Sunday, 1 September 2019

Chapter 2:- Class and Object

22:44    

Classes and Objects

 What is a class in Java
A class is a group of objects which have common properties. It is a template or blueprint from which objects are created. It is a logical entity. It can't be physical.
A class in Java can contain:
oFields
oMethods
oConstructors
oBlocks
oNested class and interface

Syntax to declare a class:
class <class_name>
{
field;
method;
}

Instance variable in Java
A variable which is created inside the class but outside the method is known as an instance variable. Instance variable doesn't get memory at compile time. It gets memory at runtime when an object or instance is created. That is why it is known as an instance variable.
new keyword in Java
The new keyword is used to allocate memory at runtime. All objects get memory in Heap memory area.

Object and Class Example: main within the class
In this example, we have created a Student class which has two data members id and name. We are creating the object of the Student class by new keyword and printing the object's value.
Here, we are creating a main() method inside the class.

class Student
{
int id;//field or data member or instance variable
String name;
public static void main(String args[])
{
Student s1=new Student();//creating an object of Student
System.out.println(s1.id);//accessing member through reference variable
System.out.println(s1.name);
}
}
Compile by: javac Student.java
Run by: java Student
0
null

Object and Class Example: main outside the class

In real time development, we create classes and use it from another class. It is a better approach than previous one. Let's see a simple example, where we are having main() method in another class.
We can have multiple classes in different Java files or single Java file. If you define multiple classes in a single Java source file, it is a good idea to save the file name with the class name which has main() method.

class Student
{
int id;
String name;
}
class TestStudent1
{
public static void main(String args[])
{
Student s1=new Student();
System.out.println(s1.id);
System.out.println(s1.name);
}
}

Compile by: javac TestStudent1.java
Run by: java TestStudent1
0
null

3 Ways to initialize object
There are 3 ways to initialize object in Java.
1.By reference variable
2.By method
3.By constructor

1)Object and Class Example: Initialization through reference
Initializing an object means storing data into the object. Let's see a simple example where we are going to initialize the object through a reference variable.
class Student
{
int id;
String name;
}

class TestStudent2
{
public static void main(String args[])
{
Student s1=new Student();
s1.id=101;
s1.name="Sonoo";
System.out.println(s1.id+" "+s1.name);//printing members with a white space
}
}
Compile by: javac TestStudent2.java
Run by: java TestStudent2
101 Sonoo


2)Object and Class Example: Initialization through method
In this example, we are creating the two objects of Student class and initializing the value to these objects by invoking the insertRecord method. Here, we are displaying the state (data) of the objects by invoking the displayInformation() method.

class Student
{ 
int rollno; 
String name; 
void insertRecord(int r, String n)
{ 
rollno=r; 
name=n; 
} 
void displayInformation()
{
System.out.println(rollno+" "+name);
} 
}
 
class TestStudent4
{ 
public static void main(String args[]){ 
Student s1=new Student(); 
Student s2=new Student(); 
s1.insertRecord(111,"Karan"); 
s2.insertRecord(222,"Aryan"); 
s1.displayInformation(); 
s2.displayInformation(); 
} 
} 

Compile by: javac TestStudent4.java
Run by: java TestStudent4
111 Karan
222 Aryan


3)Object and Class Example: Initialization through a constructor
class Employee
{ 
int id; 
String name; 
float salary; 
void insert(int i, String n, float s)
{ 
id=i; 
name=n; 
salary=s; 
} 
void display()
{
System.out.println(id+" "+name+" "+salary);
} 
} 
public class TestEmployee
{ 
public static void main(String[] args)
{ 
Employee e1=new Employee(); 
Employee e2=new Employee(); 
Employee e3=new Employee(); 
e1.insert(101,"ajeet",45000); 
e2.insert(102,"irfan",25000); 
e3.insert(103,"nakul",55000); 
e1.display(); 
e2.display(); 
e3.display(); 
} 
} 
Output:
101 ajeet 45000.0
102 irfan 25000.0
103 nakul 55000.0


