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:
- variable
- method
- 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(){
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");}
public static void main(String args[]){
Honda honda= new Honda();
honda.run();
}
}
Output:Compile Time
Error
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. }
Output:Compile Time
Error
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
- Abstract class (0 to 100%)
- 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.
5.
class Honda extends Bike{
6.
void run(){System.out.println("running safely..");}
7.
8.
public static void main(String args[]){
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{
14.
public static void main(String args[]){
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{
3.
abstract void run();
4.
void changeGear(){System.out.println("gear changed");}
5.
}
6.
7.
class Honda extends Bike{
8.
void run(){System.out.println("running safely..");}
9.
10.
public static void main(String args[]){
11.
Bike obj = new Honda();
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");}
7.
abstract void run();
8.
}
9.
10.
class Honda extends Bike{
11.
void run(){System.out.println("running safely..");}
12.
13.
public static void main(String args[]){
14.
Bike obj = new Honda();
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
1.
interface printable{
2.
void print();
3.
}
4.
5.
class A implements printable{
6.
public void print(){System.out.println("Hello");}
7.
8.
public static void main(String args[]){
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.
14. public static void main(String args[]){
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
Simple
example of package
The package keyword is used to create a package.
1. //save as Simple.java
2.
package mypack;
public class Simple{
public static void main(String args[]){
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.
Output:Welcome to package
How to access package from another package?
Using
packagename.*
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{
7. public static void main(String args[]){
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{
5. public void msg(){System.out.println("Hello");}
6. }
1. //save by B.java
2.
3. package mypack;
4. import pack.A;
5.
6. class B{
7. public static void main(String args[]){
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{
5. public void msg(){System.out.println("Hello");}
6. }
1. //save by B.java
2.
3. package mypack;
4. class B{
5. public static void main(String args[]){
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:
There are many non-access modifiers such as
static, abstract etc. Here, we will learn access modifiers.
1) private
Simple example of private access modifier
1. class A{
2. private int data=40;
3. private void msg(){System.out.println("Hello java");}
4. }
5.
6. public class Simple{
7. public static void main(String args[]){
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:
1. class A{
2. private A(){}//private constructor
3.
4. void msg(){System.out.println("Hello java");}
5. }
6.
7. public class Simple{
8. public static void main(String args[]){
9. A obj=new A();//Compile Time Error
10. }
11. }
2) default
Example of default access modifier
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{
7. public static void main(String args[]){
8. A obj = new A();//Compile Time Error
9. obj.msg();//Compile Time Error
10. }
11. }
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
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{
7. public static void main(String args[]){
8. B obj = new B();
9. obj.msg();
10. }
11. }
Output:Hello
4) public
Example of public access modifier
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{
7. public static void main(String args[]){
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.
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
7. public static void main(String args[]){
8. Simple obj=new Simple();
9. obj.msg();
10. }
11. }
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.
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
Disadvantage of Array
Types of Array
There are two types of
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{
2. public static void main(String args[]){
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{
2. public static void main(String args[]){
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.
11. public static void main(String args[]){
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{
2. public static void main(String args[]){
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{
2. public static void main(String args[]){
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
|