JAVA
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1.
Class
is a template or blueprint from which objects are created.
2.
Object
is an instance of a class.
3.
Method
is a collection of statements that are grouped together to perform an
operation.
4.
Constructor
- Constructor is a special type of method that is used to initialize the
object.
5.
Modifiers
- There are two types of modifiers access modifiers and non-access
modifiers.
6.
Inheritance
is a mechanism in which one object acquires all the properties and
behaviors of parent object.
7.
Polymorphism
is a concept by which we can perform a single action by different ways.
8.
Encapsulation
is a process of wrapping code & data together into a single unit eg. capsule.
9.
Exception
Handling handling is a mechanism to handle the runtime errors so that
normal flow of the application can be maintained.
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1. Class
It is a template or blueprint from which objects are
created. A class can contain:
·
Fields or Instance Variable
·
methods
·
constructors
Object
Object is an instance (result) of a class.
2. Instance
variable
A variable which is created inside the class but outside
the method.
3. Method
Method is a collection of statements that are grouped
together to perform an operation.
eg. When you call the System.out.println() method, the
system actually executes several statements in order to display a message on
the console.
4. Constructor
Constructor is a special type of method that is used to
initialize the object.
i. Difference between a constructor and a method:
a. A constructor doesn’t have a
return type.
b. The name of the constructor
must be the same as the name of the class.
c. A constructor is called
automatically when a new instance of an object is created.
d. Constructors are called only once for a single object while
regular methods could be called many times .
ii. Types of Constructor:
a. Default constructor (no-arg
constructor)
b. Parameterized constructor
a. Default Constructor - Default
constructor provides the default values to the object like 0, null etc.
class Bike1{
Bike1()//default constructor
{
System.out.println("Bike
is created");
}
//---------------------------------------------------------------------
public static void main(String
args[]){
Bike1 b=new Bike1(); //Default
constructor invoked
}
}
b. Parameterized constructor - A
constructor that have parameters is known as parameterized constructor.
class Student4{
int id; //field
or data member or instance variable
String name;
Student4(inti,String n){ //Constructor
having 2 parameters
id = i;
name = n;
}
void
display(){System.out.println(id+" "+name);}
//---------------------------------------------------------------------
public static void main(String
args[]){
Student4 s1 = new
Student4(111,"Karan");
Student4 s2 = new
Student4(222,"Aryan"); //Parameterized constructor invoked
s1.display();
s2.display();
}
}
iii. Constructor Overloading
Constructor overloading is a
technique in which a class can have any number of constructors that differ in
parameter lists.
class Student5{
int id;
String name;
int age;
Student5(inti,String n){
id = i;
name = n;
}
Student5(inti,Stringn,int a) //constructor
overloaded
{
id = i;
name = n;
age=a;
}
void
display(){System.out.println(id+" "+name+" "+age);}
//---------------------------------------------------------------------
public static void main(String
args[]){
Student5 s1 = new Student5(111,"Karan");
Student5 s2 = new
Student5(222,"Aryan",25);
s1.display();
s2.display();
}
}
5. Modifiers
There are two types of modifiers access modifiers and
non-access modifiers.
i. Access modifiers - specifies the scope of "instance
variable", method, constructor or class.
a. private - private access modifier is
accessible only within class. If you make any class constructor private, you
cannot create the instance of that class from outside the class.
b. default - If you don't use any
modifier, it is treated as default bydefault. The default modifier is
accessible only within package.
c. protected - The protected access
modifier is accessible within package and outside the package but through
inheritance only.
d. public - The public access modifier is
accessible everywhere.
ii. Non-access modifiers - such as static, abstract, synchronized,
native, volatile, transient etc.
a. Static - static is used for memory
management. The variable or Method
that belong to the Class rather than to any particular instance.
