How can a Class create an instance of itself??

[james dunster]

This is code from a book I am using to learn Java. This construction is used several times in the book so far and I have also seen it in uni lecture notes. In this case it creates a class called XCopy and in the middle of the Class it calls an instance of itself. Can anyone explain how this can be? How can a Class create an instance of itself before you have finished writing it? But it works.

class XCopy {

public static void main(String[] args) {

int orig =42;
XCopy x=new XCopy(); // <-----------------<< how is this poss?
int y=x.go(orig);
System.out.println(orig+" "+y);
}
int go(int arg) {

arg= arg*2;
return arg;
}
}

[Vijitha Kumara]

seen it in uni lecture notes

Please use real words when posting (uni instead University) .

In to your question,

How can a Class create an instance of itself before you have finished writing it?

you may thinking in procedural way? Think in Object oriented way.

[james dunster]

If you are going to criticise my use of vernacular English then perhaps in your posts you should use punctuation, at least one verb in a sentence and the indefinite article where required.

anyone hv a helpful answer 4 me?

[Abhijit Rai]

Hey James,
Just a couple of points

1. The code is not getting executed as soon as you finish writing the code,(First the .java file is compiled into a .class file and then the.class file is executed ).You first gotta save it with a .java extension (eg XCopy.java).

2.Next compile it .To compile the code ,one way would be using javac Xcopy.java command in DOS(ie Windows).

3.Having got the .class file you can now run it on the java virtual machine one way is would be using java XCopy again in DOS.At this step the execution actually occurs .

Now you should notice that when this step is reached there is a complete Java class (XCopy.class )available to the interpreter to make an object of .Hence an object is being created of a complete Java class .

Now when the JVM encounters " XCopy ref=new XCopy() "it creates an object of this class and assigns it to reference "ref".Think of a class as a professor's Lecture notes,now the Professor wont let you have his originals so you have to get it xeroxed .The copy of the originals can be thought of as the object .Hence you may have any number of copies(objects) of a professors original Lecture notes(Class).


Moreover you should go through the discussions about static members in any good book ,I would recommend :
Head first Java,Thinking in Java HTH

[salvin francis]

james dunster wrote:If you are going to criticise my use of vernacular English then perhaps in your posts you should use punctuation, at least one verb in a sentence and the indefinite article where required.

anyone hv a helpful answer 4 me?

Pal,
He was not critising your use of vernacular english
the point mentioned was

Vijitha Kumara wrote:Please use real words when posting

Please refer to this : http://faq.javaranch.com/java/UseRealWords

No one is here to make fun of you, we are all here to help you.

[salvin francis]

Change the perspective,

in Oops, its permissible to even do :

public class A
{
    int n;
    public A(int n){
        this.n=n;
        
        if(n>5){
            new A(n-1);
        }
        display();
    }
    /**
     * Lets Display !!
     */
    private void display()
    {
        System.out.println("Displaying n: "+n);
    }
    
    public static void main(String[] args)
    {
        new A(20);
    }
}

output:

Displaying n: 5
Displaying n: 6
Displaying n: 7
Displaying n: 8
Displaying n: 9
Displaying n: 10
Displaying n: 11
Displaying n: 12
Displaying n: 13
Displaying n: 14
Displaying n: 15
Displaying n: 16
Displaying n: 17
Displaying n: 18
Displaying n: 19
Displaying n: 20

In the above code, the Smallest Child's Display method Executes First !!!
and the Parent's Display executes Last !!

[Campbell Ritchie]

Shut up both of you. Apologies expected, please.

[james dunster]

But you can't do this:

class testAnyClass {
public static void main(String[] args) {

AnyObj a = new AnyObj();
}
}

class AnyObj {

public AnyObj() {
int k=1;
do{
AnyObj newAnyObj = new AnyObj();
k++;
} while (k<5);
}
}

Not that I want to and intuitively I can see that it wont work because the object can't finish making itself for making itself. I just can't see how it works when the object is inside its own class when created.

You split an infinitive in your message. Is that allowed?

[Charles Lyons]

Vijitha Kumara wrote:Please use real words when posting (uni instead University).

Actually, "uni" is listed in the OED as a "chiefly Australian colloquial term for university", though we've adopted it in the UK too. I therefore consider it a real word.

