Java packages

Packages
Package is a container for classes
A package is a grouping of related types
(classes and interfaces) providing access
protection and name space management.
In simple words, packages is the way we
organize files into different directories according
to their functionality, usability as well as category
they should belong to.

Packages
The JDK package from SUN: java.net

Why do we need Packages?
• One can easily determine that these types are related.
• One knows where to find types that can provide task-
related functions.
• The names of your types won't conflict with the type
names in other packages because the package creates a
new namespace.
• One can allow types within the package to have
unrestricted access to one another yet still restrict access
for types outside the package.

Java files for graphics
//in the Shape.java file
public abstract class Shape {
. . .
}
//in the Circle.java file
public class Circle extends Shape {
. . .
}
//in the Rectangle.java file
public class Rectangle extends Shape {
. . .
}

Package graphics: 1st step
• Choose a name for the package (graphics, for example)
graphics
and put a package statement with that name at the top of
every source file that contains the classes that you want to
include in the package.
• In the Shape.java file:
package graphics;
public abstract class Shape {
. . .
}
…
• In the Rectangle.java file:
package graphics;
public class Rectangle extends Shape
{
. . .
}

Package graphics: 2nd step
Put the source files in a directory whose name (graphics,
for example) reflects the name of the package to which
the type belongs:
...graphicsShape.java
...graphicsCircle.java
...graphicsRectangle.java
...graphicsCylinder.java
etc.

Package Name & Package Folder
1) A company uses its reversed Internet domain name for its package
names. The ABC company, whose Internet domain name is
ABC.com, would precede all its package names with com.ABC

package com.ABC.graphics;
2) Each component of the package name corresponds to a
subdirectory. So, if the ABC company had a com.ABC.graphics
package that contained a Shape.java source file, it would be contained
in a series of subdirectories like this:

....comABCgraphicsShape.java
....comABCgraphicsCircle.java
. . .

How to use packages
1. Referring to a package member by its qualified name:
graphics.Circle myCircle = new graphics.Circle();

2. Importing a package member:
import graphics.Circle;
...
Circle myCircle = new Circle();
graphics.Rectangle myR = new graphics.Rectangle();

3. Importing an entire package:
import graphics.*;
...
Circle myCircle = new Circle();
Rectangle myRectangle = new Rectangle();

Access to members of the classes
private (default) protected public
The same class Yes Yes Yes Yes
Subclass in the package No Yes Yes Yes
Non-subclass in the package No Yes Yes Yes
Subclass in another package No No Yes Yes
Non-subclass in another package No No No Yes

Java packages can be stored in compressed files called JAR files,
allowing classes to download faster as a group rather than one at a time.

Access modifier private
package p1 package p2

class C1 class C2 extends C1

private int x x cannot be read or
modified in C2

class C3 class C4

C1 c1; C1 c1;
c1.x cannot be read c1.x cannot be read
or modified nor modified

Default access modifier


int x x cannot be read or
modified in C2

class C3 class C4

C1 c1; C1 c1;
c1.x can be read or c1.x cannot be read
modified nor modified

Access modifier protected


protected int x x can be read or
modified in C2

class C3 class C4

C1 c1; C1 c1;
c1.x can be read or c1.x cannot be read
modified nor modified

Access modifier public


public int x x can be read or
modified in C2

class C3 class C4

C1 c1; C1 c1;
c1.x can be read or c1.x can be read nor
modified modified

Java packages

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Java packages