core java notes with examples

Core Java Notes

Final Keyword In Java

The final keyword in java is used to restrict the user. The final keyword can be used in many context.

Final can be:

1. variable

2. method

3. class

The final keyword can be applied with the variables, a final variable that have no value it is called blank

final variable or uninitialized final variable. It can be initialized in the constructor only. The blank final

variable can be static also which will be initialized in the static block only. We will have detailed learning of

these. Let's first learn the basics of final keyword.

1) final variable

If you make any variable as final, you cannot change the value of final variable(It will be constant).

Example of final variable

There is a final variable speedlimit, we are going to change the value of this variable, but It can't be

changed because final variable once assigned a value can never be changed.

class Bike{

final int speedlimit=90;//final variable

void run(){

http://4.bp.blogspot.com/-DLmsIOxEwwo/VjG4Oeo5DOI/AAAAAAAACbo/aoT8jysGcc8/s1600/a.jpg

speedlimit=400;

}

public static void main(String args[]){

Bike obj=new Bike();

obj.run();

}

}//end of class

Output:Compile Time Error

________________________________________

2) final method

If you make any method as final, you cannot override it.

Example of final method

class Bike{

final void run(){System.out.println("running");}

}

class Honda extends Bike{

void run(){System.out.println("running safely with 100kmph");}


Honda honda= new Honda();

honda.run();

}

}


________________________________________

3) final class

If you make any class as final, you cannot extend it.

Example of final class

1. final class Bike{}

2.

3. class Honda extends Bike{

4. void run(){System.out.println("running safely with 100kmph");}

5.

6. public static void main(String args[]){

7. Honda honda= new Honda();

8. honda.run();

9. }

10. }


________________________________________

Abstract class in Java

A class that is declared with abstract keyword, is known as abstract class. Before learning abstract class,

let's understand the abstraction first.

Abstraction

Abstraction is a process of hiding the implementation details and showing only functionality to the user.

Another way, it shows only important things to the user and hides the internal details for example sending

sms, you just type the text and send the message. You don't know the internal processing about the

message delivery.

Abstraction lets you focus on what the object does instead of how it does it.

Ways to achieve Abstaction

There are two ways to achieve abstraction in java

1. Abstract class (0 to 100%)

2. Interface (100%)

________________________________________

Abstract class

A class that is declared as abstract is known as abstract class. It needs to be extended and its method

implemented. It cannot be instantiated.

Syntax to declare the abstract class

1. abstract class <class_name>{}

abstract method

A method that is declared as abstract and does not have implementation is known as abstract method.

Syntax to define the abstract method

1. abstract return_type <method_name>();//no braces{}

Example of abstract class that have abstract method

In this example, Bike the abstract class that contains only one abstract method run. It implementation is

provided by the Honda class.

1. abstract class Bike{

2. abstract void run();

3. }

4.


6. void run(){System.out.println("running safely..");}

7.


9. Bike obj = new Honda();

10. obj.run();

11. }

12. }

Output:running safely..

________________________________________

Understanding the real scenario of abstract class

In this example, Shape is the abstract class, its implementation is provided by the Rectangle and Circle

classes. Mostly, we don't know about the implementation class (i.e. hidden to the end user) and object of

the implementation class is provided by the factory method.

A factory method is the method that returns the instance of the class. We will learn about the factory

method later.

In this example, if you create the instance of Rectangle class, draw method of Rectangle class will be

invoked.

1. abstract class Shape{

2. abstract void draw();

3. }

4.

5. class Rectangle extends Shape{

6. void draw(){System.out.println("drawing rectangle");}

7. }

8.

9. class Circle extends Shape{

10. void draw(){System.out.println("drawing circle");}

11. }

12.

13. class Test{


15. Shape s=new Circle();

16. //In real scenario, Object is provided through factory method

17. s.draw();

18. }

19. }

Output:drawing circle

________________________________________

Abstract class having constructor, data member, methods etc.

Note: An abstract class can have data member, abstract method, method body, constructor and even

main() method.

1. //example of abstract class that have method body

2. abstract class Bike{


4. void changeGear(){System.out.println("gear changed");}

5. }

6.



9.



12. obj.run();

13. obj.changeGear();

14. }

15. }

Output:running safely..

gear changed

1. //example of abstract class having constructor, field and method

2. abstract class Bike

3. {

4. int limit=30;

5. Bike(){System.out.println("constructor is invoked");}

6. void getDetails(){System.out.println("it has two wheels");}


8. }

9.



12.



