Unit - 4

Interfaces and Packages

Q1) What is the interface?

A1) An interface is a reference type in Java. It is similar to class. It is a collection of abstract methods. A class implements an interface, thereby inheriting the abstract methods of the interface.

Along with abstract methods, an interface may also contain constants, default methods, static methods, and nested types. Method bodies exist only for default methods and static methods.

Writing an interface is similar to writing a class. But a class describes the attributes and behaviors of an object. And an interface contains behaviors that a class implements.

Unless the class that implements the interface is abstract, all the methods of the interface need to be defined in the class.

An interface is similar to a class in the following ways −

●        An interface can contain any number of methods.

●        An interface is written in a file with a .java extension, with the name of the interface matching the name of the file.

●        The byte code of an interface appears in a .class file.

●        Interfaces appear in packages, and their corresponding bytecode file must be in a directory structure that matches the package name.

However, an interface is different from a class in several ways, including −

●        You cannot instantiate an interface.

●        An interface does not contain any constructors.

●        All of the methods in an interface are abstract.

●        An interface cannot contain instance fields. The only fields that can appear in an interface must be declared both static and final.

●        An interface is not extended by a class; it is implemented by a class.

●        An interface can extend multiple interfaces.

Syntax

Public interface XYZ   

{  

Public void method();  

}

To use an interface in a class, Java provides a keyword called implements. We provide the necessary implementation of the method that we have declared in the interface.

Q2) Write the features of interface?

A2) Features of Interface:

• We can achieve total abstraction.
• We can use multiple interfaces in a class that leads to multiple inheritance.
• It also helps to achieve loose coupling.

Q3) Compare interface and abstract class?

A3) Both abstract classes and interfaces are used to achieve abstraction so that abstract methods can be declared. Both the abstract class and the interface cannot be instantiated.

However, there are other distinctions between abstract class and interface, which are shown below.

Q4) How to declare an interface?

A4) Declaring Interfaces

The interface keyword is used to declare an interface. Here is a simple example to declare an interface −

Example

Following is an example of an interface −

/* File name : NameOfInterface.java */

Import java.lang.*;

// Any number of import statements

Public interface NameOfInterface {

// Any number of final, static fields

// Any number of abstract method declarations\

}

Interfaces have the following properties −

●        An interface is implicitly abstract. You do not need to use the abstract keyword while declaring an interface.

●        Each method in an interface is also implicitly abstract, so the abstract keyword is not needed.

●        Methods in an interface are implicitly public.

Example

/* File name : Animal.java */

Interface Animal {

Public void eat();

Public void travel();

}

Q5) How to implement the interface?

A5) Implementing Interfaces

When a class implements an interface, you can think of the class as signing a contract, agreeing to perform the specific behaviors of the interface. If a class does not perform all the behaviors of the interface, the class must declare itself as abstract.

A class uses the implements keyword to implement an interface. The implements keyword appears in the class declaration following the extends portion of the declaration.

Example

/* File name : MammalInt.java */

Public class MammalInt implements Animal {

Public void eat() {

System.out.println("Mammal eats");

}

Public void travel() {

System.out.println("Mammal travels");

}

Public int noOfLegs() {

Return 0;

}

Public static void main(String args[]) {

MammalInt m = new MammalInt();

m.eat();

m.travel();

}

}

This will produce the following result −

Output

Mammal eats

Mammal travels

When overriding methods defined in interfaces, there are several rules to be followed−

●        Checked exceptions should not be declared on implementation methods other than the ones declared by the interface method or subclasses of those declared by the interface method.

●        The signature of the interface method and the same return type or subtype should be maintained when overriding the methods.

●        An implementation class itself can be abstract and if so, interface methods need not be implemented.

When implementation interfaces, there are several rules −

●        A class can implement more than one interface at a time.

●        A class can extend only one class, but implement many interfaces.

●        An interface can extend another interface, in a similar way as a class can extend another class.

Q6) How to extend the interface?

A6) Extending Interfaces

An interface can extend another interface in the same way that a class can extend another class. The extends keyword is used to extend an interface, and the child interface inherits the methods of the parent interface.

The following Sports interface is extended by Hockey and Football interfaces.