Object and Class Example: Rectangle
class Rectangle
{ 
int length; 
 int width; 
void insert(int l, int w)
{ 
length=l; 
width=w; 
}
void calculateArea()
{
System.out.println(length*width);
} 
} 
class TestRectangle1
{ 
public static void main(String args[])
{ 
Rectangle r1=new Rectangle(); 
Rectangle r2=new Rectangle(); 
r1.insert(11,5); 
r2.insert(3,15); 
r1.calculateArea(); 
r2.calculateArea(); 
} 
} 
Output:
55
45    

Anonymous object
Anonymous simply means nameless. An object which has no reference is known as an anonymous object. It can be used at the time of object creation only.
If you have to use an object only once, an anonymous object is a good approach. For example:
1.new Calculation();//anonymous object 
Calling method through a reference:
1.Calculation c=new Calculation(); 
2.c.fact(5); 

Calling method through an anonymous object
1.new Calculation().fact(5); 

class Calculation
{ 
void fact(int  n)
{ 
int fact=1; 
for(int i=1;i<=n;i++)
{ 
fact=fact*i; 
} 
System.out.println("factorial is "+fact); 
} 
public static void main(String args[])
{ 
new Calculation().fact(5);//calling method with anonymous object 
} 
} 
Output:
Factorial is 120

Constructors in Java
It is a special type of method which is used to initialize the object.
Every time an object is created using the new() keyword, at least one constructor is called.
Rules for creating Java constructor
There are two rules defined for the constructor.
1.Constructor name must be the same as its class name
2.A Constructor must have no explicit return type
3.A Java constructor cannot be abstract, static, final, and synchronized

We can use access modifiers while declaring a constructor. It controls the object creation. In other words, we can have private, protected, public or default constructor in Java.
Types of Java constructors
There are two types of constructors in Java:
1.Default constructor (no-arg constructor)
2.Parameterized constructor

Java Default Constructor
A constructor is called "Default Constructor" when it doesn't have any parameter.
Syntax of default constructor:
1.<class_name>(){} 

//Java Program to create and call a default constructor 
class Bike1
{ 
//creating a default constructor 
Bike1(){System.out.println("Bike is created");
} 
//main method 
public static void main(String args[])
{ 
//calling a default constructor 
Bike1 b=new Bike1(); 
} 
} 
Output:-Bike is created


Java Parameterized Constructor
A constructor which has a specific number of parameters is called a parameterized constructor.
Why use the parameterized constructor?
The parameterized constructor is used to provide different values to distinct objects. However, you can provide the same values also.
//Java Program to demonstrate the use of the parameterized constructor. 
class Student4
{ 
 int id; 
String name; 
//creating a parameterized constructor 
Student4(int i,String n)
{ 
id = i; 
name = n; 
} 
//method to display the values 
void display()
{
System.out.println(id+" "+name);
} 
 public static void main(String args[])
{ 
//creating objects and passing values 
Student4 s1 = new Student4(111,"Karan"); 
Student4 s2 = new Student4(222,"Aryan"); 
//calling method to display the values of object 
s1.display(); 
s2.display(); 
} 
} 
Output:
111 Karan
222 Aryan

Does constructor return any value?
Yes, it is the current class instance (You cannot use return type yet it returns a value).

Can constructor perform other tasks instead of initialization?
Yes, like object creation, starting a thread, calling a method, etc. You can perform any operation in the constructor as you perform in the method.

Is there Constructor class in Java?
Yes.

Inheritance in Java
Inheritance in Java is a mechanism in which one object acquires all the properties and behaviors of a parent object. It is an important part of OOPs (Object Oriented programming system).
The idea behind inheritance in Java is that you can create new classes that are built upon existing classes. When you inherit from an existing class, you can reuse methods and fields of the parent class. Moreover, you can add new methods and fields in your current class also.
Inheritance represents the IS-A relationship which is also known as a parent-child relationship.
Why use inheritance in java
oFor Method Overriding (so runtime polymorphism can be achieved).
oFor Code Reusability.

