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AKGBackup - The backup program



Section 4 : Language Fundamentals




Identify correctly constructed source files, package declarations, import statements, class declarations (of all forms including inner classes), interface declarations and implementations (for java.lang.Runnable or other interface described in the test), method declarations (including the main method that is used to start execution of a class), variable declarations and identifiers.



Source Files

A source file should generally contain at most one top level public class definition. If a public class is present, the class name should match the unextended file name. A source file may contain an unlimited number of non public class definitions.

This is not a language requirement, but an implementation requirement of many compilers including the reference compilers from sun. It is therefore unwise to ignore this convention since doing so limits the portability of your source files ( but not, of course, your object files).

There are 3 top level elements that may or may not appear in a file. If they are present then the order must be

  • package declaration
  • import statements
  • class definitions

White spaces and comments may appear before or after any of these elements. 

Keywords and Identifiers 

Keywords and reserved words may not be used as identifiers. An identifier must begin with a letter, a dollar sign ($), an underscore ( _ ) or digits. You can use embedded keywords.

main method 

This method is declared public by convention. However it is a requirement that it be static so that it may be executed without the necessity of constructing an instance of the corresponding class. Its return type must be void. These conditions are necessary because when a Java program starts running, JVM looks for this method as the code to start execution of the program is placed in this method only.

Variables and Initialization

In Java, variables have two different lifetimes :

  • A member variable of a class is created when the instance is created and is accessible from any method in the class.
  • A local variable of a method is created on entry to the method, exists only during execution of the method, and is accessible only within the method.

A member variable may be initialized in its own declaration line : when this technique is used, non-static instance variables are initialized just before the class constructor is executed. Static variables are initialized at class load time.

Local variables are not initialized by the system every such variable must be explicitly initialized before being used.



section4-1 | section4-2 | section4-3 | section4-4


Sections : 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11








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