15. obj.run();

16. obj.getDetails();

17. System.out.println(obj.limit);

18. }

19. }

Output:constructor is invoked

running safely..

it has two wheels

30

Rule: If there is any abstract method in a class, that class must be abstract.

Interface in Java

An interface is a blueprint of a class. It has static constants and abstract methods.

The interface is a mechanism to achieve fully abstraction in java. There can be only abstract methods in

the interface. It is used to achieve fully abstraction and multiple inheritance in Java.

Interface also represents IS-A relationship.

It cannot be instantiated just like abstract class.

Why use Interface?

There are mainly three reasons to use interface. They are given below.

• It is used to achieve fully abstraction.

• By interface, we can support the functionality of multiple inheritance.

• It can be used to achieve loose coupling.

The java compiler adds public and abstract keywords before the interface method and public, static and

final keywords before data members.

In other words, Interface fields are public, static and final bydefault, and methods are public and abstract.

________________________________________

Understanding relationship between classes and interfaces

As shown in the figure given below, a class extends another class, an interface extends another interface

but a class implements an interface.

________________________________________

Simple example of Interface

In this example, Printable interface have only one method, its implementation is provided in the A class.

1. interface printable{

2. void print();

3. }

4.

5. class A implements printable{

6. public void print(){System.out.println("Hello");}

7.


9. A obj = new A();

10. obj.print();

11. }

12. }

Output:Hello

________________________________________

Multiple inheritance in Java by interface

If a class implements multiple interfaces, or an interface extends multiple interfaces i.e. known as multiple

inheritance.

1. interface Printable{

2. void print();

3. }

4.

5. interface Showable{

6. void show();

7. }

8.

9. class A implements Printable,Showable{

10.

11. public void print(){System.out.println("Hello");}

12. public void show(){System.out.println("Welcome");}

13.


15. A obj = new A();

16. obj.print();

17. obj.show();

18. }

19. }

Output:Hello

Welcome

Package in Java

A package is a group of similar types of classes, interfaces and sub-packages.

Package can be categorized in two form, built-in package and user-defined package. There are many

built-in packages such as java, lang, awt, javax, swing, net, io, util, sql etc.

Here, we will have the detailed learning of creating and using user-defined packages.

Advantage of Package

• Package is used to categorize the classes and interfaces so that they can be easily maintained.

• Package provids access protection.

• Package removes naming collision.

________________________________________

Simple example of package

The package keyword is used to create a package.

1. //save as Simple.java

2.

package mypack;

public class Simple{


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

}

}

How to compile the Package (if not using IDE)

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

1. javac -d directory javafilename

For example

1. javac -d . Simple.java

The -d switch specifies the destination where to put the generated class file. You can use any directory

name like /home (in case of Linux), d:/abc (in case of windows) etc. If you want to keep the package

within the same directory, you can use . (dot).

________________________________________

How to run the Package (if not using IDE)

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

________________________________________

To Compile: javac -d . Simple.java

To Run: java mypack.Simple

Output:Welcome to package

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.

________________________________________

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.

Using packagename.*

If you use package.* then all the classes and interfaces of this package will be accessible but 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.*

1. //save by A.java

2.

3. package pack;

4. public class A{

5. public void msg(){System.out.println("Hello");}

6. }

1. //save by B.java

2.

3. package mypack;

4. import pack.*;

5.

6. class B{


8. A obj = new A();

9. obj.msg();

10. }

11. }

Output:Hello

________________________________________

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

1. //save by A.java

2.

3. package pack;

4. public class A{


6. }

1. //save by B.java

2.

3. package mypack;

4. import pack.A;

5.

6. class B{


8. A obj = new A();

9. obj.msg();

10. }

11. }

Output:Hello

________________________________________

Using fully qualified name

If you use fully qualified name then only declared class of this package will be accessible. Now there is no

need to import. But you need to use fully qualified name every time when you are accessing the class or

interface.

It is generally used when two packages have same class name e.g. java.util and java.sql packages

contain Date class.

Example of package by import fully qualified name

1. //save by A.java

2.

3. package pack;

4. public class A{


6. }

1. //save by B.java

2.

3. package mypack;

4. class B{


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

7. obj.msg();

8. }

9. }

Output:Hello

Note: Sequence of the program must be package then import then class.

________________________________________

________________________________________

Access Modifiers

There are two types of modifiers in java: access modifier and non-access modifier. The access modifiers

specifies accessibility (scope) of a datamember, method, constructor or class.

There are 4 types of access modifiers:

1. private

2. default

3. protected

4. public

There are many non-access modifiers such as static, abstract etc. Here, we will learn access modifiers.

________________________________________

1) private

The private access modifier is accessible only within class.