Terms used in Inheritance
oClass: A class is a group of objects which have common properties. It is a template or blueprint from which objects are created.
oSub Class/Child Class: Subclass is a class which inherits the other class. It is also called a derived class, extended class, or child class.
oSuper Class/Parent Class: Superclass is the class from where a subclass inherits the features. It is also called a base class or a parent class.
oReusability: As the name specifies, reusability is a mechanism which facilitates you to reuse the fields and methods of the existing class when you create a new class. You can use the same fields and methods already defined in the previous class.

The syntax of Java Inheritance
class Subclass-name extends Superclass-name 
{ 
   //methods and fields 
} 
The extends keyword indicates that you are making a new class that derives from an existing class. The meaning of "extends" is to increase the functionality.

 Programmer is the subclass and Employee is the superclass. The relationship between the two classes is Programmer IS-A Employee. It means that Programmer is a type of Employee.

 class Employee
{  
float salary=40000;  
}  
class Programmer extends Employee
{  
int bonus=10000;  
public static void main(String args[])
{  
Programmer p=new Programmer();  
System.out.println("Programmer salary is:"+p.salary);  
System.out.println("Bonus of Programmer is:"+p.bonus);  
}  
}  
Programmer salary is:40000.0
Bonus of programmer is:10000

In the above example, Programmer object can access the field of own class as well as of Employee class i.e. code reusability.

Types of inheritance in java

























Single Inheritance in Java with Example
Inheritance is one of the key features of object-oriented programming (OOP).Single Inheritance enables a derived class(Sub class) to inherit properties and behavior from a single parent class(Super class). When a class extends another one class only then we  call it a single inheritance.
public class Shape
{
int length;
int breadth;
}
public class Rectangle extends Shape
{
int area;
public void calcualteArea()
{
area = length*breadth;
}
public static void main(String args[])
{
Rectangle r = new Rectangle();
//Assigning values to Shape class attributes
r.length = 10;
r.breadth = 20;
//Calculate the area
r.calcualteArea();
System.out.println("The Area of rectangle of length \""
+r.length+"\" and breadth \""+r.breadth+"\" is \""+r.area+"\"");
}
}

Output :
The Area of rectangle of length "10" and breadth "20" is "200"

Multiple Inheritance in Java Example
Multiple Inheritance in Java is nothing but one class extending more than one class. Java does not have this capability. As the designers considered that multiple inheritance will to be too complex to manage, but indirectly you can achieve Multiple Inheritance in Java using Interfaces.
Multiple Inheritance” refers to the concept of one class extending (Or inherits) more than one base class. Here we have two interfaces Car and Bus.
•Car interface has a attribute speed and a method defined distanceTravelled()
•Bus interface has a attribute distance and method speed()
The Vehicle class implements both interface Car and Bus and provides implementation.

interface Car
{
int  speed=60;
public void distanceTravelled();
}
interface Bus
{
int distance=100;
public void speed();
}
public class Vehicle  implements Car,Bus
{
int distanceTravelled;
int averageSpeed;
public void distanceTravelled()
{
distanceTravelled=speed*distance;
System.out.println("Total Distance Travelled is : "+distanceTravelled);
}
public void speed()
{
int averageSpeed=distanceTravelled/speed;
System.out.println("Average Speed maintained is : "+averageSpeed);
}
public static void main(String args[])
{
Vehicle v1=new Vehicle();
v1.distanceTravelled();
v1.speed();
}
}
Output :
Total Distance Travelled is : 6000
Average Speed maintained is : 100

Multilevel Inheritance in Java with Example
In Java Multilevel Inheritance sub class will be inheriting a parent class and as well as the sub class act as the parent class to other class

When a class extends a class, which extends anther class then this is called multilevel inheritance.

