Java Source File Structure

Java Source File Structure

A Java source file can have the following elements that, if present, must be specified in the following order

• An optional package declaration to specify a package name.
• Zero or more import declarations. Since import declarations introduce class and interface names in the source code, they must be placed before any type declarations.
• Any number of top-level class and interface declarations. Since these declarations belong to the same package, they are said to be defined at the top level, which is the package level.
• The classes and interfaces can be defined in any order. Class and interface declarations are collectively known as type declarations. Technically, a source file need not have any such definitions, but that is hardly useful.

The Java 2 SDK imposes the restriction that at the most one public class definition per source file can be defined. If a public class is defined, the file name must match this public class. If the public class name is MyApp, then the file name must be MyApp.java.

The main() Method 

• The Java interpreter executes a method called main in the class specified on the command line.
• Any class can have a main() method, but only the main() method of the class specified to the Java interpreter is executed to start a Java application.
• The main() method must have public accessibility so that the interpreter can call it.
• It is a static method belonging to the class, so that no object of the class is required to start the execution.
• It does not return a value, that is, it is declared void.
• It always has an array of String objects as its only formal parameter. This array contains any arguments passed to the program on the command line.
• All this adds up to the definition of the main() method:

public static void main(String[] args) { // ... }