Does JAVA automatically intern strings?

10 Message(s) by 8 Author(s) originally posted in java programmer

From: pedro.checo

Date: Friday, October 26, 2007

Hi,

From 1.5 and on, does JAVA automatically intern string

<programming> A sequence of {data} values, usually {bytes},
	which usually stand for {characters} (a "character string").
	The {mapping} between values and characters is determined by
	the {character set} which is itself specified implcitly or
	explicitly by the environment in which the string is being
	interpreted.

	The most common character set is {ASCII} but, since the late
	1990s, there has been increased interest in larger character
	sets such as {Unicode} where each character is represented by
	more than eight {bits}.

	Most programming languages consider strings (e.g.
	"124:shabooya:\n", "hello world") basically distinct from
	numbers which are typically stored in fixed-length {binary} or
	{floating-point} representation.

	A {bit string} is a sequence of {bits}.

	(1999-12-21)

During a recent team discussion, we thought we read somewhere that
this was added to 1.5 as default behaviour, but we weren't sure. By
automatically interning I mean that if you've String a = new
String("ABC") then

String str = new String("ABC")

is equivalent to

String str = a.intern();

Thus far, we've found nothing to say that this was added.

Thanks in advance for your input.

From: Eric Sosman

Date: Friday, October 26, 2007

wrote in message

In {object-oriented programming} sending a message to an
	{object} (to invoke a {method}) is equivalent to calling a
	{procedure} in traditional programming languages, except that
	the actual code executed may only be selected at run time
	depending on the {class} of the object.  Thus, in response to
	the message "drawSelf", the method code invoked would be
	different if the target object were a circle or a square.

	(1995-02-16)

On 10/26/07 17:40,:

Hi,
From 1.5 and on, does JAVA automatically intern strings?
During a recent team discussion, we thought we read somewhere that
this was added to 1.5 as default behaviour, but we weren't sure. By
automatically interning I mean that if you've String a = new
String("ABC") then
String str = new String("ABC")
is equivalent to
String str = a.intern();
Thus far, we've found nothing to say that this was added.
Thanks in advance for your input.

Why not just try the experiment?

String s1 = "ABC";
String s2 = new String("ABC");
System.out.println(s1 == s2);

For extra amusement, you could check a few other things:

System.out.println(s1 == s1.intern());
System.out.println(s2 == s2.intern());

--
Eric.Sosman@xxxxxxxxxxx

From: pedro.checo

Date: Friday, October 26, 2007

wrote in message:
wrote in message On 10/26/07 17:40,:
> Hi,
From 1.5 and on, does JAVA automatically intern strings?
> During a recent team discussion, we thought we read somewhere that
> this was added to 1.5 as default behaviour, but we weren't sure. By
> automatically interning I mean that if you've String a = new
> String("ABC") then
> String str = new String("ABC")
> is equivalent to
> String str = a.intern();
> Thus far, we've found nothing to say that this was added.
> Thanks in advance for your input.
Why not just try the experiment?
String s1 = "ABC";
String s2 = new String("ABC");
System.out.println(s1 == s2);
For extra amusement, you could check a few other things:
System.out.println(s1 == s1.intern());
System.out.println(s2 == s2.intern());
--
Eric.Sos...@xxxxxxxxxxx Hide quoted text

1. Executable code, especially a "pure code" portion shared
	between multiple instances of a program running in a
	{multitasking} {operating system}.

	Compare {English}.

	2. Textual material in the mainstream sense; data in ordinary
	{ASCII} or {EBCDIC} representation (see {flat ASCII}).  "Those
	are text files; you can review them using the editor."

	These two contradictory senses confuse hackers too.

	[{Jargon File}]

	(1995-03-16)

-
- Show quoted text -

Good point. I should've been more specific. If that isn't the
default behaviour, then is there a flag

1. <programming> A variable or quantity that can take on one
	of two values; a bit, particularly one that is used to
	indicate one of two outcomes or is used to control which of
	two things is to be done.  "This flag controls whether to
	clear the screen before printing the message."  "The program
	status word contains several flag bits."  See also {hidden
	flag}, {mode bit}.

	2. {command line option}.

	[{Jargon File}]

	(1998-05-02)

(or any other way) to make it
so? We'd have to change a legacy library

<programming, library> A collection of {subroutines} and
	{functions} stored in one or more files, usually in compiled
	form, for linking with other programs.  Libraries are one of
	the earliest forms of organised {code reuse}.  They are often
	supplied by the {operating system} or {software development
	environment} developer to be used in many different programs.
	The routines in a library may be general purpose or designed
	for some specific function such as three dimensional animated
	graphics.

	Libraries are linked with the user's program to form a
	complete {executable}.  The linking may be {static linking}
	or, in some systems, {dynamic linking}.

	(1998-11-21)

in order to take advantage of
String.intern() and'd like to avoid making code

<software> Instructions for a computer in some programming
	language, often {machine language}.  The word "code" is often
	used to distinguish instructions from {data} (e.g. "The code
	is marked 'read-only'") whereas "{software}" is used in
	contrast with "{hardware}" and may consist of more than just
	code.