How can a Class create an instance of itself before you have finished writing it?

I can see how this could be confusing. Remember the difference between the definition of a class (and its methods) and its implementation. This allows the Java compiler to first construct a definition of every class (which lists its class name and package, properties, and method signatures) ahead of compiling the implementations for each of those methods. If it does this for each class, it will then know the complete structure of the entire application. On a second pass, it can compile the implementations of each method and reference the signatures it has stored. This two-step compilation is one way to ensure symbols are resolved correctly (it may not actually be the method used by the Java compiler, but it is a sensible approach).

This can be more confusing, yet also more obvious, in C++. There files are included in one-another, rather than referenced by package names. The C++ compiler tends to do compilation in a linear manner, and code like this can be problematic:

class MyContainer {
  private:
    ContainerChild* el;
};

class ContainerChild {
  private:
    MyContainer* parent;
};

Now in this case, MyContainer needs to know about ContainerChild, but ContainerChild also needs to know about MyContainer. Since the file is parsed linearly, the latter isn't a problem. But the former is: ContainerChild is only declared (and here also implemented) after MyContainer. The solution in C++ is to first declare the class, then define it later. Like this:

class ContainerChild;

class MyContainer {
  private:
    ContainerChild* el;
};

class ContainerChild {
  private:
    MyContainer* parent;
};

This is an explicit example of how a compiled language can differentiate between declarations (one-line classes), definitions (the class structure, usually in a header file) and implementations (the body of the code, usually in a cpp file).

Campbell Ritchie wrote:Apologies expected, please.

I'm sorry

[james dunster]

Campbell Ritchie wrote:Shut up both of you. Apologies expected, please.

Is this really what it is like in here? I was expecting better. Why are people so anal about one minor infraction of the use-real-words dictum in a beginner's first ever post? I am already a bit discouraged because of difficulty with the subject, I really don't need someone whom I dont' know demanding apologies! Or am I taking this seriously when I shouldn't? So fire me.

[james dunster]

Abhijit Rai wrote:Hey James,
Just a couple of points

1. The code is not getting executed as soon as you finish writing the code,(First the .java file is compiled into a .class file and then the.class file is executed ).You first gotta save it with a .java extension (eg XCopy.java).

2.Next compile it .To compile the code ,one way would be using javac Xcopy.java command in DOS(ie Windows).

3.Having got the .class file you can now run it on the java virtual machine one way is would be using java XCopy again in DOS.At this step the execution actually occurs .

Now you should notice that when this step is reached there is a complete Java class (XCopy.class )available to the interpreter to make an object of .Hence an object is being created of a complete Java class .

Now when the JVM encounters " XCopy ref=new XCopy() "it creates an object of this class and assigns it to reference "ref".Think of a class as a professor's Lecture notes,now the Professor wont let you have his originals so you have to get it xeroxed .The copy of the originals can be thought of as the object .Hence you may have any number of copies(objects) of a professors original Lecture notes(Class).


Moreover you should go through the discussions about static members in any good book ,I would recommend :
Head first Java,Thinking in Java HTH

Thank you. The piece of code is actually from Head First Java. I just read the chapter on Numbers and Statics again and I'm even more confused. As I (mis)understand it main() is a static method which is called without creating an instance of the class in which it sits but, in the code XCopy, main does create an instance of the class in which it sits ! So we have a static method creating an object, not of some other class, but of itself. I have been staring at this thing for hours and get my mind around the concept.

[Charles Lyons]

james dunster wrote:Or am I taking this seriously when I shouldn't?

I would guess so... this is mostly a very friendly place! Though the occasional polite dispute/debate has been known.

[james dunster]

[

How can a Class create an instance of itself before you have finished writing it?

I can see how this could be confusing. Remember the difference between the definition of a class (and its methods) and its implementation. This allows the Java compiler to first construct a definition of every class (which lists its class name and package, properties, and method signatures) ahead of compiling the implementations for each of those methods. If it does this for each class, it will then know the complete structure of the entire application. On a second pass, it can compile the implementations of each method and reference the signatures it has stored. This two-step compilation is one way to ensure symbols are resolved correctly (it may not actually be the method used by the Java compiler, but it is a sensible approach).

