Java   View all facts   Glossary   Help

specification > class > Object class

Object class
subject			fact

Object class	has definition A class that is at the top of the class hierarchy
	has no superclass because it is at the top of the class hierarchy
	has specification
	implements some of the behaviour that every class in the Java system needs
	is the most general of all classes
	is a subtopic of Example Classes
	is a subtopic of Objects
	is an instance of class
	is defined in java.lang
class	a class is a member of the default package unless its source file contains a package statement
	if a class has one or more abstract methods or the class does not implement all of its superclass's abstract methods then the class must be declared abstract
	if a class has no explicit constructor then the default constructor is used
	if a class implements an interface then the class must provide implementations for all methods declared in the interface and its superinterfaces or the class must be declared abstract
	if a class is not declared public then it is accessible only to members of the same package
	adds 0 or more variables to the variables it inherits from its superclass
	adds 0 or more methods to the methods it inherits from its superclass
	can access any public class in other packages
	can be imported from a package
	can extend only one superclass
	can override methods that are inherited from the class's superclass
	can protect its members from access by other classes or objects using an access modifier
	cannot inherit method implementations from an interface
	contains all of the code that relates to its objects including code describing how the objects of the class are structured - i.e. the data stored in each object that implement the properties the procedures, called methods, that implement the behaviour of the objects
	contains data associated with each object
	declares a list of variables, called instance variables, corresponding to data that will be present in each instance
	defines 0 or more class methods 0 or more class variables 0 or more instance methods 0 or more instance variables
	defines a class type whose instances are the values of the class type
	has benefit reusability modularity
	has example public class HelloWorld { public static void main(String[] args) { System.out.println("Hello World!"); } }
	has part class name
	has part code
	has part constructor
	has syntax class classname { // declarations of variables // declarations of constructors // declarations of other methods with public ones first }
	inherits 0 or more methods from its superclass
	inherits 0 or more variables from its superclass
	inherits behaviour from its superclass
	is a descendant of Object class
	is specified by 1 class definition
	may have access modifier public or private or protected
	provides implementation for all its instance methods unless the class is abstract
	represents several similar objects
	should be a member of 1 named package rather than the default package
	should be named after a thing its instances represent in the real world
	should be placed in its own source file
	should not be named after the internals of a computer system such as 'Record', 'Table', 'Data', 'Structure', or 'Information'
	should order elements as follows: class variables instance variables constructors the most important public methods methods that are simply used to access variables private methods
	to instantiate you create an instance of it
	uses an implements clause to declare that it contains methods for each of the operations specified by the interface
access unit	has access mode
syntactic unit	has syntax rule bold = mandatory italic = non-terminal normal font = optional

Next class: programmer-created exception class    Up: class    Previous class: nested class