	(2000-04-08)

changes unless
absolutely necessary.

From: Eric Sosman

Date: Friday, October 26, 2007

wrote in message On 10/26/07 18:02,:
wrote in message:
wrote in message On 10/26/07 17:40,:

Hi,

From 1.5 and on, does JAVA automatically intern strings?

During a recent team discussion, we thought we read somewhere that
this was added to 1.5 as default behaviour, but we weren't sure. By
automatically interning I mean that if you've String a = new
String("ABC") then

String str = new String("ABC")

is equivalent to

String str = a.intern();

Thus far, we've found nothing to say that this was added.

Thanks in advance for your input.

Why not just try the experiment?

String s1 = "ABC";
String s2 = new String("ABC");
System.out.println(s1 == s2);
Good point. I should've been more specific. If that isn't the
default behaviour, then is there a flag (or any other way) to make it
so? We'd have to change a legacy library in order to take advantage of
String.intern() and'd like to avoid making code changes unless
absolutely necessary.

I'm not a language

1. <language, programming> {programming language}.

	2. <human language> {natural language}.

	(1998-09-07)

lawyer, but I think the `new'
operator is *required* to produce a brand-new object

<object-oriented> In {object-oriented programming}, an
	instance of the data structure and behaviour defined by the
	object's {class}.  Each object has its own values for the
	{instance variables} of its class and can respond to the
	{methods} defined by its class.

	For example, an object of the "Point" class might have
	instance variables "x" and "y" and might respond to the "plot"
	method by drawing a dot on the screen at those coordinates.

	(2004-01-26)

distinct from all existing objects. Check the JAVA
Language Specification if you want to find out for
sure.

... but what are these changes you contemplate,
and how do you propose to "take advantage" of interned
Strings? If all Strings were in fact interned so that
`s1.equals(s2) == (s1 == s2)' always held, you could
get away with using == instead of .equals(). But the
legacy library'd still get the right answer with
.equals(), just as it always did, and it wouldn't be
"absolutely necessary" to change it. So what's up?

--
Eric.Sosman@xxxxxxxxxxx

From: kohlerm

Date: Friday, October 26, 2007

Hi,

wrote in message:
wrote in message On 10/26/07 18:02,:
[snip]
sure.
... but what are these changes you contemplate,
and how do you propose to "take advantage" of interned
Strings? If all Strings were in fact interned so that
`s1.equals(s2) == (s1 == s2)' always held, you could
get away with using == instead of .equals(). But the
legacy library'd still get the right answer with
.equals(), just as it always did, and it wouldn't be
"absolutely necessary" to change it. So what's up?
--
Eric.Sos...@xxxxxxxxxxx Zitierten Text ausblenden -
- Zitierten Text anzeigen -

knowing when Strings are interned and when new copies of the
underlying char[] is very important to avoid consuming too much
memory

<storage> These days, usually used synonymously with {Random
	Access Memory} or {Read-Only Memory}, but in the general sense
	it can be any device that can hold {data} in
	{machine-readable} format.

	(1996-05-25)

.
Check my blog at https://www.sdn.sap.com/irj/sdn/weblogs?blog=/pub/wlg/5095

Regards,
Markus

From: A. Bolmarcich

Date: Friday, October 26, 2007

wrote in message:
wrote in message On 10/26/07 17:40,:
Hi,

From 1.5 and on, does JAVA automatically intern strings?

During a recent team discussion, we thought we read somewhere that
this was added to 1.5 as default behaviour, but we weren't sure. By
automatically interning I mean that if you've String a = new
String("ABC") then

String str = new String("ABC")

is equivalent to

String str = a.intern();

Thus far, we've found nothing to say that this was added.

Thanks in advance for your input.
Why not just try the experiment?
String s1 = "ABC";
String s2 = new String("ABC");
System.out.println(s1 == s2);
For extra amusement, you could check a few other things:
System.out.println(s1 == s1.intern());
System.out.println(s2 == s2.intern());

For even extra amusement

System.out.println(("A"+"B"+"C") == s1);

From: Patricia Shanahan

Date: Friday, October 26, 2007

wrote in message:

Hi,
wrote in message:
wrote in message On 10/26/07 18:02,:
[snip]
sure.

... but what are these changes you contemplate,
and how do you propose to "take advantage" of interned
Strings? If all Strings were in fact interned so that
`s1.equals(s2) == (s1 == s2)' always held, you could
get away with using == instead of .equals(). But the
legacy library'd still get the right answer with
.equals(), just as it always did, and it wouldn't be
"absolutely necessary" to change it. So what's up?

--
Eric.Sos...@xxxxxxxxxxx Zitierten Text ausblenden -

- Zitierten Text anzeigen -
knowing when Strings are interned and when new copies of the
underlying char[] is very important to avoid consuming too much
memory.
Check my blog at https://www.sdn.sap.com/irj/sdn/weblogs?blog=/pub/wlg/5095

Your blog seems to be about memory consumption and the sharing of the
underlying char[]. Perhaps you could explain the connection you seem to
see between that and intern?