Example

// Filename: Sports.java

Public interface Sports {

Public void setHomeTeam(String name);

Public void setVisitingTeam(String name);

}

// Filename: Football.java

Public interface Football extends Sports {

Public void homeTeamScored(int points);

Public void visitingTeamScored(int points);

Public void endOfQuarter(int quarter);

}

// Filename: Hockey.java

Public interface Hockey extends Sports {

Public void homeGoalScored();

Public void visitingGoalScored();

Public void endOfPeriod(int period);

Public void overtimePeriod(int ot);

}

The Hockey interface has four methods, but it inherits two from Sports; thus, a class that implements Hockey needs to implement all six methods. Similarly, a class that implements Football needs to define the three methods from Football and the two methods from Sports.

Q7) How to access implementation through interface references?

A7) Variables can be declared as object references that use an interface instead of a class.

Such a variable can refer to any instance that implements the stated interface.

When you use one of these references to invoke a method, the right version is used based on the actual instance of the interface being referred to.

This is one of the most important characteristics of interfaces. The method to be executed is dynamically looked up at runtime, allowing classes to be built after the code that calls methods on them.

This is analogous to accessing a subclass object using a superclass reference.

The callback() function is invoked in the following example using an interface reference variable:

Interface Callback {

Void callback(int param);

}

Class Client implements Callback {

// Implement Callback's interface

Public void callback(int p) {

System.out.println("callback called with " + p);

}

Void aa() {

System.out.println("hi.");

}

}

Public class Main{

Public static void main(String args[]) {

Callback c = new Client();

c.callback(42);

}

}

The following is the program's output:

Callback called with 42

Despite the fact that the variable c is of the interface type Callback, it has been assigned an instance of Client.

The callback() method can be used using c, but it cannot access any other members of the Client class.

Only the methods declared by the interface declaration are known to an interface reference variable.

As a result, because aa() is defined by Client but not Callback, c could not be used to access it.

Q8) Define package?

A8) Package

Advantage of Java Package

1) Java package is used to categorize the classes and interfaces so that they can be easily maintained.

2) Java package provides access protection.

3) Java package removes naming collision.

How to compile java package

If you are not using any IDE, you need to follow the syntax given below:

Javac -d directory javafilename 

How to run java package program

You need to use fully qualified name e.g. Mypack.Simple etc to run the class.

Q9) Write a simple example of java package?

A9) The package keyword is used to create a package in java

//save as Simple.java  

Package mypack;  

Public class Simple

{  

Public static void main(String args[]){  

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

}  

}  

To Compile: javac -d. Simple.java

To Run: java mypack.Simple

The -d is a switch that tells the compiler where to put the class file i.e. it represents destination. The. Represents the current folder.

OUTPUT: Welcome to package

Q10) How to access the package?

A10) Accessing Packages

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.*

1. If you use package.* then all the classes and interfaces of this package will be accessible but not subpackages.
2. 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.class \name 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");}  

}  

//save by B.java  

Package mypack;  

Import pack.A;  

Class B{  

Public static void main(String args[]){  

A obj = new A();  

Obj.msg();  

}  

}  

OUTPUT:  Hello

Using fully qualified name

Example of package by import fully qualified name

//save by A.java  

Package pack;  

Public class A

{  

Public void msg()

{   System.out.println("Hello");  }  

}  

//save by B.java  

Package mypack;  

Class B{  

Public static void main(String args[]){  

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

Obj.msg();  

}  

} 

OUTPUT: Hello

Q11) Describe classpath?

A11) CLASSPATH is an environment variable which is used by Application ClassLoader to locate and load the .class files. The CLASSPATH defines the path, to find third-party and user-defined classes that are not extensions or part of Java platform. Include all the directories which contain .class files and JAR files when setting the CLASSPATH.

You need to set the CLASSPATH if:

●        You need to load a class that is not present in the current directory or any sub-directories.

●        You need to load a class that is not in a location specified by the extensions mechanism.

The CLASSPATH depends on what you are setting the CLASSPATH. The CLASSPATH has a directory name or file name at the end. The following points describe what should be the end of the CLASSPATH.

●        If a JAR or zip, the file contains class files, the CLASSPATH end with the name of the zip or JAR file.

●        If class files placed in an unnamed package, the CLASSPATH ends with the directory that contains the class files.

