Package

• Java provides a mechanism for partitioning the class name space into more manageable sections known as the package.
• Packages are container for classes that are used to keep the class name space compartmentalized.
• Packages are stored in a hierarchical manner and are explicitly imported into new class definition.
• The package provides both a naming and a visibility control mechanism.
• We can define classes inside a package that are not accessible by the code outside that package.
• We can also define class members that are accessible within the same package.
• It helps to hide implementation of the classes from the out side of the package.

Defining a Package

• To create a package, simply include a package keyword as the first statement in java source file.
• Any classes declared within that file will belong to the specified package.
• It defines a name space in which classes are stored.
• If no package is defined than class names are put into the default package, which has no name.

General form of package statement:

Package pkg_name;

• Where, pkg is the name of the package.
• package must be stored in to the directory and the directory name must match the package name exactly.
• More than one file can include the same package statement.
• Hierarchy of packages can also be created as follows:

package pkg1[ .pkg2 [.pkg3]  ];

Example:             package java.awt.image;

• This package must be stored in java/awt/image directory.
• package can not be renamed without renaming the directory in which the classes are stored.

package MyPack;
class Balance
{
	String name;
	double bal;
	Balance(String n, double b)
	{
		name=n;
		bal=b;
	}
	void show()
	{
		if (bal>0)
		System.out.println("Name is:" + name +":$" + bal);
	}
}
class AccountBalance
{
	public static void main(String args[])
	{
		Balance current[]=new Balance[3];
		current[0]=new Balance("K.J.Fielding",123.23);
		current[1]=new Balance("will tell",157.02);
		current[2]=new Balance("Tom",-12.33);
		for(int i=0;i<3;i++)
		{
			current[i].show();
		}
	}
}

Compilation of program:

 C:\javaprogs\MyPack>javac AccountBalance.java

C:\javaprogs>java MyPack.AccountBalance

Access Protection

• Anything declared as public can be accessed from anywhere.
• private cannot be accessed from outside of its class.
• Default: when a member does not have an explicit access specification, it is visible to sub-classes as well as to other classes in the same package.
• Protected can be accessed outside the current package, but only to the subclass
• A class only has two possible access level: default and public. When a class is declared as public, it is accessible by any other code. If a class has default access, then it can only be accessed by other code within the same package.