Class X
{
public void methodX()
{
System.out.println("Class X method");
}
}
Class Y extends X
{
public void methodY()
{
System.out.println("class Y method");
}
}
Class Z extends Y
{
public void methodZ()
{
System.out.println("class Z method");
}
public static void main(String args[])
{
Z obj = new Z();
obj.methodX(); //calling grand parent class method
obj.methodY(); //calling parent class method
obj.methodZ(); //calling local method
}
}

Hierarchical Inheritance:-In such kind of inheritance one class is inherited by many sub classes
class A
{
public void methodA()
{
System.out.println("method of Class A");
}
}
class B extends A
{
public void methodB()
{
System.out.println("method of Class B");
}
}
class C extends A
{
public void methodC()
{
System.out.println("method of Class C");
}
}
class D extends A
{
public void methodD()
{
System.out.println("method of Class D");
}
}
class JavaExample
{
public static void main(String args[])
{
B obj1 = new B();
C obj2 = new C();
D obj3 = new D();
//All classes can access the method of class A
obj1.methodA();
obj2.methodA();
obj3.methodA();
}
}

Output:
method of Class A
method of Class A
method of Class A

Hybrid Inheritance
In simple terms you can say that Hybrid inheritance is a combination of Single and Multiple inheritance.
class C
{
public void disp()
{
System.out.println("C");
}
}
class A extends C
{
public void disp()
{
System.out.println("A");
}
}
class B extends C
{
public void disp()
{
System.out.println("B");
}
}
class D extends A
{
public void disp()
{
System.out.println("D");
}
public static void main(String args[]){
D obj = new D();
obj.disp();
 }
}
Output:
D

Interface in Java
An interface in java is a blueprint of a class. It has static constants and abstract methods.
The interface in Java is a mechanism to achieve abstraction. There can be only abstract methods in the Java interface, not method body. It is used to achieve abstraction and multiple inheritance in Java.
In other words, you can say that interfaces can have abstract methods and variables. It cannot have a method body.
Java Interface also represents the IS-A relationship.It cannot be instantiated just like the abstract class.
Since Java 8, we can have default and static methods in an interface.Since Java 9, we can have private methods in an interface.

Why use Java interface?
There are mainly three reasons to use interface. They are given below.
oIt is used to achieve abstraction.
oBy interface, we can support the functionality of multiple inheritance.
oIt can be used to achieve loose coupling.
How to declare an interface?
An interface is declared by using the interface keyword. It provides total abstraction; means all the methods in an interface are declared with the empty body, and all the fields are public, static and final by default. A class that implements an interface must implement all the methods declared in the interface.
Eg. Multiple inheritance(see above)

Abstraction in Java
Abstraction is a process of hiding the implementation details and showing only functionality to the user.
Ways to achieve Abstraction
There are two ways to achieve abstraction in java
1.Abstract class (0 to 100%)
2.Interface (100%)

Abstract class in Java
A class which is declared as abstract is known as an abstract class. It can have abstract and non-abstract methods. It needs to be extended and its method implemented. It cannot be instantiated.

Points to Remember
oAn abstract class must be declared with an abstract keyword.
oIt can have abstract and non-abstract methods.
oIt cannot be instantiated.
oIt can have constructors and static methods also.
oIt can have final methods which will force the subclass not to change the body of the method.

Example of abstract class
1.abstract class A{} 

Abstract Method in Java
A method which is declared as abstract and does not have implementation is known as an abstract method.
Example of abstract method
1.abstract void printStatus();//no method body and abstract 

Example of Abstract class that has an abstract method
In this example, Bike is an abstract class that contains only one abstract method run. Its implementation is provided by the Honda class.
Example of Abstract class that has an abstract method
In this example, Bike is an abstract class that contains only one abstract method run. Its implementation is provided by the Honda class.

abstract class Bike
{ 
abstract void run(); 
} 
class Honda4 extends Bike
{ 
void run()
{
System.out.println("running safely");
} 
public static void main(String args[])
{ 
Bike obj = new Honda4(); 
obj.run(); 
} 
} 
Compile by: javac Honda4.java
Run by: java Honda4
running safely..

