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specification > class > System

System
subject			fact

System	has definition A class that is part of the API of Java and which provides system-independent access to system-dependent functionality
	has purpose to provide system-independent access to system-dependent functionality
	is part of the API of the Java programming language
	is part of java.lang
	is a subtopic of Example Classes
	is a subtopic of How Java Works
	is an instance of class
	is imported automatically into every program
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 have a main method
	can have instances
	can have more than one constructor each of which has different sets of arguments
	can have the same name as another class if the two classes are not in the same package and their packages are never imported into the same file
	can implement more than one interface
	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 a fully qualified name that includes its package name and a simple name that does not include its package name
	has 0 or more subclasses
	has 1 superclass except Object class which has no superclass
	has behaviour that is specified by its instance methods
	has more specialized behaviour than a class farther up in the class hierarchy
	has public interface which contains its public instance variables and instance methods
	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 abstract if it has one or more abstract methods
	is the unit of data abstraction in an object-oriented program
	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 have a comment at the top describing the purpose of the class, how it should be used, its authors and its history of modification
	should have a unique name since somebody in the future might want to import the packages containing both classes and hence create a name clash
	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

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