OOPs

Question 1

Entity

Answer 1

an entity is a single person, place, or thing about which data can be stored.

Question 2

Characterictics of object

Answer 2

An object has three charactertics:

1. State : value of object
2. Behaviour : functionality of the object
3. Identity : Unique ID. For JVM to identify each object uniquely. (Values of the id is not visible to the external user)

For example:- Pen is an object. Its name is Reynolds; color is white, known as its state. It is used to write, so writing is its behaviour

Question 3

Object

Any one definition would do…

Answer 3

An object is a real-world entity.
An object is a runtime entity.
The object is an entity which has state and behavior.
The object is an instance of a class.

example:-chair, bike, marker, pen, table, car, etc.

Question 4

Class
Field
Method

Answer 4

Class:
A class is a group of objects which have common properties. Its like a blueprint of the object. Before we create an object, we need to first create a class

Field:
Used to store data

Method:
Used to perform some operations

Question 5

Syntax for class

Answer 5

class ClassName {
  // fields
  // methods
}

Question 6

Instance variable

Answer 6

Variable which is created inside the class but outside the method. It gets the memory at runtime when object/instance is created, that is why it is known as instance variable

Question 7

What can a class contain in Java

Answer 7

A class in Java can contain:

1. Fields
2. Methods
3. Constructors
4. Blocks
5. Nested class and interface

Question 8

new keyword

Answer 8

The new keyword is used to allocate memory at runtime

Question 9

Constructor

Definition, syntax

Answer 9

In java constructor is just like a method but without a return type

Syntax:-
class Test{
         //creating a constructor
	Test(){
	//constructor of the body
	}
}

Note:- constructor name should be same as that of the class name

Question 10

return type

Answer 10

Return type is basically a data type declared as a part of the function

Example:-
nt multiply(int a,int b)

{

int c=a*b;

return(c);

}

Here int in a return type

Question 11

Types of Java constructor

Answer 11

1. Default constructor(no-arg constructor)

2. Parameterized constructor

Question 12

Constructor overloading

Answer 12

It is a technique in Java of having more than one constructors with different parameters

Each constructor performs a different task

They are differentiated by the compiler by the number of parameters in their lists and their types

Question 13

Constructor vs Method

Answer 13

1. No return type
2. Return type
3. Used to initialize the state of the object(combination of the original values and the modifications made to them)
4. A method is used to expose the behavior(function) of the object
5. Invoked implicitly
6. Invoked explicitly

Question 14

Method

Answer 14

Collection of instructions that performs a specific task.
It provides the re usability of the code
It provides easy modification and readability of the code, just by adding and removing chunk of code.
The method is executed only when we invoke it

Question 15

Method declaration

Answer 15

It has six components that are known as method header
1. Method signature :-
It includes method name and parameter list

2. Access specifier/modifier:-
 return type of the method. Specifies the visibility of the method. Java provides four types of access specifier:
1. public 
2. private
3. protected
4. default
3

3. Return type:-
   data type that the method returns
4. Method Name: It is a unique name that is used to define the name of a method. A method is invoked by its name.
5. Parameter List: It contains the data type and variable name. optional
6. Method Body: It contains all the actions to be performed

Question 16

Types of method

Answer 16

1. Pre-defined method
Methods that are already present in Java class libraries 
 Also known as standard library or built in method. Some pre-defined methods are length() equals, compareTo(), sqrt(), etc.

2. User defined method
   This method is written by the programmer. These methods are modified according to the requirement

Question 17

How to call or invoke a User defined method?

Answer 17

The calling of the method is simple. When you call, the program control is transferred to the called method

method_name(parameters)

Question 18

Static method
(def, advantage)

Answer 18

A method that has a static keyword is known as static method. 
It belongs to class and not to the object in Java

Advantage:-
We can call static method without creating an object
It can access static data members and also change the value of it.

Question 19

Instance methods

Answer 19

Methods that require object of its class to be created before calling it is called as instance methods.

To invoke a method, we have to create an Object of the class within which it is defined