Dan's Q - char to byte

[Cathy Song]

class JSC201 {
static byte m1() {
final char c = '\u0001';
return c; // 1
}
static byte m3(final char c) {
c = 's'; //I added this line
return c;} // 2
public static void main(String[] args) {
char c = '\u0003';
System.out.print(""+m1()+m3(c));
}
}
I dont understand why line 2 is a compiler error. The method m3() is returning c, which is a compile time constant, since its declared as final,
AND I am assigning a value to it.

[Marlene Miller]

Hi Cathy.
A simple name is a compile-time constant if (1) it is declared final and (2) it has a variable initializer and (3) that initalizer is a compile-time constant.
final int x1 = 10;
final int x2 = x1;
final int x3 = x1 + x2;
final String s1 = "abc";
final String s2 = s1;
A final parameter has a value at run-time - the argument of the method invocation.
final int y1; // no
final int y2 = y1; // no
final Object o1 = null; // no
final Object o2 = new Object(); // no
final String s3 = new String("abc"); // no
[ October 16, 2003: Message edited by: Marlene Miller ]

[Agandhi]

Hi Marlene,
I understood your explaination, except for final String s3 = new String("abc"). Why can't we call it as compile-time constant?
Thanks..

[Anthony Roy]

The JLS says this:
15.28 Constant Expression
ConstantExpression:
Expression
A compile-time constant expression is an expression denoting a value of primitive type or a String that is composed using only the following:
* Literals of primitive type and literals of type String
* Casts to primitive types and casts to type String
...
i.e. Objects cannot be constant expressions, and new String("abc") is an Object (and not merely a string literal).
It seems to me that a good way of thinking about strings is somewhere between primitives and Objects, String objects seem to me to be a sort of wrapper around a string literal, much like Integer is a wrapper around int, but with automatic wrapping and unwrapping enabled (much as the Integer and other wrapper classes will behave in J2SDK1.5).
Of course the technical details may be much different to this, but I find it a good way to understand what is going on in these cases.

[Cathy Song]

Thanks Marlene and Anthony.

[Dan Chisholm]

Originally posted by ArchanaG Reddy:
Hi Marlene,
I understood your explaination, except for final String s3 = new String("abc"). Why can't we call it as compile-time constant?
Thanks..

The String referenced by s3 is not created until run-time. Since it does not exist at compile-time, it can not be a compile-time constant.

[Marlene Miller]

String objects seem to me to be a sort of wrapper around a string literal,

I think it would be safer to think of a string as a wrapper around a char array. Take a peek at java.lang.String.

private char value[];
    public String() {
        value = new char[0];
    }

Now a char array is also an object, they say. This object has unnamed variables of type char, they say. So here is my abstract image of these ideas (It looks better in Visio.)

String s = new String(�abc�);
                                      ---------------
 ---          --------------         |  ---+---+---  |
| s | -----> | char[] value | -----> | | a | b | c | |
 ---          --------------         |  ---+---+---  |
                                      ---------------
variable      String object            array of char

And I know it is hard to get used to, but the JLS says a string literal is a reference to an instance of class String.
That is, the source code �abc� represents a (32-bit or whatever) number that identifies some object on the heap. At some point the virtual machine replaces the source code string �abc� with a real reference.
Every now and then these details help me to analyze some tricky problems.
[ October 17, 2003: Message edited by: Marlene Miller ]

[pola]

Hi,
I am new to this forum and i would be thankful if you could tell me what Dans'Q....is it a mock exam or ??....give me the link .....

[Harwinder Bhatia]

Hi Vamsi
Welcome to the Ranch. Here's the link to Dan's website:
http://www.danchisholm.net
Good luck !
Regards
Harwinder