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String comparisons

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  String comparisons Merciadri Luca <Luca.Merciadri@student.ulg.ac.be> - 2011-04-02 22:26 +0200
    Re: String comparisons Aéris <aeris@imirhil.fr> - 2011-04-02 23:23 +0200
      Re: String comparisons Aéris <aeris@imirhil.fr> - 2011-04-02 23:27 +0200
      Re: String comparisons Merciadri Luca <Luca.Merciadri@student.ulg.ac.be> - 2011-04-03 18:59 +0200
        Re: String comparisons Lew <noone@lewscanon.com> - 2011-04-06 12:19 -0400
    Re: String comparisons Roedy Green <see_website@mindprod.com.invalid> - 2011-04-02 21:17 -0700
    Re: String comparisons Merciadri Luca <Luca.Merciadri@student.ulg.ac.be> - 2011-04-03 12:07 +0200

#477 — String comparisons

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Hi,

I'm given the following Java code snippet:
==
String d = new String("abcd");
String s = new String("abcd");
String c = d;
String n = c.clone(); // see discussion here after
==

Using System.out.println for the following args:

1. d == s
2. d == c
3. c == n
4. d.equals(s)
5. s.equals(n)
6. d.equals(null)

gives respectively
1. FALSE
2. TRUE
3. TRUE // supposed
4. TRUE
5. FALSE // supposed
6. FALSE

where supposed comments are there because my compiler does not accept
the

==
String n = c.clone();
==
instruction in the main program (because

clone() has protected access in java.lang.Object

).


First, I want to understand which values are compared in 1., 2. and 3.

Second, I want to understand what I should change to get no compiler
error using the clone method on c.

Thanks.
- -- 
Merciadri Luca
See http://www.student.montefiore.ulg.ac.be/~merciadri/
- -- 

The ends justify the means.
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#480

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Le 02/04/2011 22:26, Merciadri Luca a écrit :
> 1. d == s
> 2. d == c
> 3. c == n
> 4. d.equals(s)
> 5. s.equals(n)
> 6. d.equals(null)
> 
> gives respectively
> 1. FALSE
> 2. TRUE
> 3. TRUE // supposed
> 4. TRUE
> 5. FALSE // supposed
> 6. FALSE

This is totally wrong

d, s and n are 3 different instances, so d == s and c == n are all
FALSE. c is the same instance than d, so d == c is TRUE.

But all represent the same string "abcd" so all .equals() all TRUE.


For the compiler error, Object.clone() is protected so you can't use it
until you override it.

And String is immutable in Java, so there is no interest to clone
String. If you want to copy String, use new String(c) instead, but I
don't see the goal?

- -- 
Aeris
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#481

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Le 02/04/2011 23:23, Aéris a écrit :
> But all represent the same string "abcd" so all .equals() all TRUE.

Except for the d.equals(null), of course?

- -- 
Aeris
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#513

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I can explain why 1., 2., 4. and 6. are respectively FALSE, TRUE,
TRUE and FALSE.

Considering ints apart, the == operator returns TRUE e.g. in a == b if
a and b are same objects. As a result, d != s is true, and d == s is
false because d and s are not the same objects.

For question 2., d == c is true because of the affectation String c =
d. That is, d and c are exactly the same object.

This was not what I had understood from reading Aéris' message at
first sight, but re-looking at it, this is the same
explanation. Thanks, Aéris. I would have better understood if one had
said that == tests if the two objects are the same!

- -- 
Merciadri Luca
See http://www.student.montefiore.ulg.ac.be/~merciadri/
- -- 

If all else fails, try the obvious.
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#553

Merciadri Luca wrote:
> I can explain why 1., 2., 4. and 6. are respectively FALSE, TRUE,
> TRUE and FALSE.
>
> Considering ints apart, the == operator returns TRUE e.g. in a == b if
> a and b are same objects. As a result, d != s is true, and d == s is
> false because d and s are not the same objects.

More precisely, they are not objects at all but pointers to objects.

> For question 2., d == c is true because of the affectation String c =
> d. That is, d and c are exactly the same object.

Not true.  They are pointers to the same object.

> This was not what I had understood from reading Aéris' message at
> first sight, but re-looking at it, this is the same
> explanation. Thanks, Aéris. I would have better understood if one had
> said that == tests if the two objects are the same!

That's in the tutorials.

You should read the tutorials.  We've been recommending them for a long time.

-- 
Lew
Honi soit qui mal y pense.
http://upload.wikimedia.org/wikipedia/commons/c/cf/Friz.jpg

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#484

On Sat, 02 Apr 2011 22:26:27 +0200, Merciadri Luca
<Luca.Merciadri@student.ulg.ac.be> wrote, quoted or indirectly quoted
someone who said :

>String d = new String("abcd");
>String s = new String("abcd");
>String c = d;
>String n = c.clone(); // see discussion here after

new String almost never needed.  Just use "abcd".

You can't  use String.clone because it is a protected method and
String is final. So any speculation about its behaviour is
theological.


See http://mindprod.com/jgloss/gotchas.html
especially the one on string comparison.

See also http://mindprod.com/jgloss/string.html
http://mindprod.com/jgloss/interned.html
-- 
Roedy Green Canadian Mind Products
http://mindprod.com
Doing what the user expects with respect to navigation is absurdly important for user satisfaction.
~ anonymous Google Android developer

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#493

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Thanks for all the answers. Well, this is not `totally wrong.' What
you meant is that the two values I supposed (i.e. 3. and 5.) are
incorrect. That is, 3. should be FALSE, and 5. should be TRUE.

Thanks for the explanations.
- -- 
Merciadri Luca
See http://www.student.montefiore.ulg.ac.be/~merciadri/
- -- 

First deserve, then desire.
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