Simple example of private access modifier

In this example, we have created two classes A and Simple. A class contains private data member and

private method. We are accessing these private members from outside the class, so there is compile time

error.

1. class A{

2. private int data=40;

3. private void msg(){System.out.println("Hello java");}

4. }

5.

6. public class Simple{


8. A obj=new A();

9. System.out.println(obj.data);//Compile Time Error

10. obj.msg();//Compile Time Error

11. }

12. }

Role of Private Constructor:

If you make any class constructor private, you cannot create the instance of that class from outside the

class. For example:

1. class A{

2. private A(){}//private constructor

3.

4. void msg(){System.out.println("Hello java");}

5. }

6.

7. public class Simple{


9. A obj=new A();//Compile Time Error

10. }

11. }

________________________________________

2) default

If you don't use any modifier, it is treated as default bydefault. The default modifier is accessible only

within package.

Example of default access modifier

In this example, we have created two packages pack and mypack. We are accessing the A class from

outside its package, since A class is not public, so it cannot be accessed from outside the package.

1. //save by A.java

2.

3. package pack;

4. class A{

5. void msg(){System.out.println("Hello");}

6. }

1. //save by B.java

2.

3. package mypack;

4. import pack.*;

5.

6. class B{


8. A obj = new A();//Compile Time Error

9. obj.msg();//Compile Time Error

10. }

11. }

In the above example, the scope of class A and its method msg() is default so it cannot be accessed from

outside the package.

________________________________________

3) protected

The protected access modifier is accessible within package and outside the package but through

inheritance only.

The protected access modifier can be applied on the data member, method and constructor. It can't be

applied on the class.

Example of protected access modifier

In this example, we have created the two packages pack and mypack. The A class of pack package is

public, so can be accessed from outside the package. But msg method of this package is declared as

protected, so it can be accessed from outside the class only through inheritance.

1. //save by A.java

2.

3. package pack;

4. public class A{

5. protected void msg(){System.out.println("Hello");}

6. }

1. //save by B.java

2.

3. package mypack;

4. import pack.*;

5.

6. class B extends A{


8. B obj = new B();

9. obj.msg();

10. }

11. }

Output:Hello

________________________________________

4) public

The public access modifier is accessible everywhere. It has the widest scope among all other modifiers.

Example of public access modifier

1. //save by A.java

2.

3. package pack;

4. public class A{


6. }

1. //save by B.java

2.

3. package mypack;

4. import pack.*;

5.

6. class B{


8. A obj = new A();

9. obj.msg();

10. }

11. }

Output:Hello

Understanding all java access modifiers

Let's understand the access modifiers by a simple table.

Access Modifier within class within package outside package by subclass only outside package

Private Y N N N

Default Y Y N N

Protected Y Y Y N

Public Y Y Y Y

________________________________________

Applying access modifier with method overriding

If you are overriding any method, overridden method (i.e. declared in subclass) must not be more

restrictive.

1. class A{

2. protected void msg(){System.out.println("Hello java");}

3. }

4.

5. public class Simple extends A{

6. void msg(){System.out.println("Hello java");}//C.T.Error


8. Simple obj=new Simple();

9. obj.msg();

10. }

11. }

The default modifier is more restrictive than protected. That is why there is compile time error.

Encapsulation in Java

Encapsulation is a process of wrapping code and data together into a single unit e.g. capsule i.e mixed of

several medicines.

We can create a fully encapsulated class by making all the data members of the class private. Now we

can use setter and getter methods to set and get the data in it.

Array in Java

Normally, array is a collection of similar type of elements that have contiguous memory location.

In java, array is an object the contains elements of similar data type. It is a data structure where we store

similar elements. We can store only fixed elements in an array.

Array is index based, first element of the array is stored at 0 index.

Advantage of Array

• Code Optimization: It makes the code optimized, we can retrieve or sort the data easily.

• Random access: We can get any data located at any index position.

________________________________________

Disadvantage of Array

• Size Limit: We can store only fixed size of elements in the array. It doesn't grow its size at runtime. To

solve this problem, collection framework is used in java.

________________________________________

Types of Array

There are two types of array.

• Single Dimensional Array

• Multidimensional Array

________________________________________

Single Dimensional Array

Syntax to Declare an Array in java

1. dataType[] arrayRefVar; (or)

2. dataType []arrayRefVar; (or)

3. dataType arrayRefVar[];

Instantiation of an Array in java

1. arrayRefVar=new datatype[size];

Example of single dimensional java array

Let's see the simple example of java array, where we are going to declare, instantiate, initialize and

traverse an array.

1. class B{


3.

