Packages

                                              Packages

It is encapsulation mechanism to group related classes and interfaces into a single unit, which is nothing but a package.

eg-:
1-:All classes and interfaces which is required for database operations are grouped into a single package which is nothing but a java.sql package.
2-:all classes and interfaces which are used fully for file io operations are grouped into a separate package which is nothing but the java.io package.

The main advantages of the package are-:

• To resolve name conflict that is a unique identification of our component
• it improved the modularity of the application 
• it improves the maintainability of the application.
• it provides security for our components.

There is one universal naming convention for packages which is to use an internet domain name in reverse.

eg-: com.icicibank.loan.housing.Account;

com.icicibank-: client's internet domain name in reverse.

loan-:module name.

housing-:sub module name.

Account-: class name.

package com.code.java;

public class Test{

  public static void main(String[] args){

     System.out.print("hello");

  }

}

javac Test.java 

Generated .class file will be placed in the current working directory.

javac -d . Test.java

-d -: destination to place generated .class file

(.)-: CWD(current working directory)

• generated .class file will be placed in corresponding package structure.

• if the corresponding package structure is not already available then this command itself will create the corresponding package structure.
• as destination instead of dot we can take any valid directory name example javac -d F: Test.java
• if the specify the directory not already available the we will get compile time error .

                eg-: javac -d Z: Test.java 

•      if z : not available then we will get compile time error saying directory not found Z:

•      at the time of execution we have to use fully qualified name

                  eg-: javac  com.code.java.Test

Conclusion-:

• In any java source file there can be almost one package statement that is more than one package statement not allow otherwise we will get compile time error.

        eg-:

                package pack1;

                package pack2;

                public class Test{

                    public static void main(String[] args){

                        System.out.print("hello");

                    }

                }

compile time error saying -:class, interface or enum expected.

• in any java program the first non-comment should package statement (if it is available) otherwise we will get compile time error.

                    eg-:

                     import java.util.*

                     package pack2;

                     public class Test{

                            

                     }

                    compile time error saying -:class, interface or enum expected.

the following is valid java source file structure .

• package statement
• import statement
• class name or interface or enum declaration 

note-:an empty source file is a valid java program hence the following are valid java source files.