●        If class files placed in a named package, the CLASSPATH ends with the directory that contains the root package in the full package name, that is the first package in the full package name.

The default value of CLASSPATH is a dot (.). It means the only current directory searched. The default value of CLASSPATH overrides when you set the CLASSPATH variable or using the -classpath command (for short -cp). Put a dot (.) in the new setting if you want to include the current directory in the search path.

If CLASSPATH finds a class file which is present in the current directory, then it will load the class and use it, irrespective of the same name class presents in another directory which is also included in the CLASSPATH.

If you want to set multiple classpaths, then you need to separate each CLASSPATH by a semicolon (;).

The third-party applications (MySQL and Oracle) that use the JVM can modify the CLASSPATH environment variable to include the libraries they use. The classes can be stored in directories or archives files. The classes of the Java platform are stored in rt.jar.

There are two ways to ways to set CLASSPATH: through Command Prompt or by setting Environment Variable.

Q12) How to Set CLASSPATH in Windows Using Command Prompt?

A12) Type the following command in your Command Prompt and press enter.

1. Set CLASSPATH=%CLASSPATH%;C:\Program Files\Java\jre1.8\rt.jar;  

In the above command, The set is an internal DOS command that allows the user to change the variable value. CLASSPATH is a variable name. The variable enclosed in percentage sign (%) is an existing environment variable. The semicolon is a separator, and after the (;) there is the PATH of rt.jar file.

How ext folder works in Java

The ext directory works a bit like the CLASSPATH. Ext directory is the part of the class loading mechanism. The classes which are available within JARs in the ext directory are available to Java applications.

Q13) How to set the CLASSPATH of a MySQL database?

A13) Let's see how to set the CLASSPATH of a MySQL database:

Step 1: Click on the Windows button and choose Control Panel. Select System.

Step 2: Click on Advanced System Settings.

Step 3: A dialog box will open. Click on Environment Variables.

Step 4: If the CLASSPATH already exists in System Variables, click on the Edit button then put a semicolon (;) at the end. Paste the Path of MySQL-Connector Java.jar file.

If the CLASSPATH doesn't exist in System Variables, then click on the New button and type Variable name as CLASSPATH and Variable value as C:\Program Files\Java\jre1.8\MySQL-Connector Java.jar;.;

Remember: Put ;.; at the end of the CLASSPATH.

Q14) How to import a package?

A14) There are many packages to choose from. In the previous example, we used the Scanner class from the java.util package. This package also contains date and time facilities, random-number generator and other utility classes.

To import a whole package, end the sentence with an asterisk sign (*). The following example will import ALL the classes in the java.util package:

Example

Import java.util.*;

User-defined Packages

To create your own package, you need to understand that Java uses a file system directory to store them. Just like folders on your computer:

Example

└── root

└── mypack

└── MyPackageClass.java

To create a package, use the package keyword:

MyPackageClass.java

Package mypack;

Class MyPackageClass {

Public static void main(String[] args) {

System.out.println("This is my package!");

}

}

Save the file as MyPackageClass.java, and compile it:

C:\Users\Your Name>javac MyPackageClass.java

Then compile the package:

C:\Users\Your Name>javac -d . MyPackageClass.java

This forces the compiler to create the "mypack" package.

The -d keyword specifies the destination for where to save the class file. You can use any directory name, like c:/user (windows), or, if you want to keep the package within the same directory, you can use the dot sign ".", like in the example above.

Note: The package name should be written in lower case to avoid conflict with class names.

When we compiled the package in the example above, a new folder was created, called "mypack".

To run the MyPackageClass.java file, write the following:

C:\Users\Your Name>java mypack.MyPackageClass

The output will be:

This is my package!

Q15) Write a Program to create and use a user defined package in Java?

A15) Program

// Vehicle.java

Package vehicles;

Interface Vehicle

{

Public void run();

Public void speed();

}

//Bike.java

Package vehicles;

Public class Bike implements Vehicle

{

Public void run()

{

System.out.println("Bike is running.");

}

Public void speed()

{

System.out.println("Speed of Bike: 50 Km/h");

}

Public static void main(String args[])

{

Bike bike = new Bike();

Bike.run();

Bike.speed();

}

}

Compile: 

Javac –d . Vehicle.java

Javac –d . Bike.java

Run:

Java vehicles.Bike

Output:

Bike is running

Speed of Bike: 50 Km/h

Q16) Write a sample program to implement an interface in Java?

