Please help me! About the instance initialization block!

[yen hoang]

Pleaon explain for me why the assign a certain value to a variable in instance initialization block is ok but increment ( or st like that) is not

    
public class A
{
       {
              x=8; // it's ok
              y++;// compile error
        }
        int x,y;
        publlic A()
        {
               System.out.println("x= " +x +" y=" +y);
        }
        public static void main(String[] arg)
        {
                A a= new A();
         }
}

I thought that the instance initialization block is called after all super classes of this class are called. At that time all instance variables have been initialized. So why I can't increment or add a value to it?

[Mahima Chhinchani]

Every thing is fine except the way you have declared the variable , the compiler will complain about illigal forward refrencing, just declare the variable before the initialization block , there will be no compilation problem

[Ankit Garg]

you cannot access the value of a field before its declaration (as Mahima mentioned). That's why you are getting an error. Try these too

class ForwardReferencing {
    {
        a = 10;
        a = b = 20;
        //a = b;
        method(b);
    }
    void method() {
        System.out.println(b);  //20
    }
    int a,b;
}

[Welly Tambunan]

Ankit Garg wrote:you cannot access the value of a field before its declaration (as Mahima mentioned). That's why you are getting an error. Try these too

class ForwardReferencing {
    {
        a = 10;
        [b]a = b = 20; // interesting line[/b]
        //a = b;
        method(b);
    }
    void method() {
        System.out.println(b);  //20
    }
    int a,b;
}

but why the bold part is not cause compile error.
we read value of b and assigned to a ..?
confused...

[Bob Wheeler]

Assigning a variable:
So for assigning a value to a variable in an initialization block I don't need to declare the variable before. I can do it afterwards.
But if I do it outside of the initialization block I MUST declare the variable before the assignment:

  a = 20; // ERROR
	int a;  
    { x=20; // WORKS
     } 
    int x; 

Accessing a variable:
But if I access a value, e.g. by using it as a parameter, I have to declare the variable before accessing it, independently if it is used within
or outside an initialization block:

      { x++; // ERROR
                } 
    int x;   

These statements are true for static or instance initialization block ???
Any errors in my statements?

Thanks
Bob

[yen hoang]

Welly Tambunan wrote:

Ankit Garg wrote:you cannot access the value of a field before its declaration (as Mahima mentioned). That's why you are getting an error. Try these too

class ForwardReferencing {
    {
        a = 10;
        [b]a = b = 20; // interesting line[/b]
        //a = b;
        method(b);
    }
    void method() {
        System.out.println(b);  //20
    }
    int a,b;
}

but why the bold part is not cause compile error.
we read value of b and assigned to a ..?
confused...

The problem makes me be confused is the time at which instance initialization block ewxecuted. Theoretically,it's executed right after the call to supper() in a constructor. But at that time,all instance variables have been initialised.so why they can't be accessed.
And another question is: if they can be accessed,why they can be assigned valued?

[Ankit Garg]

Welly Tambunan wrote:

Ankit Garg wrote:you cannot access the value of a field before its declaration (as Mahima mentioned). That's why you are getting an error. Try these too

class ForwardReferencing {
    {
        a = 10;
        [b]a = b = 20; // interesting line[/b]
        //a = b;
        method(b);
    }
    void method() {
        System.out.println(b);  //20
    }
    int a,b;
}

but why the bold part is not cause compile error.
we read value of b and assigned to a ..?
confused...

When you say

a = b = 20;

then you are not assigning the value of b to a. You are assigning 20 to a. It is equivalent to

b = 20;
a = 20;

There are some ways to confirm it but I don't want to confuse you...

[Welly Tambunan]

Ankit Garg wrote:

b = 20;
a = 20;

There are some ways to confirm it but I don't want to confuse you...

oo.. i see. So i was wrong about that code. Can you give me another way to confirm it?
I'm interesting to take a look at it.

[Ankit Garg]

Well if you want to know, then take a look here...

[yen hoang]

Excuse me! The problem makes me be confused is the time at which instance initialization block ewxecuted. Theoretically,it's executed right after the call to supper() in a constructor. But at that time,all instance variables have been initialised.so why they can't be accessed.
And another question is: if they can be accessed,why they can be assigned valued?

[Marvin Soliven]

yen hoang wrote:Pleaon explain for me why the assign a certain value to a variable in instance initialization block is ok but increment ( or st like that) is not

    
public class A
{
       {
              x=8; // it's ok
              y++;// compile error
        }
        int x,y;
        publlic A()
        {
               System.out.println("x= " +x +" y=" +y);
        }
        public static void main(String[] arg)
        {
                A a= new A();
         }
}

JLS (§8.3.2.3) has this to say :

The declaration of a member needs to appear before it is used only if the member is an instance (respectively static) field of a class or interface C and all of the following conditions hold:

1. The usage occurs in an instance (respectively static) variable initializer of C or in an instance (respectively static) initializer of C.

2. The usage is not on the left hand side of an assignment.

3. C is the innermost class or interface enclosing the usage.

By incrementing (y++) which is equivalent to (y = y+1) , you implicitly used y at the right side of an assignment. Just IMHO though :-)

Interestingly, if you change y to static int in your declaration, the compiler will not complain anymore See JLS (§8.3.2.2 )

[yen hoang]

JLS (§8.3.2.3) has this to say :

The declaration of a member needs to appear before it is used only if the member is an instance (respectively static) field of a class or interface C and all of the following conditions hold:

1. The usage occurs in an instance (respectively static) variable initializer of C or in an instance (respectively static) initializer of C.

2. The usage is not on the left hand side of an assignment.

3. C is the innermost class or interface enclosing the usage.

By incrementing (y++) which is equivalent to (y = y+1) , you implicitly used y at the right side of an assignment. Just IMHO though :-)

Interestingly, if you change y to static int in your declaration, the compiler will not complain anymore See JLS (§8.3.2.2 )

thank you very much. I understood!