L.J.
Institute of Engineering & Technology Semester: V (2016)
Subject Name: Object Oriented Programming using JAVA - QUESTION BANK SOLUTION
Subject Code: 2150704 Unit-5 And 9
Faculties: MS. HEMALI MOJIDRA (SHAH)
UNIT-5 PACKAGES
Use of Package, CLASSPATH, Import statement, Static import, Access control
1 Explain package in java. List out all packages with short description. (June-12) [LJIET] 3/4/7
OR What is package? List various built in package used in java. (May-15) [LJIET]
Ans :
Java Package :
package is a group of similar types of classes, interfaces and sub-packages.
Package in java can be categorized in two form
o built-in package - java, lang, awt, javax, swing, net, io, util, sql etc.
o user-defined package.
Advantage of Java Package:
Java package is used to categorize the classes and interfaces so that they can be
easily maintained.
Java package provides access protection.
Java package removes naming collision. You can create class having same name
but they should be in different package.
Simple example of java package:
The package keyword is used to create a package in java.
//save as LJCE.java
package ljce;
public class LJCE {
public static void main(String args[]){
System.out.println("Welcome to package ljce ");
}
}
How to run java package program
You need to use fully qualified name e.g. ljce.LJCE etc to run the class.
Output:Welcome to package ljce
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.
1) Using packagename.*
If you use package.* then all the classes and interfaces of this package will be accessible but
Object Oriented Programming using JAVA (2150704)By Prof. Hemali Mojidra(Shah)2016 Page 1
L.J. Institute of Engineering & Technology Semester: V (2016)
not subpackages.
The import keyword is used to make the classes and interface of another package accessible
to the current package.
Example of package that import the packagename.*
//save by A.java
package pack;
public class A{
public void msg(){System.out.println("Hello");}
}
//save by B.java
package mypack;
import pack.*;
class B{
public static void main(String args[]){
A obj = new A();
obj.msg();
}
}
Output:Hello
2) Using packagename.classname
If you import package.classname then only declared class of this package will be accessible.
Example of package by import package.classname
//save by A.java
package pack;
public class A{
public void msg(){System.out.println("Hello");}
}
Built-in Packages
These packages consists of a large number of classes which are a part of Java API. For e.g,
we have used java.io package previously which contain classes to support input / output
operations in Java. Similarly, there are other packages which provides different functionality.
Some of the commonly used built-in packages are shown in the table below :
Package
Description
Name
Contains language support classes ( for e.g classes which defines primitive
java.lang data types, System class, math operations, etc.) . This package is
automatically imported.
java.io Contains classes for supporting input / output operations.
Contains utility classes which implement data structures like Linked List,
java.util
Hash Table, Vector, Dictionary, etc and support for Date / Time operations.
java.applet Contains classes for creating Applets. –GUI programming
Object Oriented Programming using JAVA (2150704)By Prof. Hemali Mojidra(Shah)2016 Page 2
L.J. Institute of Engineering & Technology Semester: V (2016)
Contains classes for implementing the components of graphical user interface
java.awt
( like buttons, menus, etc. ).
Contains classes for supporting networking operations and network
java.net programming
2 What is package? Explain steps to create package with example. (Dec-13,May-16) [LJIET] 3/4/7
Ans: Ans -1
3 Explain package and interface by giving examples. (June-11, June-14) [LJIET] OR 3/4/7
(ii) Explain packages. (Dec-15) [LJIET]
Ans: Package – Ans-1
Intrface: An interface in java is a blueprint of a class. It is fully abstract class. It has static
constants(final) and abstract methods only.
Interface fields are public, static and final by default, and methods are public and
abstract.
The interface in java is a mechanism to achieve fully abstraction. There can be only
abstract methods in the java interface not method body. It is used to achieve fully abstraction
and multiple inheritance in Java.
Java Interface also represents IS-A relationship.
It cannot be instantiated just like abstract class.(OR It is not possible to create object of
interface)
Use Of Java interface:
It is used to achieve fully abstraction.
It is used to give basic structure to the child classes.
By interface, we can support the functionality of multiple inheritance.
It can be used to achieve loose coupling. (Overriding and dynamic method dispatch
and runtime polymorphism)
Note : The java compiler adds public and abstract keywords before the interface
method and public, static and final keywords before data members.
Understanding relationship between classes and interfaces
Example:
Object Oriented Programming using JAVA (2150704)By Prof. Hemali Mojidra(Shah)2016 Page 3
L.J. Institute of Engineering & Technology Semester: V (2016)
interface A {
int i=10;
void m1();
void m1(int a);
void m2(); interface A
}
class B implements A
{
class B
public void m1() {
System.out.println("m1 called");
}
public void m1(int a) {
System.out.println("m1(int) called");
}
public void m2() {
System.out.println("m2 called");
}
}
class UseInerfaceDemo
{
public static void main(String[] args)
{
A a = new B();
a.m1();
a.m1(6);
a.m2();
}
}
Output:
m1 called
m1(int) called
m2 called
Multiple inheritance in Java by interface
interface A1 {
int i=10;
void m1();
void m1(int a);
void m2();
}
interface A2 {
void m3();
Object Oriented Programming using JAVA (2150704)By Prof. Hemali Mojidra(Shah)2016 Page 4
L.J. Institute of Engineering & Technology Semester: V (2016)
}
class B implements A1,A2
{
public void m1() {
System.out.println("m1 called");
}
public void m1(int a) {
System.out.println("m1(int) called");
}
public void m2() {
System.out.println("m2 called");
}
public void m3() {
System.out.println("m3 called");
}
}
class UseInerfaceDemo
{
public static void main(String[] args)
{
B b = new B();
b.m1();
b.m1(6);
b.m2();
b.m3();
}
}
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