inheritance confusion

[Ashkon]

Can someone please help explain the following behaviour, or point me
to a post that explains it?

While i was debugging one of my company's programs i noticed some
objects behaving strangely. I was having a very difficult time
finding the source of the problem. Finally, when i realized what was
going on i wrote the following three classes to try to understand what
happens in the JVM.

Given:
/**
* @author Ashkon Sabet
* www.ashkon.net
*/

public class Parent
{
protected boolean parentBoolean = false;

public Parent()
{
System.out.println("Entering Parent() Constructor");

init();

System.out.println("Exiting Parent() Constructor");
}

protected void init()
{
System.out.println("Entering Parent.init()");

parentBoolean = true;

System.out.println("Value of parentBool "+parentBoolean);

System.out.println("Exiting Parent.init()");
}
}

/**
* @author Ashkon Sabet
* www.ashkon.net
*/
public class Child extends Parent
{
protected boolean childBoolean = false;

public Child()
{
super();
}

protected void init()
{
System.out.println("Entering Child.init()");

super.init();

setChildBoolean();

System.out.println("Child.childBoolean = "+childBoolean);

System.out.println("Exiting Child.init()");
}

public void setChildBoolean()
{
if (super.parentBoolean == true)
this.childBoolean = true;
}
}

/**
*
* @author Ashkon Sabet
* www.ashkon.net
*/

public class FamilyTester {

/** Creates a new instance of FamilyTester */
public FamilyTester() {
}

/**
* @param args the command line arguments
*/
public static void main(String[] args) throws Exception{
Child myFirstChild = new Child();
System.out.println("My First Child Boolean ==
"+myFirstChild.childBoolean);

// Child mySecondChild = new Child();
// mySecondChild.setChildBoolean();
// System.out.println("My Second Child Boolean ==
"+mySecondChild.childBoolean);

}
}

When you run FamilyTester you get the following output:

Entering Parent() Constructor
Entering Child.init()
Entering Parent.init()
Value of parentBool true
Exiting Parent.init()
Child.childBoolean = true
Exiting Child.init()
Exiting Parent() Constructor
My First Child Boolean == false

*******************************
I would have expected the last line to read:
My First Child Boolean == true

Thank you for you help.
-Ash

[Raymond DeCampo]

Yes, I will try to explain this. Before the Child instance is
initialized, the Parent instance within that Child instance is
constructed. So, the Parent constructor is called, then the
initializers for the Child class are run, then the Child constructor is
executed. For the variable childBoolean, this is what happens:

1) Parent constructor is run. Parent constructor calls init(), which
results in Child.init() being executed. So childBoolean is set to true.

2) Child initializers are executed. childBoolean is then set to false.

3) Child constructor is executed, no change for childBoolean.

To see this in action, try the following modification and note when the
new message is displayed:

class Parent
{
protected boolean parentBoolean = false;

public Parent()
{
System.out.println("Entering Parent() Constructor");

init();

System.out.println("Exiting Parent() Constructor");
}

protected void init()
{
System.out.println("Entering Parent.init()");

parentBoolean = true;

System.out.println("Value of parentBool "+parentBoolean);

System.out.println("Exiting Parent.init()");
}
}

class Child extends Parent
{
protected boolean childBoolean = false;
private String msg = getMsg();

public Child()
{
super();
}

protected void init()
{
System.out.println("Entering Child.init()");

super.init();

setChildBoolean();

System.out.println("Child.childBoolean = "+childBoolean);

System.out.println("Exiting Child.init()");
}

public void setChildBoolean()
{
if (super.parentBoolean == true)
this.childBoolean = true;
}

private String getMsg()
{
String result = "childBoolean was just initialized to false";
System.out.println(result);
return result;
}
}

public class FamilyTester {

/** Creates a new instance of FamilyTester */
public FamilyTester() {
}

/**
* @param args the command line arguments
*/
public static void main(String[] args) throws Exception{
Child myFirstChild = new Child();
System.out.println("My First Child Boolean == "
+myFirstChild.childBoolean);

}
}

Ray

[Neomorph]

On 28 Aug 2003 23:56:26 -0700, as****@hotmail.com (Ashkon) two-finger
typed:

Ray,

Thank you for your insight. I understand what is happening now.

However, the flow of the source code makes it seem more intuitive to
have Child's global data members created and initialized before
Parent's constructor is run and not in between the running of Parent's
constructor and Child's constructor. From your post though, i
understand that Parent's constructor is executed before Child's global
data memebers are defined.