So that explains why in Salvin Francis' piece of code you can see the value of n go from 20 to 5 before the new objects are made and then from 5 to 20 after/ Not quite sure why it delivers them in opposite order from how they were made; I could ask him.

[Vijitha Kumara]

Charles Lyons wrote:

Vijitha Kumara wrote:Please use real words when posting (uni instead University).

Actually, "uni" is listed in the OED as a "chiefly Australian colloquial term for university", though we've adopted it in the UK too. I therefore consider it a real word.

Thanks Charles Lyons, for pointing that.

@james dunster

I think my intention was pointed out by salvin francis earlier. Anyway don't get discouraged. Everyone here are nice people trying to help others.

[Charles Lyons]

james dunster wrote:So that explains why in Salvin Francis' piece of code you can see the value of n go from 20 to 5 before the new objects are made and then from 5 to 20 after/ Not quite sure why it delivers them in opposite order from how they were made.

That is basically an example of recursion where the recursive function is the constructor for the object. Recursion is fundamentally a procedural concept, but the key here (which makes this OOP rather than procedural) is that the recursion happens inside the constructor and before the display() method is invoked on each object.

So we start with a constructor A(20), this then creates a new A(19), which creates a new A(18) etc. until we get to A(5) which then calls display() and prints 5, returns from that constructor so the A(6) constructor completes its execution with a display() and prints 6, returns from that constructor back into the A(7) constructor which completes and display() prints 7 etc. all the way back to the original A(20) constructor which does its display() and prints 20. Execution then returns to the main() method which completes and terminates the program.

This has nothing to do with how the compiler does its compilation, and nothing to do with definitions or implementations. It is just recursion of the same function (here a constructor) with a little OOP thrown in. If we weren't doing OOP, you could easily have done the same with:

public class A {
  public static void show(int n) {
    if(n>5) show(n-1);
    System.out.println(n);
  }
  public static void main(String[] args) {
    show(20);
  }
}

[Vijitha Kumara]

So that explains why in Salvin Francis' piece of code you can see the value of n go from 20 to 5 before the new objects are made and then from 5 to 20 after/ Not quite sure why it delivers them in opposite order from how they were made;

Well, those calls inside the constructor A(int n) won't returned until the condition evaluates to false for the first time(i.e: n==5). At that time the last one (one with the value 5 for n) completes it's constructor and returns, so does the rest.

[fred rosenberger]

Please, everyone, take a deep breath, and calm down. Believe it or not, we're all trying to help here.

the reason we ask everyone to 'use real words' is that we have a large international community. For a lot of folks, English is not their first language, and using colloquialisms like 'uni' instead of 'university' does not make it easy for them to follow. Heck, for a few years after I started spending time here, I didn't know what 'uni' meant - unicycle? universe? unit?

So, the request to use the full spelling of the word is to make it easier for someone reading your post to help YOU. If someone reads a post, would you rather they spend time helping you find the answer, or spend time trying to figure out what your abbreviations mean?

Almost everyone who comes here posts from a full keyboard. Saving 7 keystrokes in a post several hundred long seems rather silly, don't you think?

[james dunster]

Charles Lyons wrote:

james dunster wrote:So that explains why in Salvin Francis' piece of code you can see the value of n go from 20 to 5 before the new objects are made and then from 5 to 20 after/ Not quite sure why it delivers them in opposite order ~snip

I understood all of that and now also Silvan Francis' post for which thanks. To those of you still bleating on about the use of the word uni ............wel nvm.

[Campbell Ritchie]

Charles Lyons wrote: I'm sorry

It wasn't you I meant. You didn't do anything wrong.

I think Fred has said all that is required, while I was away from the keyboard.

[salvin francis]

Silvan

Actually i wanted to demonstrate that the code approach is not linear.


Deviating from main topic:

Most simplest explaination:
Every statement in A is executed.
so when the statement says create an object of A .......
program execution kinda halts there (not really - just for explaination) and the child Object is created (and its constructor is invoked).
This happens recursively until the condition is false. (Always have a terminating condition for recursive calls!!)


Now back to the main topic:
A class can create an instance of itself, rather in real world an anology is that a virus can replicate or an animal can give birth to an offspring

As for the location in code confusion:
A class is defined within a set of curly braces, so everything related to it would occur in that.
Hence the code to create another instance would occur within the class:
In your example: in a static method
In mine : in the constructor.