OOPL Comparison

The following table shows some similarities and differences in features and the use of terms and concepts in Smalltalk, C++, and Java.

OO Concept/syntax	Smalltalk	C++	Java
Abstract class	Concept exists, no mechanism for enforcement	A class with a pure virtual function can't be instantiated	Classes may be declared abstract
Assignment operator	:=	=	=
Character constant	$c	'c'	'c'
Class	Class	Class	Class
Class method (corresponds to a message sent to the class rather than an instance)	Class method	Static function member	Class (or static) method
Class variable (data associated with the class, not an instance)	Class variable	Static data member	Class field
Comment	"Comment"	// Comment /* Comment */	// Comment /* Comment */ /** Javadoc Comment **/
Dynamic binding	Unrestricted	Only subclasses of the declared class	Subclasses of the declared class or implementations of the declared interface
Equality test (see also identity test)	=	==	equals()
Finalization of instances before deletion (e.g., to release operating system resources)	Some implementations support finalize method	Destructor function	finalize method is standard
Garbage collection (automatic memory management)	Garbage collection is standard	No garbage collection - programmer explicitly manages memory	Garbage collection is standard
Identity test (see also Equality test)	==	same as equality	==
Inheritance	Single inheritance only	Multiple inheritance	Single inheritance only
Initialization of instances	Override new method	Constructor function	Constructor function
Instance data or state	Instance variable	Data member	Instance field
Interface contract (specifies the public interface to classes that implement it)	No corresponding construct	No corresponding construct	Interface
Message	Message	Member function call	Method invocation
Message send syntax	anObject doIt	anObject.doIt() (direct) pObject->doIt() (pointer)	anObject.doIt()
Method	Method	Member function	Method
Method argument syntax	methodName: argument (keyword method)	memberFunctionName(argument)	methodName(argument)
Method return	^ (caret)	return	return
Object (root of all classes)	Class Object	No corresponding class	Class Object
Operator overloading (an aspect of polymorphism; different operator behavior based on receiver type)	Any operator can be overloaded	Any operator can be overloaded	Only methods can be overloaded, not built-in operators such as +, - , *
Pointers	No explicit storage pointers	Explicit use of storage pointers and pointer arithmetic	No explicit storage pointers
Self (the object executing a method)	self	this (usually not written)	this (usually not written)
String constant	'string constant'	"string constant"	"string constant"
Subclass	Subclass	Derived class	Subclass
Superclass	Superclass	Base class	Superclass
Superclass method invocation	super	:: (scoping operator, more general than Smalltalk or Java capability)	super()