Difference between abstract class and interface
Abstract class and interface both are used to achieve abstraction where we can declare the abstract methods. Abstract class and interface both can't be instantiated.

But there are many differences between abstract class and interface that are given below.

Abstract class  Interface
1)Abstract class can have abstract and non-abstractmethods.Interface can have only abstract methods. Since Java 8, it can have default and static methods also.
2)Abstract class doesn't support multiple inheritance.Interface supports multiple inheritance.
3)Abstract class can have final, non-final, static and non-static variables.Interface has only static and final variables.
4)Abstract class can provide the implementation of interface.Interface can't provide the implementation of abstract class.
5)The abstract keyword is used to declare abstract class.The interface keyword is used to declare interface.
6)An abstract class can extend another Java class and implement multiple Java interfaces.  An interface can extend another Java interface only.
7)An abstract class can be extended using keyword "extends".An interface can be implemented using keyword "implements".
8)A Java abstract class can have class members like private, protected, etc. Members of a Java interface are public by default.

9)Example:
public abstract class Shape
{
public abstract void draw();
}
Example:
public interface Drawable
{
void draw();
}
Simply, abstract class achieves partial abstraction (0 to 100%) whereas interface achieves fully abstraction (100%).

Example of abstract class and interface in Java
//Creating interface that has 4 methods 
interface A
{ 
void a();//bydefault, public and abstract 
void b(); 
void c(); 
void d(); 
} 
 
//Creating abstract class that provides the implementation of one method of A interface 
abstract class B implements A
{ 
public void c(){System.out.println("I am C");} 
} 
 
//Creating subclass of abstract class, now we need to provide the implementation of rest of the methods 
class M extends B
{ 
public void a(){System.out.println("I am a");} 
public void b(){System.out.println("I am b");} 
public void d(){System.out.println("I am d");} 
} 
 
//Creating a test class that calls the methods of A interface 
class Test5
{ 
public static void main(String args[]){ 
A a=new M(); 
a.a(); 
a.b(); 
a.c(); 
a.d(); 
}} 
Output:
 I am a
 I am b
 I am c
 I am d

Types of polymorphism in java- Runtime and Compile time polymorphism

types of polymorphism. There are two types of polymorphism in java:
1)Static Polymorphism also known as compile time polymorphism
2)Dynamic Polymorphism also known as runtime polymorphism

Polymorphism is one of the OOPs feature that allows us to perform a single action in different ways.
Runtime Polymorphism (or Dynamic polymorphism)
It is also known as Dynamic Method Dispatch. Dynamic polymorphism is a process in which a call to an overridden method is resolved at runtime, thats why it is called runtime polymorphism.
class ABC
{
public void myMethod()
{
System.out.println("Overridden Method");
 }
}
public class XYZ extends ABC
{
public void myMethod()
{
System.out.println("Overriding Method");
}
public static void main(String args[]){
ABC obj = new XYZ();
obj.myMethod();
  }
}
Output:
Overriding Method


Compile time Polymorphism (or Static polymorphism)
Polymorphism that is resolved during compiler time is known as static polymorphism. Method overloading is an example of compile time polymorphism.

Method Overloading: This allows us to have more than one method having the same name, if the parameters of methods are different in number, sequence and data types of parameters

class SimpleCalculator
{
int add(int a, int b)
{
return a+b;
}
int  add(int a, int b, int c) 
{
return a+b+c;
}
}
public class Demo
{
public static void main(String args[])
{
SimpleCalculator obj = new SimpleCalculator();
System.out.println(obj.add(10, 20));
System.out.println(obj.add(10, 20, 30));
}
}
Output:
30
60

