl>}* <expression>
              program (class-decls body)

<class-decl> ::= class <identifier> extends <identifier> {field <identifier>}* {<method-decl>}*
                 class-decl (class-name super-name field-ids method-decls)

<method-decl> ::= method <identifier> ({<identifier>}*) <expression>
                  method-decl (method-name ids body)

<expression> ::= new <identifier> ({<identifier>}*)
                 new-object-exp (class-name rands)

<expression> ::= send <expression> <identifier> ({<expression>}*)
                 method-app-exp (obj-exp method-name rands)

<expression> ::= super <identifier> ({<expression>}*)
                 super-call-exp (method-name rands)