Styjun

I undertand the reason behind my output is run time polymorphism. But I am unable to figure it out how ?
Please Explain how run time polymorphism, constructors and inheritance are related.

This is JAVA code. I have triend understanding run time polymorphism but I have failed to figure it out.

package df;

class A1

A1()
 System.out.println("Constructor A() called"); 

void m1() 
 System.out.println("Inside A's m1 method"); 
 

class B2 extends A1

B2()
 System.out.println("Constructor B() called"); 


void m1() 
 System.out.println("Inside B's m1 method"); 
 

class C3 extends A1

C3()
 System.out.println("Constructor C()"); 

void m1() 
 System.out.println("Inside C's m1 method"); 
 

public class Cldf

// object of type A 
public static void main(String[] args) 
 A1 a = new B2(); 

I expected result as below

Constructor B() called.

but result is

Constructor A() called
Constructor B() called

super() is by default called from the subclass when inheriting. This is done by the compiler.

This is done because if the superclass initializes any private variable inside its constructor, it needs to be initialized. Having said that, it doesn't mean 2 objects are created. Only one instance of the subclass is created.

In your case, B2 Constructor called the A1 constructor by default.

Try doing below and you will encounter a compilation error.

Class A1
 public A1(String msg)
 


Class B2 extends A1
 public B2()
 

Constructor of a class is called when a object of that class is initialized. In your example class B2 extends class A1. When B2 object is created B2() constructor is called. Ideally you have to call super constructor in your B2 constructor like

 B2()
 super();
 System.out.println("Constructor B() called"); 
 

So, when you B2() is invoked during object creation it will call its super class constructor. If this is not implemented by default java compiler will call superclass constructor. So your result has Constructor A() called and then Constructor B() called.
In the below case reference is of parent class and object is of child class.
A1 a = new B2();
As object is B2, constructor B2() is called, which internally calls A1() by default. Now if you call a.m1() it will print 'Inside A's m1 method'. This is because reference is of parent type and parent reference can access only parent class methods. If your parent class doesn't have m1 and your class has m1, then you will get a compile time error.