Dan exam doubt 19

[amit taneja]

hii in one question dan

class I {
private String name;
protected void finalize() {System.out.print(name);}
public I(String s) {name = s;}
}
class J {
private static void m1(I[] a1) {
a1[0] = a1[1] = a1[2] = null;
}
public static void main (String[] args) {
I[] a1 = new I[3]; // 1
a1[0] = new I("A"); // 2
a1[1] = new I("B"); // 3
a1[2] = new I("C"); // 4
m1(a1);
System.gc();
}}

After method m1 returns, the object created on which line is not eligible for garbage collection?

a. 1
b. 2
c. 3
d. 4
e. None of the above
f. Compile-time error
g. Run-time error

the answer is "a"
with expanation...
Please note that this question asks which objects are NOT eligible for garbage collection after method m1 returns. After method m1 returns, the array a1 created on line 1 is not eligible for garbage collection. Method m1 sets all elements of the array to null; so the objects created on lines 2, 3 and 4 are eligible for garbage collection when method m1 returns.


i didn't understand why the elements of a[0], a[1], a[2] become null...when there refernce in main method is still there ?

[Bajji Pat]

Hi Amit,

In your case, there is one "Array Object" created on the heap which can hold four references(or whatever the number).
In this case the elements of the array have been nulled but the array object has itslef not been nulled.

This code is similiar to your case :

         Sample s[] = new Sample[3];

s[0] = null;
s[1] = null;
s[2] = null;
// Here we see that the Array object is not null
                
                System.out.println("*****"+(s==null));
System.out.println(s instanceof Object);

Regards
Balaji Pattabhiraman
[ April 22, 2005: Message edited by: Bajji Pat ]

[Sunyaluk Boonmas]

This is the sample of pass by reference problem.

a1 in main method and a1 in argument has the same reference.

So, if we change any attribute of them, it will effect each other value.

However array object in line 1 still reachble by live thread.

[amit taneja]

i m still not clear Sir


so can you please tell me what is pass by refernce ?
is it like this


class A{
Stringbuffer a = new Stringbuffer("abc");
Stringbuffer b;

public static void main(String args[])
{
b=a;
b=b.append("xyz");
System.out.println(a +" "+b);
}
}

will produce answer abcxyz abcxyz

because both refer to same object ?


is this is fine eg of "pass by reference" ?

[Sunyaluk Boonmas]

I revise a little in your code here.

 
public class A{
StringBuffer a = new StringBuffer("abc");
StringBuffer b;
 
public static void main(String args[])
{
A obj = new A();
    obj.doIt();
}
 
void doIt(){
b=a;
b=b.append("xyz");
    System.out.println(a +" "+b);
}
 
}
 
 
 

This is the sample of the variable that refer to the sam object.So,change one will effect one.It like pass by reference,but not.

Pass by reference in my understand is the Object that pass to argument.The argument variable will keep the copy of reference(same refer as the original variable).So, if you change the attribute of the object thatpass to method.It will effect to the original variable that keep this referenvce too.And I can explain like this code.

  
public class A{
StringBuffer a = new StringBuffer("abc");
 
public static void main(String args[])
{
A obj = new A();
    obj.doIt(obj.a);
}
 
void doIt(StringBuffer b){
b=b.append("xyz");
    System.out.println(a +" "+b);
}
 
}
 

If I miss please comment me.

Thank you.
[ April 22, 2005: Message edited by: Sunyaluk Boonmas ]

[amit taneja]

ya
please anybody justfiy the above code..and
explain pass by reference in more details...


thanx

[Sunyaluk Boonmas]

This explanation very clear, I think.

http://www.javaranch.com/campfire/StoryPassBy.jsp

And this flash playing is interesting.I think it is very good to show how it work.


http://www.geocities.com/mcglonec1978/javacert/paramPassing_highres.html

This it main url.
http://www.geocities.com/mcglonec1978/javacert/javacert.html#param_passing

Hope it useful for you.

However if I am wrong in any point please tell me.

I will exam next week.

cheer...