Thank you. You are genius! :)

So, what now? What if i need to override Parent's init() method but
still call it and still set global data memeber values? What's a good
solution?

Why not try this:

public class Parent {

protected boolean parentBoolean = false;

public Parent() {
Parent.init(this);
}

static void init(Parent p) {
p.parentBoolean = true ;
}
}

public class Child {

protected boolean childBoolean = false;

public Child() {
super();
Child.init(this);
}

static void init(Child p) {
p.childBoolean = true ;
}
}

This way the parent only ever initializes parent variables, and the child
does the parent first, and then only its own variables.

Cheers.

[Raymond DeCampo]

Ashkon wrote:

Ray,

Thank you for your insight. I understand what is happening now.

However, the flow of the source code makes it seem more intuitive to
have Child's global data members created and initialized before
Parent's constructor is run and not in between the running of Parent's
constructor and Child's constructor. From your post though, i
understand that Parent's constructor is executed before Child's global
data memebers are defined.

Thank you. You are genius! :)

So, what now? What if i need to override Parent's init() method but
still call it and still set global data memeber values? What's a good
solution?

Glad I could help.

There are many ways to do what you want. I suspect that the example is
too simplified to really understand what you need. Perhaps if you try
to explain the context a little more.

Ray

[JGod]

Good example, and I'm only taking an educated guess at what's going on here.

I compiled your example and saw your results.

I found that if you change this line:

protected boolean parentBoolean = false;

to

protected boolean parentBoolean;

you WILL get the result you expect ("My First Child Boolean = true").

This tells me that fields are initialized to their DEFAULT values
immediately upon entering the constructor but then to their declared
initializations immediately AFTER a call to super(), if one exists (and
which of course has to be the first line of the constructor if it were to
exist at all).

I can't explain the rationalization for why the JVM chooses (if it was even
intended at all!) this behaivor, but it's clearly true.

Btw, your example would be cleaner for explaining the point at hand if you
changed Child to simply be:

class Child extends Parent
{
protected boolean childBoolean = false;

public Child() {
super(); // will call Child.init()
}

protected void init() {
childBoolean= true;
}
}

You would see the same result: false returned as is; but true returned if
the initialization (which appears to be harmless duplication) were removed.
"Ashkon" <as****@hotmail.com> wrote in message
news:d8**************************@posting.google.c om...

Can someone please help explain the following behaviour, or point me
to a post that explains it?

While i was debugging one of my company's programs i noticed some
objects behaving strangely. I was having a very difficult time
finding the source of the problem. Finally, when i realized what was
going on i wrote the following three classes to try to understand what
happens in the JVM.

Given:
/**
* @author Ashkon Sabet
* www.ashkon.net
*/

public class Parent
{
protected boolean parentBoolean = false;

public Parent()
{
System.out.println("Entering Parent() Constructor");

init();

System.out.println("Exiting Parent() Constructor");
}

protected void init()
{
System.out.println("Entering Parent.init()");

parentBoolean = true;

System.out.println("Value of parentBool "+parentBoolean);

System.out.println("Exiting Parent.init()");
}
}

/**
* @author Ashkon Sabet
* www.ashkon.net
*/
public class Child extends Parent
{
protected boolean childBoolean = false;

public Child()
{
super();
}

protected void init()
{
System.out.println("Entering Child.init()");

super.init();

setChildBoolean();

System.out.println("Child.childBoolean = "+childBoolean);

System.out.println("Exiting Child.init()");
}

public void setChildBoolean()
{
if (super.parentBoolean == true)
this.childBoolean = true;
}
}

/**
*
* @author Ashkon Sabet
* www.ashkon.net
*/

public class FamilyTester {

/** Creates a new instance of FamilyTester */
public FamilyTester() {
}

/**
* @param args the command line arguments
*/
public static void main(String[] args) throws Exception{
Child myFirstChild = new Child();
System.out.println("My First Child Boolean ==
"+myFirstChild.childBoolean);

// Child mySecondChild = new Child();
// mySecondChild.setChildBoolean();
// System.out.println("My Second Child Boolean ==
"+mySecondChild.childBoolean);

}
}

When you run FamilyTester you get the following output:

Entering Parent() Constructor
Entering Child.init()
Entering Parent.init()
Value of parentBool true
Exiting Parent.init()
Child.childBoolean = true
Exiting Child.init()
Exiting Parent() Constructor
My First Child Boolean == false

*******************************
I would have expected the last line to read:
My First Child Boolean == true

Thank you for you help.
-Ash