Method Overloading in Java
If a class has multiple methods having same name but different in parameters, it is known as Method Overloading.
If we have to perform only one operation, having same name of the methods increases the readability of the program.
Advantage of method overloading
Method overloading increases the readability of the program.
Different ways to overload the method
There are two ways to overload the method in java
1.By changing number of arguments
2.By changing the data type

Method Overloading: changing no. of arguments

class Adder
{ 
static int add(int a,int b){return a+b;} 
static int add(int a,int b,int c){return a+b+c;} 
} 
class TestOverloading1
{ 
public static void main(String[] args)
{ 
System.out.println(Adder.add(11,11)); 
System.out.println(Adder.add(11,11,11)); 
}
} 

22
33

Method Overloading: changing data type of arguments
class Adder
{ 
static int add(int a, int b)
{
return a+b;
} 
static double add(double a, double b)
{
return a+b;
} 
} 
class TestOverloading2
{ 
public static void main(String[] args)
{ 
System.out.println(Adder.add(11,11)); 
System.out.println(Adder.add(12.3,12.6)); 
}
} 

22
24.9

Method Overriding in Java
If subclass (child class) has the same method as declared in the parent class, it is known as method overriding in Java.

In other words, If a subclass provides the specific implementation of the method that has been declared by one of its parent class, it is known as method overriding.

Usage of Java Method Overriding
oMethod overriding is used to provide the specific implementation of a method which is already provided by its superclass.
oMethod overriding is used for runtime polymorphism
Rules for Java Method Overriding
1.The method must have the same name as in the parent class
2.The method must have the same parameter as in the parent class.
3.There must be an IS-A relationship (inheritance).

//Java Program to demonstrate why we need method overriding 
//Here, we are calling the method of parent class with child 
//class object. 
//Creating a parent class 

class Vehicle
{ 
void run()
{
System.out.println("Vehicle is running");
} 
} 
//Creating a child class 
class Bike extends Vehicle
{ 
public static void main(String args[]){ 
//creating an instance of child class 
Bike obj = new Bike(); 
//calling the method with child class instance 
obj.run(); 
 } 
} 
Output:
Vehicle is running

Can we override static method?
No, a static method cannot be overridden. It can be proved by runtime polymorphism, so we will learn it later.

Why can we not override static method?
It is because the static method is bound with class whereas instance method is bound with an object. Static belongs to the class area, and an instance belongs to the heap area.

Can we override java main method?
No, because the main is a static method.

Difference between method overloading and method overriding in java

1)Method overloading is used to increase the readability of the program.Method overriding is used to provide the specific implementation of the method that is already provided by its super class.
2)Method overloading is performed within class.Method overriding occurs in two classes that have IS-A (inheritance) relationship.
3)In case of method overloading, parameter must be different.In case of method overriding, parameter must be same.
4)Method overloading is the example of compile time polymorphism.Method overriding is the example of run time polymorphism.
5)In java, method overloading can't be performed by changing return type of the method only. Return type can be same or different in method overloading. But you must have to change the parameter.Return type must be same or covariant in method overriding.
Java Method Overloading example
class OverloadingExample
{ 
static int add(int a,int b)
{
return a+b;
} 
static int add(int a,int b,int c)
{
return a+b+c;
} 
}

Java Method Overriding example
class Animal
{ 
void eat()
{
System.out.println("eating...");
} 
} 
class Dog extends Animal
{ 
void eat()
{
System.out.println("eating bread...");
} 
}  

Java Inner Classes
Java inner class or nested class is a class which is declared inside the class or interface.
We use inner classes to logically group classes and interfaces in one place so that it can be more readable and maintainable.
Additionally, it can access all the members of outer class including private data members and methods.
Syntax of Inner class
class Java_Outer_class
{ 
//code 
class Java_Inner_class
{ 
//code 
} 
}

Advantage of java inner classes

There are basically three advantages of inner classes in java. They are as follows:
1)Nested classes represent a special type of relationship that is it can access all the members (data members and methods) of outer class including private.
2)Nested classes are used to develop more readable and maintainable code because it logically group classes and interfaces in one place only.
3)Code Optimization: It requires less code to write.