6. public static
intmethodName(int a, int b) {
}
·
public static − modifier
·
int − return type
·
methodName − name of the method
·
a, b − formal parameters
·
int a, int b − list of parameters
7. The this
keyword
This keyword is
used as a reference to the object of the current class.
8. super keyword
in java
super keyword is is used as a reference to the object of immediate
parent class.
9. Inheritance
Inheritance is a
mechanism in which one object acquires all the properties and behaviors of
parent object.
·
For Method Overriding (so runtime polymorphism
can be achieved).
·
For Code Reusability.
Syntax of 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.
10. Polymorphism
Polymorphism is a concept by which we can perform a
single action by different ways.
Polymorphism means many forms. There are two types of polymorphism:
·
compile time polymorphism (Method overloading)
·
runtime polymorphism (Method overriding)
We can perform polymorphism by method overloading and
method overriding.
i.
Method
Overloading
If a class has multiple methods
having same name but different in parameters, it is known as Method
Overloading. Method overloading increases the readability of the program.
There are two ways to overload
the method in java
·
By changing number of arguments
·
By changing the data type
ii.
Method Overriding
If subclass (child class) has the
same method as declared in the parent class, it is known as method overriding
in java.
Usage of Java Method Overriding
·
Method overriding is used to provide specific
implementation of a method that is already provided by its super class.
·
Method overriding is used for runtime polymorphism
Rules for Java Method Overriding
·
method must have same name as in the parent class
·
method must have same parameter as in the parent
class.
·
must be IS-A relationship (inheritance).
iii. Final
The final keyword in java is
used to restrict the user. Final can be:
1. variable - If you make any variable as
final, you cannot change the value of final variable(It will be constant).
2. method - If you make any method as
final, you cannot override it.
3. class - If you make any class as final,
you cannot extend it.
Can we declare a constructor final?
No, because constructor is
never inherited.
11. Abstraction
is a process of hiding the implementation details.
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.
There are two ways
to achieve abstraction in java
·
Abstract class (0 to 100%)
·
Interface (100%)
i.
Abstract
class
A class that is declared with
abstract keyword is known as abstract
class. It needs to be extended and
its method implemented. It cannot be instantiated.
Example - abstract class A{}
Abstract method
A method that is declared with
abstract keyword and does not have implementation is known as abstract method.
Example - abstract void
printStatus();//no body and abstract (declaring signature)
ii.
Interface
An interface is a blueprint of a
class. It has static constants and abstract methods. There are mainly three
reasons to use interface.
·
It is used to achieve abstraction.
·
By interface, we can support the functionality
of multiple inheritance.
iii. Difference between abstract class and interface
1) Abstract class can have
abstract and non-abstract methods.
Interface can have only abstract
methods.
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) 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%).
12. Encapsulation
Encapsulation is a process of wrapping code and data
together into a single unit, for example capsule i.e. mixed of several
medicines.
We can create a fully encapsulated class in java by
making all the data members of the class private. We can use setter and getter
methods to set and get the data in it.
get and set methods
They serve to set and get values from class variables
that are defined as 'private' (because of security issue), but these method are
defined as 'public'.
13. Exception
Handling
The exception handling is a mechanism to handle the
runtime errors so that normal flow of the application can be maintained.
The advantage of exception handling is to maintain the
normal flow of the application.
Suppose there is 10 statements in your program and there
occurs an exception at statement 5, rest of the code will not be executed i.e.
statement 6 to 10 will not run. If we perform exception handling, rest of the
statement will be executed. That is why we use exception handling in java.
Types of Exception
i. Checked Exception - Checked exceptions are checked at
compile-time. Checked Exception uses "throws" keyword.
e.g. IOException, SQLException
ii. Unchecked Exception - Unchecked exceptions checked at runtime.
ArithmeticException
NullPointerException
NumberFormatException
There are 5 keywords used in java exception handling.
·
try
·
catch
·
finally
·
throw
·
throws
Java try-catch
Java try block is used to
enclose the code that might throw an exception.
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