A16) Program

Interface Results

{

Final static float pi = 3.14f;

Float areaOf(float l, float b);

}

Class Rectangle implements Results

{

Public float areaOf(float l, float b)

{

Return (l * b);

}

}

Class Square implements Results 

{

Public float areaOf(float l, float b)

{

Return (l * l);

}

}

Class Circle implements Results

{

Public float areaOf(float r, float b)

{

Return (pi * r * r);

}

}

Public class InterfaceDemo

{

Public static void main(String args[])

{

Rectangle rect = new Rectangle();

Square square = new Square();

Circle circle = new Circle();

System.out.println("Area of Rectangle: "+rect.areaOf(20.3f, 28.7f));

System.out.println("Are of square: "+square.areaOf(10.0f, 10.0f));

System.out.println("Area of Circle: "+circle.areaOf(5.2f, 0));

}

}

Output:

Area of Rectangle: 582.61

Are of square: 100.0

Area of Circle: 84.905594

Q17) Write a sample program to implement multiple inheritance?

A17) Program

Interface Vehicle

{ 

Void run(); 

} 

Interface Bike extends Vehicle

{ 

Void stop(); 

} 

Public class Demo implements Bike

{ 

Public void run()

{

System.out.println("Vehicle is running.");

} 

Public void stop()

{

System.out.println("Bike is stop.");

} 

Public static void main(String args[])

{ 

Demo obj = new Demo(); 

Obj.run(); 

Obj.stop(); 

} 

}

Output:

Vehicle is running.

Bike is stop.

Q18) Write a difference between package and interface?

A18) Key difference between Package and Interface

●      An interface is a collection of abstract methods, whereas a package is a collection of classes and interfaces.

●      A keyword package is used to construct a package, whereas a keyword interface is used to build an interface.

●      If a package's class or interface is to be utilized, the package must be imported, and the interface must be implemented.

Unit - 4

Interfaces and Packages

Q1) What is the interface?

A1) An interface is a reference type in Java. It is similar to class. It is a collection of abstract methods. A class implements an interface, thereby inheriting the abstract methods of the interface.

Along with abstract methods, an interface may also contain constants, default methods, static methods, and nested types. Method bodies exist only for default methods and static methods.

Writing an interface is similar to writing a class. But a class describes the attributes and behaviors of an object. And an interface contains behaviors that a class implements.

Unless the class that implements the interface is abstract, all the methods of the interface need to be defined in the class.

An interface is similar to a class in the following ways −

●        An interface can contain any number of methods.

●        An interface is written in a file with a .java extension, with the name of the interface matching the name of the file.

●        The byte code of an interface appears in a .class file.

●        Interfaces appear in packages, and their corresponding bytecode file must be in a directory structure that matches the package name.

However, an interface is different from a class in several ways, including −

●        You cannot instantiate an interface.

●        An interface does not contain any constructors.

●        All of the methods in an interface are abstract.

●        An interface cannot contain instance fields. The only fields that can appear in an interface must be declared both static and final.

●        An interface is not extended by a class; it is implemented by a class.

●        An interface can extend multiple interfaces.

Syntax

Public interface XYZ   

{  

Public void method();  

}

To use an interface in a class, Java provides a keyword called implements. We provide the necessary implementation of the method that we have declared in the interface.

Q2) Write the features of interface?

A2) Features of Interface:

• We can achieve total abstraction.
• We can use multiple interfaces in a class that leads to multiple inheritance.
• It also helps to achieve loose coupling.

Q3) Compare interface and abstract class?

A3) Both abstract classes and interfaces are used to achieve abstraction so that abstract methods can be declared. Both the abstract class and the interface cannot be instantiated.

However, there are other distinctions between abstract class and interface, which are shown below.

Q4) How to declare an interface?