Difference between nested class and inner class in Java
Inner class is a part of nested class. Non-static nested classes are known as inner classes.

Types of Nested classes
There are two types of nested classes non-static and static nested classes.The non-static nested classes are also known as inner classes.
oNon-static nested class (inner class)
1.Member inner class
2.Anonymous inner class
3.Local inner class

oStatic nested class

Member Inner Class:-A class created within class and outside method.
Anonymous Inner Class:-A class created for implementing interface or extending class. Its name is decided by the java compiler.
Local Inner Class:-A class created within method.
Static Nested Class:-A static class created within class.
Nested Interface:-An interface created within class or interface


Inner class
An inner class is declared inside the curly braces of another enclosing class. Inner class is coded inside a Top level class as shown below:-
//Top level class definition
class MyOuterClassDemo
{
private int myVar= 1;
// inner class definition
class MyInnerClassDemo
{
public void seeOuter ()
{
System.out.println("Value of myVar is :" + myVar);
}
} // close inner class definition
} // close Top level class definition

Anonymous Inner Classes
It is a type of inner class which
1.has no name
2.can be instantiated only once
3.is usually declared inside a method or a code block ,a curly braces ending with semicolon.
4.is accessible only at the point where it is defined.
5.does not have a constructor simply because it does not have a name
6.cannot be static

Example:
package beginnersbook.com;
class Pizza
{
public void eat()
{
System.out.println("pizza");
}
}
class Food
{
/* There is no semicolon(;) 
* semicolon is present at the curly braces of the method end.
*/
Pizza p = new Pizza()
{
public void eat()
{
System.out.println("anonymous pizza");
}
};
}

Access Modifiers in Java
There are two types of modifiers in Java: access modifiers and non-access modifiers.
The access modifiers in Java specifies the accessibility or scope of a field, method, constructor, or class. We can change the access level of fields, constructors, methods, and class by applying the access modifier on it.
There are four types of Java access modifiers:
1.Private: The access level of a private modifier is only within the class. It cannot be accessed from outside the class.
2.Default: The access level of a default modifier is only within the package. It cannot be accessed from outside the package. If you do not specify any access level, it will be the default.
3.Protected: The access level of a protected modifier is within the package and outside the package through child class. If you do not make the child class, it cannot be accessed from outside the package.
4.Public: The access level of a public modifier is everywhere. It can be accessed from within the class, outside the class, within the package and outside the package.

Examples of Private
class A
{ 
private int data=40; 
private void msg()
{
System.out.println("Hello java");
} 
} 
public class Simple
{ 
public static void main(String args[])
{ 
A obj=new A(); 
System.out.println(obj.data);//Compile Time Error 
obj.msg();//Compile Time Error 
} 
} 

Default
//save by A.java 
package pack; 
class A
{ 
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();//Compile Time Error 
obj.msg();//Compile Time Error 
} 
}

Protected
//save by A.java 
package pack; 
public class A{ 
protected void msg(){System.out.println("Hello");} 
} 
//save by B.java 
package mypack; 
import pack.*; 
 
class B extends A
{ 
public static void main(String args[]){ 
B obj = new B(); 
obj.msg(); 
} 

}  

Public
//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(); 
 } 
} 

non-access modifiers
•The static modifier for creating class methods and variables.
•The final modifier for finalizing the implementations of classes, methods, and variables.
•The abstract modifier for creating abstract classes and methods.
•The synchronized and volatile modifiers, which are used for threads.