4. int a[]=new int[5];//declaration and instantiation

5. a[0]=10;//initialization

6. a[1]=20;

7. a[2]=70;

8. a[3]=40;

9. a[4]=50;

10.

11. //printing array

12. for(int i=0;i<a.length;i++)//length is the property of array

13. System.out.println(a[i]);

14.

15. }}

Output: 10

20

70

40

50

________________________________________

Declaration, Instantiation and Initialization of Java Array

We can declare, instantiate and initialize the java array together by:

1. int a[]={33,3,4,5};//declaration, instantiation and initialization

Let's see the simple example to print this array.

1. class B{


3.

4. int a[]={33,3,4,5};//declaration, instantiation and initialization

5.

6. //printing array

7. for(int i=0;i<a.length;i++)//length is the property of array

8. System.out.println(a[i]);

9.

10. }}

Output:33

3

4

5

________________________________________

Passing Java Array in the method

We can pass the array in the method so that we can reuse the same logic on any array.

Let's see the simple example to get minimum number of an array using method.

1. class B{

2. static void min(int arr[]){

3. int min=arr[0];

4. for(int i=1;i<arr.length;i++)

5. if(min>arr[i])

6. min=arr[i];

7.

8. System.out.println(min);

9. }

10.


12.

13. int a[]={33,3,4,5};

14. min(a);//passing array in the method

15.

16. }}

Output:3

________________________________________

Multidimensional array

In such case, data is stored in row and column based index (also known as matrix form).

Syntax to Declare Multidimensional Array in java

1. dataType[][] arrayRefVar; (or)

2. dataType [][]arrayRefVar; (or)

3. dataType arrayRefVar[][]; (or)

4. dataType []arrayRefVar[];

Example to instantiate Multidimensional Array in java

1. int[][] arr=new int[3][3];//3 row and 3 column

Example to initialize Multidimensional Array in java

1. arr[0][0]=1;

2. arr[0][1]=2;

3. arr[0][2]=3;

4. arr[1][0]=4;

5. arr[1][1]=5;

6. arr[1][2]=6;

7. arr[2][0]=7;

8. arr[2][1]=8;

9. arr[2][2]=9;

Example of Multidimensional java array

Let's see the simple example to declare, instantiate, initialize and print the 2Dimensional array.

1. class B{


3.

4. //declaring and initializing 2D array

5. int arr[][]={{1,2,3},{2,4,5},{4,4,5}};

6.

7. //printing 2D array

8. for(int i=0;i<3;i++){

9. for(int j=0;j<3;j++){

10. System.out.print(arr[i][j]+" ");

11. }

12. System.out.println();

13. }

14.

15. }}

Output:1 2 3

2 4 5

4 4 5

________________________________________

________________________________________

Copying an array

We can copy an array to another by the arraycopy method of System class.

Syntax of arraycopy method

1. public static void arraycopy(

2. Object src, int srcPos,Object dest, int destPos, int length

3. )

Example of arraycopy method

1. class ArrayCopyDemo {

2. public static void main(String[] args) {

3. char[] copyFrom = { 'd', 'e', 'c', 'a', 'f', 'f', 'e',

4. 'i', 'n', 'a', 't', 'e', 'd' };

5. char[] copyTo = new char[7];

6.

7. System.arraycopy(copyFrom, 2, copyTo, 0, 7);

8. System.out.println(new String(copyTo));

9. }

10. }

Output:caffein

________________________________________

Addition 2 matrices

Let's see a simple example that adds two matrices.

1. class AE{


3. //creating two matrices

4. int a[][]={{1,3,4},{3,4,5}};

5. int b[][]={{1,3,4},{3,4,5}};

6.

7. //creating another matrix to store the sum of two matrices

8. int c[][]=new int[2][3];

9.

10. //adding and printing addition of 2 matrices

11. for(int i=0;i<2;i++){

12. for(int j=0;j<3;j++){

13. c[i][j]=a[i][j]+b[i][j];

14. System.out.print(c[i][j]+" ");

15. }

16. System.out.println();//new line

17. }

18.