A4) Declaring Interfaces

The interface keyword is used to declare an interface. Here is a simple example to declare an interface −

Example

Following is an example of an interface −

/* File name : NameOfInterface.java */

Import java.lang.*;

// Any number of import statements

Public interface NameOfInterface {

// Any number of final, static fields

// Any number of abstract method declarations\

}

Interfaces have the following properties −

●        An interface is implicitly abstract. You do not need to use the abstract keyword while declaring an interface.

●        Each method in an interface is also implicitly abstract, so the abstract keyword is not needed.

●        Methods in an interface are implicitly public.

Example

/* File name : Animal.java */

Interface Animal {

Public void eat();

Public void travel();

}

Q5) How to implement the interface?

A5) Implementing Interfaces

When a class implements an interface, you can think of the class as signing a contract, agreeing to perform the specific behaviors of the interface. If a class does not perform all the behaviors of the interface, the class must declare itself as abstract.

A class uses the implements keyword to implement an interface. The implements keyword appears in the class declaration following the extends portion of the declaration.

Example

/* File name : MammalInt.java */

Public class MammalInt implements Animal {

Public void eat() {

System.out.println("Mammal eats");

}

Public void travel() {

System.out.println("Mammal travels");

}

Public int noOfLegs() {

Return 0;

}

Public static void main(String args[]) {

MammalInt m = new MammalInt();

m.eat();

m.travel();

}

}

This will produce the following result −

Output

Mammal eats

Mammal travels

When overriding methods defined in interfaces, there are several rules to be followed−

●        Checked exceptions should not be declared on implementation methods other than the ones declared by the interface method or subclasses of those declared by the interface method.

●        The signature of the interface method and the same return type or subtype should be maintained when overriding the methods.

●        An implementation class itself can be abstract and if so, interface methods need not be implemented.

When implementation interfaces, there are several rules −

●        A class can implement more than one interface at a time.

●        A class can extend only one class, but implement many interfaces.

●        An interface can extend another interface, in a similar way as a class can extend another class.

Q6) How to extend the interface?

A6) Extending Interfaces

An interface can extend another interface in the same way that a class can extend another class. The extends keyword is used to extend an interface, and the child interface inherits the methods of the parent interface.

The following Sports interface is extended by Hockey and Football interfaces.

Example

// Filename: Sports.java

Public interface Sports {

Public void setHomeTeam(String name);

Public void setVisitingTeam(String name);

}

// Filename: Football.java

Public interface Football extends Sports {

Public void homeTeamScored(int points);

Public void visitingTeamScored(int points);

Public void endOfQuarter(int quarter);

}

// Filename: Hockey.java

Public interface Hockey extends Sports {

Public void homeGoalScored();

Public void visitingGoalScored();

Public void endOfPeriod(int period);

Public void overtimePeriod(int ot);

}

The Hockey interface has four methods, but it inherits two from Sports; thus, a class that implements Hockey needs to implement all six methods. Similarly, a class that implements Football needs to define the three methods from Football and the two methods from Sports.

Q7) How to access implementation through interface references?

A7) Variables can be declared as object references that use an interface instead of a class.

Such a variable can refer to any instance that implements the stated interface.

When you use one of these references to invoke a method, the right version is used based on the actual instance of the interface being referred to.

This is one of the most important characteristics of interfaces. The method to be executed is dynamically looked up at runtime, allowing classes to be built after the code that calls methods on them.

This is analogous to accessing a subclass object using a superclass reference.

The callback() function is invoked in the following example using an interface reference variable:

Interface Callback {

Void callback(int param);

}

Class Client implements Callback {

// Implement Callback's interface

Public void callback(int p) {

System.out.println("callback called with " + p);

}

Void aa() {

System.out.println("hi.");

}

}

Public class Main{

Public static void main(String args[]) {

Callback c = new Client();

c.callback(42);

}

}

The following is the program's output:

Callback called with 42

Despite the fact that the variable c is of the interface type Callback, it has been assigned an instance of Client.

The callback() method can be used using c, but it cannot access any other members of the Client class.

Only the methods declared by the interface declaration are known to an interface reference variable.

As a result, because aa() is defined by Client but not Callback, c could not be used to access it.

Q8) Define package?

A8) Package

Advantage of Java Package

1) Java package is used to categorize the classes and interfaces so that they can be easily maintained.

2) Java package provides access protection.

3) Java package removes naming collision.

