a question regrading new operator

[Li Wenfeng]

given the following sample code:
class A{
int i=getJ();
int j=10;
int getJ(){return j;}
public static void main(String[] args){
A a = new A();
System.out.println("i="+a.i+" j="+a.j);
}
}
Why does it output that i is 0, rather than 10?
thanks!
[ September 22, 2002: Message edited by: Li Wenfeng ]
[ September 24, 2002: Message edited by: Li Wenfeng ]

[Jeremy Hynoski]

Hi,
A look at the decompiled class file shows a few secrets...
Here is the constructor for your program once the compiler gets its filthy mitts on it:
Method A()
0 aload_0
1 invokespecial #1 <Method java.lang.Object()>
4 aload_0
5 aload_0
6 invokevirtual #2 <Method int getJ()>
9 putfield #3 <Field int i>
12 aload_0
13 bipush 10
15 putfield #4 <Field int j>
18 return
Now, ignoring the convoluted garble, check out the order. putfield is where a value is assigned into a variable. Also note where "j" is actually set... line 13 and 15 where the integer 10 is pushed and put into field 4. So what's in field j when the method getJ() is called? Nothing? Why? Because all of the assignments happen in order.
When an instance is constructed, the constuctor's parent constructor is called first (see line 1 in the decompile?). After that, the instance variables are all initialised IN ORDER OF DEFINITION. Then the instance blocks are run, and after that, the actual constructor code. Its important to note this order, and also the guarantee that everything will be defined when you try to run code - you will never get an uninitialised error when creating an instance because the specification simply won't let you. However, it has set up j with a zero value before it gets around to putting in the stuff you want in it. This is why "i" will return zero.
Hope that helps!
Jeremy
[ September 24, 2002: Message edited by: Jeremy Hynoski ]

[Barkat Mardhani]

First, the code will not compile, as you are referencing instance variables i and j from a static method.

The code does compile and run as posted. The reason i is printing 0 is as follows (in my mind):
1. In first pass (when Object is being create), compiler is will define and assign default values to all member variables. So i and j both get zeros.
2 In second pass, compiler will actually evaluate the programmer's assigned value for each member, in the textual order. While doing so for i, it needs the value of j. At that time j is zero, so i gets 0. Next, j is assined 10. If you put a print statement after it, you see j is 10.
Hope this helps
Barkat

[Li Wenfeng]

thanks greatly for your warmhearted replies!

[Shishio San]

Hi,
I think you meant: why does i print 0 instead of 10 ?
The i's initializer expression gets excuted be j's initialization to 10 and at that time the method getJ() returns the default value of J which is 0.
N.B
Before executing any intializer expression, instance members are constructed and assigned to their default values.