19. }}

Output:2 6 8

6 8 10

Lab exercises

Model 2(Lab)

1.Wrire a Proram to creat a class that stores the train reservation details. In addition, define a

method that will display the stored details.

public class Reservation {

int ticketID;

String name;

String source;

String destination;

Reservation() {

ticketID = 80;

name ="Harry";

source = "Chicago"

destination = "Dallas";

}

public void showTicket () {

System.out.println("The Ticket ID is "+ ticketID);

System.out.println("The Passenger Name is "+ name);

System.out.println("The Sourse is" + sourse);

System.out.println("The Destination is" + Destination);

System.out.println("\nChoose the option: ");

public static void main(String[] args) {

Reservation g = new Reservation();

g.showTicket();

}

}

Q 2. Write a program to create a class named EmployeeDetails and display a menue similar to the

following menu:

--------------Menu--------

1. Enter Data

2. Display Data

3. Exit

Choose the option

Thereafter, invoke the respective method according to the given menu input. The methods will

contain appropriate message, such as the displayData()

method will contain the message, displayData method is invoked

public class EmployeeDetails {

public void showMenue () {

int option;

System.out.println("------------Menu--------");

System.out.println("1. Enter Data");

System.out.println("2. Display Data");

System.out.println("3. Exit");

System.out.println("\nChoose the option: ");

Option = 2;

switch (option) {

case 1:

enterData();

break;

case2:

DisplayData();

break;

case3:

exitMenue();

break;

defalt:

System.out.println("Incorrect menu option");

showMenu();

break;

}

}

public void enterData() {

System.out.println("enterData method is invoked");

}


System.out.println("displayData method is invoked");

}


System.out.println("exitMenu method is invoked");

System.exit(0);

}

public static void main(String[] args) {

EmployeeDetails obj = new EmployeeDetails();

obj.showMenu();

}

}

3.Write a program to create a class that stores the grocery details. In addition, define three methods that

will add the weight, remove the weight, and display the current weight, respectively..

4.Write a program to create a class that declares a member variable and a member method. The

member method will display the value of the member variable.

Ans.Q 4. Write a program that will store the employee records, such as employee ID, employee

name, department, designation,

date of joining, date of birth, and marital status, in an array. In addition, the stored record needs to

http://4.bp.blogspot.com/-jMveP4bNMrc/Vm2b5m-VzsI/AAAAAAAAC9c/eD6gqxAs54I/s1600/03.png

be displayed.

Ans

public class employeerecord

string employeeDetails [] [] = new String [1] [7];

public void storeData() {

employeeDeatils [0] [0] = "A101";

employeeDeatils [0] [1] = "john mathew";

employeeDetails [0] [2] = "admin";

employeeDetails [0] [3] = "manager";

employeeDetails [0] [4] = "05/04/1998";

employeeDetails [0] [5] = "11/09/1997";

employeeDetails [0] [6] = "married";

}

public void displayData() {

system.out.println ("Employee Details:");

for ( int i = 0; i < employeeDetails,length;

for ( int j = 0; j < employeeDetails[i].length; j++ {

system.out.println(employeeDetails[i] [j] );

}

}

}

public static void main (String [] args) {

EmployeeRecord obj = new EmployeeRecord();

obj.storeData();

obj.displayData();

}

}

5.Write a program to identify whether the given character is a vowel or a consonant.

Ans.

6.Write a program to display the following numeric pattern:

12345

1234

123

12

1

Ans

http://3.bp.blogspot.com/-8JiaDismS7g/Vm2cbIYGoTI/AAAAAAAAC9g/xeWpDxOBN6g/s1600/04.png

.7.Using the NetBeans IDE, create an Employee class, create a class with a main method to test the

Employee class, compile and run your application, and print the results to the command line output.

Ans.

1. start the NetBeans IDE by using the icon from desktop.

2. create a new project employeepractice in the D:\labs\02-review\practices directory with anemployeetest

main class in the com.example package.

3. Set the source/binary format to JDK 7.

a. right-click the project and select properties.

b. select JDK 7 from the drop-down list for SOurc/binary format.

c. click ok.

4. create another package called com.example.domain.

5. Add a java class called Employee in the com.example.domain package.

http://4.bp.blogspot.com/-8qanGSGCSxc/Vm2ctEc7R3I/AAAAAAAAC9o/oc3UDVP0oBk/s1600/05.png

6. code the employee class.

a. add the following data fields to the employee class-use your judgment as to what you want to call these

fields in the class. Refer to the lesson material for ideas on the fields names and the syntax if you are not

sure. Use public as the access modifier.

7. create a no-arg constructor for the employee class.

Netbeans can format your code at any time for you. Right-click in the class and select format, or press the

Alt-Shift-F key combination.

8. Add accessor/mutator methods for each of the fields.

9. write code in the employeetest class to test your employee class.

a. construct an instance of employee.

b. use the setter mothods to assign the following values to the instance:

c. in the body of the main methoed, use the system.out.printIn method to write the values of the employee

fields to the console output.

d. rsolve any missing import statements.

e. save the employeetest class.

10. run the EmployeePractice project.

11. Add some additional employee instances to your test class.

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core java notes with examples

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