How to compile java package

If you are not using any IDE, you need to follow the syntax given below:

Javac -d directory javafilename 

How to run java package program

You need to use fully qualified name e.g. Mypack.Simple etc to run the class.

Q9) Write a simple example of java package?

A9) The package keyword is used to create a package in java

//save as Simple.java  

Package mypack;  

Public class Simple

{  

Public static void main(String args[]){  

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

}  

}  

To Compile: javac -d. Simple.java

To Run: java mypack.Simple

The -d is a switch that tells the compiler where to put the class file i.e. it represents destination. The. Represents the current folder.

OUTPUT: Welcome to package

Q10) How to access the package?

A10) Accessing Packages

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.*

1. If you use package.* then all the classes and interfaces of this package will be accessible but not subpackages.
2. 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.class \name 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");}  

}  

//save by B.java  

Package mypack;  

Import pack.A;  

Class B{  

Public static void main(String args[]){  

A obj = new A();  

Obj.msg();  

}  

}  

OUTPUT:  Hello

Using fully qualified name

Example of package by import fully qualified name

//save by A.java  

Package pack;  

Public class A

{  

Public void msg()

{   System.out.println("Hello");  }  

}  

//save by B.java  

Package mypack;  

Class B{  

Public static void main(String args[]){  

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

Obj.msg();  

}  

} 

OUTPUT: Hello

Q11) Describe classpath?

A11) CLASSPATH is an environment variable which is used by Application ClassLoader to locate and load the .class files. The CLASSPATH defines the path, to find third-party and user-defined classes that are not extensions or part of Java platform. Include all the directories which contain .class files and JAR files when setting the CLASSPATH.

You need to set the CLASSPATH if:

●        You need to load a class that is not present in the current directory or any sub-directories.

●        You need to load a class that is not in a location specified by the extensions mechanism.

The CLASSPATH depends on what you are setting the CLASSPATH. The CLASSPATH has a directory name or file name at the end. The following points describe what should be the end of the CLASSPATH.

●        If a JAR or zip, the file contains class files, the CLASSPATH end with the name of the zip or JAR file.

●        If class files placed in an unnamed package, the CLASSPATH ends with the directory that contains the class files.

●        If class files placed in a named package, the CLASSPATH ends with the directory that contains the root package in the full package name, that is the first package in the full package name.

The default value of CLASSPATH is a dot (.). It means the only current directory searched. The default value of CLASSPATH overrides when you set the CLASSPATH variable or using the -classpath command (for short -cp). Put a dot (.) in the new setting if you want to include the current directory in the search path.

If CLASSPATH finds a class file which is present in the current directory, then it will load the class and use it, irrespective of the same name class presents in another directory which is also included in the CLASSPATH.

If you want to set multiple classpaths, then you need to separate each CLASSPATH by a semicolon (;).

The third-party applications (MySQL and Oracle) that use the JVM can modify the CLASSPATH environment variable to include the libraries they use. The classes can be stored in directories or archives files. The classes of the Java platform are stored in rt.jar.

There are two ways to ways to set CLASSPATH: through Command Prompt or by setting Environment Variable.

Q12) How to Set CLASSPATH in Windows Using Command Prompt?

A12) Type the following command in your Command Prompt and press enter.

1. Set CLASSPATH=%CLASSPATH%;C:\Program Files\Java\jre1.8\rt.jar;  

In the above command, The set is an internal DOS command that allows the user to change the variable value. CLASSPATH is a variable name. The variable enclosed in percentage sign (%) is an existing environment variable. The semicolon is a separator, and after the (;) there is the PATH of rt.jar file.

How ext folder works in Java

The ext directory works a bit like the CLASSPATH. Ext directory is the part of the class loading mechanism. The classes which are available within JARs in the ext directory are available to Java applications.

Q13) How to set the CLASSPATH of a MySQL database?

A13) Let's see how to set the CLASSPATH of a MySQL database:

Step 1: Click on the Windows button and choose Control Panel. Select System.

Step 2: Click on Advanced System Settings.

Step 3: A dialog box will open. Click on Environment Variables.

Step 4: If the CLASSPATH already exists in System Variables, click on the Edit button then put a semicolon (;) at the end. Paste the Path of MySQL-Connector Java.jar file.

