Doubt on constructors

[Dinesh Tahiliani]

public Cert1
{
public Cert1()
{
System.out.print("1");
}
}

public Cert2 extends Cert1
{

public Cert2()
{
System.out.print("2");
}
}

public Cert3 extends Cert3
{

public Cert3()
{
System.out.print("3");
}
}

public class certdemo
{
public static void main(String args[])
{
new Cert3();
}
}

o/p -- 123

Why not 321 as Cert3 extends Cert2 which extends Cert1

Can anyone focus some light on this please

Source " My own code"

[ansuman mohapatra]

because the flow of the constructor goes from the super class till the other sub classes who extend that super class.

Here when c3 is called it looks up to find its immediate superclass i.e c2...c2 extends the class c1....So from there the flow of the constructor starts. Here the flow starts from C1 because it is the constructor of the superclass..Hence the output 123.

Hope that helps.
[ March 13, 2008: Message edited by: ansuman mohapatra ]

[sridhar row]

Are you sure your code will even compile?

[ansuman mohapatra]

class Cert1
{
public Cert1()
{
System.out.print("1");
}
}

class Cert2 extends Cert1
{
public Cert2()
{
System.out.print("2");
}
}

class Cert3 extends Cert2
{
public Cert3()
{
System.out.print("3");
}
}

public class Constructor
{
public static void main(String args[])
{
new Cert3();
}
}

You can try with this code.

[Dinesh Tahiliani]

Why what happen

[Abdul Kader]

the order make sense, because if we print 321, then we have to first execute all the statement in the class 3, this make us into trouble because without knowing what your parent variables are doing you cant execute you class variables.

do you think it make sense?

[Dinesh Tahiliani]

I am not getting can you illustrate your words with example which prints 321.

Please

[Uttara Rishi]

Hi Dinesh,

Here's the explanation

First whenever a constructor runs there is an implicit call to its super constructor in the first line itself if there is no call to the other overloaded constructors.

In your example, cert3 calls cert2 which calls cert1 which calls the Object constructor.

All the super constructor runs and then the subclass constructor constructor runs!

For a more detailed explanation checkout K & B.

Hope that helps.


Uttara Rishi.

[sandhi mridul]

Hi Dinesh,

Whenever any of the construtor get called weather its subclass constructor or superclass the execution always start from the default constructor of the superclass.
That is why the o/p is 123.
Your code is not OK.I did changes.

 public class Cert1
{
public  Cert1()
{
System.out.print("1");
}
}
 
public [B]class[/B] Cert2 extends Cert1
{
 
public Cert2()
{
System.out.print("2");
}
}
 
public [B]class[/B] Cert3 extends [B]Cert2[/B]
{
 
public Cert3()
{
System.out.print("3");
}
}
 
public class certdemo
{
public static void main(String args[])
{
new Cert3();
}
}
   

Execution goes like this

In main() method the Cert3() get called.
class hierarchy goes like this
cert 1
|
cert 2
|
cert 3

Now as the cert3()default constructor is at the last in the hierarchy so the call goes automatically to cert1() default constructor that is super class.
so now the output will be like
cert1() ->cert2()->cert3()

o/p 123

Hope this help


Sandhya
[ March 13, 2008: Message edited by: sandhi mridul ]