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5. Packages

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Topics:

Need for packages

What are packages; package declaration in

Java

Import statement in Java

How do packages resolve name clashes?

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Package Organize class name space

Hierarchical division must be reflected in directory structure,i.e.,

packagecyc.sys;

import java.io.*;

class Login { /* in fact, this is cyc.sys.Login*/ }

Default packages (library) -- JDK java.lang - Object, Thread, Exception, String,...

java.io - InputStream, OutputStream, ...

java.net - Networking

support for Socket & URL-based objects

java.awt - Abstract window toolkit defines a simple, restricted windowing API

java.util - Hashtable, Vector, BitSet, Regexp, ..

java.tools - Compilation, Debugging, Documentation

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Naming a Package

Package name should start with small alpha bateonly.

It can be any user defined name.

If super package is available then it must precedesub package name.

Package statement must be the first statement in

.java file

There can be only one package statement in one

.java file.

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Accessing package

If a package needs to be access outside currentpackage it must be imported by using import

statement.

The syntax is import

root_pkg.sub_pkg.Class/Interface name.

There can be multiple import statements within

same class.

You need not import java.lang (imported bydefault).

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User defined package

Create A package:->packagemypack;

publicclassSimple{

publicstaticvoidmain(String args[]){

System.out.println("Welcome to package");}

}

Compiling the package:-> javac -d . Simple.java

Run the package as:-> java mypack.Simple

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How to access package from another package?

There are three ways to access the package from outside the package:

1. import package.*;2. import package.classname;

3. fully qualified name.

(If you use package.* then all the classes and interfaces of this

package will be accessible but not subpackages.)

EX:

//save by A.java

packagepack;

publicclassA{

publicvoidmsg(){

System.out.println("Hello");

}

}

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//save by B.java

packagemypack;

importpack.*;

classB{

publicstaticvoidmain(String args[]){

A obj = newA();

obj.msg();

}

}

----OR----

pack.A obj = newpack.A();//using fully qualified name

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Access specifierspublic:Any Class or any package.

private:class access (same as C++)

protected:sub class access + any class in same

package

Default :package access. static: only one copy for all instances

abstract: left unimplemented

final: not overridden

native: methods implemented in native code.

synchronized: described in Thread

transient: data that is not persistent

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Comparison

Specifier Class Subclass Package World

private X

protected X X X

public X X X Xpackage X X

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Thank You