basics of java in layman term for beginner

Basics of java in layman term for beginner

What is an object ?

object is a real world entity.

Object is an instance of the class.It means something that is alive. Let us take an example of CAR:-

concept of a car is class. I mean we all know that it is having four tyres , one engine , four doors , a seat e.t.c. but we can't do anything just by the concept we need real Car to perform operations on it . This real car is what we call is an object.


Explain about main() method in java ?

main() method is an entry point of a program.
public static void main(String[] args)

Why main() method is public, static and void in java ?

public: It can be accessed from anywhere.
static: no class object need to call this method.
void: it is not returning anything in back.

What is JIT compiler ?

JIT is just in time compiler.JIT compiler runs after the program has started and compiles the code (usually bytecode or some kind of VM instructions) on the fly (or just-in-time, as it's called) into a form that's usually faster,

What is bytecode in java ?

Bytecode is the code that we get after Java file converted into class file( Compile once run anywhere concept is based on this only , we need to convert java code into bytecode or class file after that it can be executed anywhere on any machine)


What is a method in java ?

Method is any function to define the certain operation.

access specifier return type function name parameters
access specifier :public/private/protected
returntype:any data structue
name:any name
parameters:any number of parameters of any type separated by comma.

Example:-
public void getMomoms();
private int noMomos(String yourname);
publib int[] getList(int matches, String[] playernames);

Please refer this code snippet for understanding below concepts:-

package Basics;

public class ParentCar {

 //Declared instances of a class
 int carnumber;
 String carname;
 String cartype;
 
 int speed;
 
 //declaring it as static so that we no need of object calling its value
  String agegroup="18-48";
 
 //Declared Constructor of a class
 public ParentCar(int carnumber, String carname, String cartype) {
  //super keyword is used to call parent constructor
  super();
  //this keyword means current class
  this.carnumber = carnumber;
  this.carname = carname;
  this.cartype = cartype;
 }
 
 //declared function to set the speed
 public void setSpeed()
 {
  this.speed=400;
  System.out.println("speed of a parent car is:"+speed);
 }
 
 
 
 
}


package Basics;

public class ChildCar extends ParentCar{

 String specialFeature;
 
 String agegroup="12-18";

 //constructor of a child car
 public ChildCar(int carnumber, String carname, String cartype, String specialFeature) {
  super(carnumber, carname, cartype);
  this.specialFeature = specialFeature;
 }

 /*
  * Method overwriding
  * Function having same name and return type as of parent class
  * Body is different
  * Also we call it as a Run Time Polymorphism
  */
 public void setSpeed()
 {
  this.speed=260;
  System.out.println("speed of a child car is:"+speed);
 }
 
 public void printAgegroup()
 {
  System.out.println("child class age group"+this.agegroup);
  System.out.println("Parent class age group"+super.agegroup);
 }
 
 //Method overloading method 1
 public void permit(String weekday)
 {
  System.out.println("Ok to drive on"+ weekday);
 }
 
 //Method overloading method 2
 public void permit(String Holiday, String weekend)
 {
  if(weekend.equalsIgnoreCase("saturday"))
  {
   System.out.println("allow on "+weekend+"Holiday ");
  }
  else{
   
   System.out.println("Not allowed on"+weekend+"Holiday ");
  }
 }
 
 public static void main(String[] args)
 {
  //initializing parent class object
  ParentCar pcar=new ParentCar(700, "parent", "Big");
  
  //initializing child class object
  ChildCar ccar=new ChildCar(100, "child", "small", "children functions");
  
  //parent car calling its set speed
  pcar.setSpeed();
  //child class calling its own set speed
  ccar.setSpeed();
  
  //object is of parent but instance is of child hence will
  //call child class method
  ParentCar cpcar=new ChildCar(300, "combined car", "both", "fast");
  cpcar.setSpeed();
  
  //super keyword
  
  ccar.printAgegroup();
  
  //method overloading
  
  ccar.permit("Monday");
  ccar.permit("YES", "Saturday");
  ccar.permit("YES", "Sunday");
  ccar.permit("YES", "Weekday");
 }

}

What is a constructor in java ?

Constructor is a method having the same name as that of a class and is not having any return type.

Constructor can have parameters but one should be must without parameters.

Constructor is used to creating an object of a class.

Child class can call it super class constructor using super.



How to call one constructor from the other constructor ?

using a super keyword.

What is method overriding in java ?

Method Overriding :- It means changing a definition of a method at Run time. This concept we can understand by Inheritance:-

ParentCar is a super class.
ChildCar is a child class.

setSpeed is a method we are overriding. Depending on the instance and situations body will change for the same method of initializing speed.

What is method overloading in java ?

Method overloading is nothing but having the same name but different parameter or different returnType.

Basically name will be same but things will change depending upon parameters and return type.It is also called compile type polymorphism as we know which method to call at runtime.

What is a super keyword in java and difference between this() and super() in java ?

As we have used super() keyword in our last example to call the things that exist in the parent class.To be clearer :-

We have declared the same variable name in both parent and child and will see what calls what?

printAgegroup() calling both time agegroup only but depending upon this or super it is changing value.


Difference between method overloading and method overriding in java ?

Overloading: Compile time polymorphism same name but different signature.
Overwriding: Run time polymorphism same name as well as same signature but body is different.