Final Keyword In Java
The final keyword in java is used to restrict the user. The java 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.
Java final variable
If you make any variable as final, you cannot change the value of final variable(It will be constant).
class Bike9
{ 
final int speedlimit=90;//final variable 
void run()
{ 
speedlimit=400; 
} 
public static void main(String args[])
{ 
Bike9 obj=new  Bike9(); 
obj.run(); 
} 
}//end of class 

Output:Compile Time Error

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

Example of final class
final class Bike{} 
 
class Honda1 extends Bike
{ 
void run(){System.out.println("running safely with 100kmph");} 
public static void main(String args[]){ 
Honda1 honda= new Honda1(); 
honda.run(); 
 } 
} 
Output:Compile Time Error

Is final method inherited?
Ans) Yes, final method is inherited but you cannot override it. For Example:
class Bike
{ 
 inal void run(){System.out.println("running...");} 
} 
class Honda2 extends Bike
{ 
public static void main(String args[]){ 
new Honda2().run(); 
} 
}

Java static keyword
The static keyword in Java is used for memory management mainly. We can apply java static keyword with variables, methods, blocks and nested class. The static keyword belongs to the class than an instance of the class.
The static can be:
1.Variable (also known as a class variable)
2.Method (also known as a class method)



//Java Program to demonstrate the use of a static method. 
class Student
{
int rollno; 
String name; 
static String college = "ITS"; 
//static method to change the value of static variable 
static void change()
{ 
college = "BBDIT"; 
} 
//constructor to initialize the variable 
Student(int r, String n)
{ 
rollno = r; 
name = n; 
} 
//method to display values 
void display()
{
System.out.println(rollno+" "+name+" "+college);
} 
} 
//Test class to create and display the values of object 
public class TestStaticMethod
{ 
public static void main(String args[]){ 
Student.change();//calling change method 
//creating objects 
Student s1 = new Student(111,"Karan"); 
Student s2 = new Student(222,"Aryan"); 
Student s3 = new Student(333,"Sonoo"); 
//calling display method 
s1.display(); 
s2.display(); 
s3.display(); 
} 
} 
Output:111 Karan BBDIT
       222 Aryan BBDIT
       333 Sonoo BBDIT

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

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.

Types of packages in Java
As mentioned in the beginning of this guide that we have two types of packages in java.
1 User defined package: The package we create is called user-defined package.
2)Built-in package: The already defined package like java.io.*, java.lang.* etc are known as built-in packages.

Wrapper classes in Java
The wrapper class in Java provides the mechanism to convert primitive into object and object into primitive.
Since J2SE 5.0, autoboxing and unboxing feature convert primitives into objects and objects into primitives automatically. The automatic conversion of primitive into an object is known as autoboxing and vice-versa unboxing.

Use of Wrapper classes in Java
Java is an object-oriented programming language, so we need to deal with objects many times like in Collections, Serialization, Synchronization, etc. Let us see the different scenarios, where we need to use the wrapper classes.

oChange the value in Method: Java supports only call by value. So, if we pass a primitive value, it will not change the original value. But, if we convert the primitive value in an object, it will change the original value.
oSerialization: We need to convert the objects into streams to perform the serialization. If we have a primitive value, we can convert it in objects through the wrapper classes.
oSynchronization: Java synchronization works with objects in Multithreading.
ojava.util package: The java.util package provides the utility classes to deal with objects.
oCollection Framework: Java collection framework works with objects only. All classes of the collection framework (ArrayList, LinkedList, Vector, HashSet, LinkedHashSet, TreeSet, PriorityQueue, ArrayDeque, etc.) deal with objects only.
The eight classes of the java.lang package are known as wrapper classes in Java. The list of eight wrapper classes are given below:
Primitive Type Wrapper class
boolean            Boolean
char     Character
byte     Byte
short    Short
int        Integer
long     Long
float     Float
double Double

Wrapper class Example: Primitive to Wrapper
//Java program to convert primitive into objects 
//Autoboxing example of int to Integer 
public class WrapperExample1
{ 
public static void main(String args[])
{ 
//Converting int into Integer 
int a=20; 
Integer i=Integer.valueOf(a);//converting int into Integer explicitly 
Integer j=a;//autoboxing, now compiler will write Integer.valueOf(a) internally 
System.out.println(a+" "+i+" "+j); 
}
} 
Output:
20 20 20