If the CLASSPATH doesn't exist in System Variables, then click on the New button and type Variable name as CLASSPATH and Variable value as C:\Program Files\Java\jre1.8\MySQL-Connector Java.jar;.;

Remember: Put ;.; at the end of the CLASSPATH.

Q14) How to import a package?

A14) There are many packages to choose from. In the previous example, we used the Scanner class from the java.util package. This package also contains date and time facilities, random-number generator and other utility classes.

To import a whole package, end the sentence with an asterisk sign (*). The following example will import ALL the classes in the java.util package:

Example

Import java.util.*;

User-defined Packages

To create your own package, you need to understand that Java uses a file system directory to store them. Just like folders on your computer:

Example

└── root

└── mypack

└── MyPackageClass.java

To create a package, use the package keyword:

MyPackageClass.java

Package mypack;

Class MyPackageClass {

Public static void main(String[] args) {

System.out.println("This is my package!");

}

}

Save the file as MyPackageClass.java, and compile it:

C:\Users\Your Name>javac MyPackageClass.java

Then compile the package:

C:\Users\Your Name>javac -d . MyPackageClass.java

This forces the compiler to create the "mypack" package.

The -d keyword specifies the destination for where to save the class file. You can use any directory name, like c:/user (windows), or, if you want to keep the package within the same directory, you can use the dot sign ".", like in the example above.

Note: The package name should be written in lower case to avoid conflict with class names.

When we compiled the package in the example above, a new folder was created, called "mypack".

To run the MyPackageClass.java file, write the following:

C:\Users\Your Name>java mypack.MyPackageClass

The output will be:

This is my package!

Q15) Write a Program to create and use a user defined package in Java?

A15) Program

// Vehicle.java

Package vehicles;

Interface Vehicle

{

Public void run();

Public void speed();

}

//Bike.java

Package vehicles;

Public class Bike implements Vehicle

{

Public void run()

{

System.out.println("Bike is running.");

}

Public void speed()

{

System.out.println("Speed of Bike: 50 Km/h");

}

Public static void main(String args[])

{

Bike bike = new Bike();

Bike.run();

Bike.speed();

}

}

Compile: 

Javac –d . Vehicle.java

Javac –d . Bike.java

Run:

Java vehicles.Bike

Output:

Bike is running

Speed of Bike: 50 Km/h

Q16) Write a sample program to implement an interface in Java?

A16) Program

Interface Results

{

Final static float pi = 3.14f;

Float areaOf(float l, float b);

}

Class Rectangle implements Results

{

Public float areaOf(float l, float b)

{

Return (l * b);

}

}

Class Square implements Results 

{

Public float areaOf(float l, float b)

{

Return (l * l);

}

}

Class Circle implements Results

{

Public float areaOf(float r, float b)

{

Return (pi * r * r);

}

}

Public class InterfaceDemo

{

Public static void main(String args[])

{

Rectangle rect = new Rectangle();

Square square = new Square();

Circle circle = new Circle();

System.out.println("Area of Rectangle: "+rect.areaOf(20.3f, 28.7f));

System.out.println("Are of square: "+square.areaOf(10.0f, 10.0f));

System.out.println("Area of Circle: "+circle.areaOf(5.2f, 0));

}

}

Output:

Area of Rectangle: 582.61

Are of square: 100.0

Area of Circle: 84.905594

Q17) Write a sample program to implement multiple inheritance?

A17) Program

Interface Vehicle

{ 

Void run(); 

} 

Interface Bike extends Vehicle

{ 

Void stop(); 

} 

Public class Demo implements Bike

{ 

Public void run()

{

System.out.println("Vehicle is running.");

} 

Public void stop()

{

System.out.println("Bike is stop.");

} 

Public static void main(String args[])

{ 

Demo obj = new Demo(); 

Obj.run(); 

Obj.stop(); 

} 

}

Output:

Vehicle is running.

Bike is stop.

Q18) Write a difference between package and interface?

A18) Key difference between Package and Interface

●      An interface is a collection of abstract methods, whereas a package is a collection of classes and interfaces.

●      A keyword package is used to construct a package, whereas a keyword interface is used to build an interface.

●      If a package's class or interface is to be utilized, the package must be imported, and the interface must be implemented.