Patricia

From: Lew

Date: Friday, October 26, 2007

wrote in message:

knowing when Strings are interned and when new copies of the
underlying char[] is very important to avoid consuming too much
memory.

Actually, it probably does not make a whole lot of difference except in corner
cases.

As a rule of thumb, the JVM

<language, architecture> (JVM) A specification for software
	which interprets {Java} programs that have been compiled into
	{byte-codes}, and usually stored in a ".class" file.  The JVM
	{instruction set} is {stack}-oriented, with variable
	instruction length.  Unlike some other instruction sets, the
	JVM's supports {object-oriented} programming directly by
	including instructions for object {method} invocation (similar
	to {subroutine} call in other instruction sets).

	The JVM itself is written in {C} and so can be {ported} to run
	on most {platforms}.  It needs {thread} support and {I/O} (for
	{dynamic class loading}).  The Java byte-code is independent
	of the platform.

	There are also some hardware implementations of the JVM.

	{Specification
	(http://www.javasoft.com/docs/books/vmspec/html/VMSpecTOC.doc.html)}.

	{Sun's Java chip
	(http://news.com/News/Item/0,4,9328,00.html)}.

	[Documentation?  Versions?]

	(2000-01-03)

and API

<programming> (API, or "application programming interface")
	The interface (calling conventions) by which an {application
	program} accesses {operating system} and other services.  An
	API is defined at {source code} level and provides a level of
	{abstraction} between the application and the {kernel} (or
	other privileged utilities) to ensure the {portability} of the
	code.

	An API can also provide an interface between a {high level
	language} and lower level utilities and services which were
	written without consideration for the {calling conventions}
	supported by compiled languages.  In this case, the API's main
	task may be the translation of parameter lists from one format
	to another and the interpretation of {call-by-value} and
	{call-by-reference} arguments in one or both directions.

	(1995-02-15)

(application programming

1. The art of debugging a blank sheet of paper (or, in these
	days of on-line editing, the art of debugging an empty file).

	2. A pastime similar to banging one's head against a wall, but
	with fewer opportunities for reward.

	3. The most fun you can have with your clothes on (although
	clothes are not mandatory).

	[{Jargon File}]

	(2003-02-12)

interface)are very smart about managing String
literals, although they'll be essentially permanent. That should not sweat
your memory consumption unless you've a true crapload of literals.

That leaves

<mathematics, data> (Or "terminal node") In a {tree}, a node
	which has no {daughter}.

	(1998-11-14)

String intermediate values and new-allocated instance

<programming> An individual {object} of a certain {class}.
	While a class is just the type definition, an actual usage of
	a class is called "instance".  Each instance of a class can
	have different values for its {instance variables}, i.e. its
	{state}.

	(1998-03-06)

s. API (application programming interface)
method

<programming> The name given in {Smalltalk} and other
	{object-oriented languages} to a procedure or routine
	associated with one or more {classes}.  An {object} of a
	certain class knows how to perform actions, e.g. printing
	itself or creating a new instance of itself, rather than the
	function (e.g. printing) knowing how to handle different types
	of object.

	Different classes may define methods with the same name
	(i.e. methods may be {polymorphic}).  The term "method" is used
	both for a named operation, e.g. "PRINT" and also for the code
	which a specific class provides to perform tha
	t operation.

	Most methods operate on objects that are instances of a
	certain class.  Some object-oriented languages call these
	"object methods" to distinguish then from "{class methods}".

	In {Smalltalk}, a method is defined by giving its name,
	documentation, temporary local variables and a sequence of
	expressions separated by "."s.

	(2000-03-22)

s like String.substring() tend to be pretty smart about reusing existing
char [] backing arrays, but it's pretty hard, too, to predict how the
optimizer will handle

1. <programming, operating system> A simple item of data that
	identifies a resource.  For example, a {Unix} file handle
	identifies an open file and associated data such as whether it
	was opened for read or write and the current read/write
	position.  On the {Macintosh}, a handle is a pointer to a
	pointer to some dynamically-allocated memory.  The extra level
	of indirection allows on-the-fly {memory compaction} or
	{garbage collection} without invalidating application program
	references to the allocated memory.

	2. <jargon> An alias used intended to conceal a user's true
	identity in an electronic message.  The term is common on
	Citizen's Band and other amateur radio but, in that context
	usually means the user's real name as {FCC} rules forbid
	concealing one's identity.

	Use of grandiose handles is characteristic of {crackers},
	{weenies}, {spods}, and other lower forms of network life;
	true hackers travel on their own reputations.

	Compare {nick}.

	[{Jargon File}]

	(2004-07-20)

situations. Inlining, loop

<programming> A sequence of {instructions} that the
	{processor} repeats, either until some condition is met, or
	indefinitely.

	In an {structured language} (e.g. {C}, {Pascal}, {BASIC}, or
	{Fortran}), a loop is usually achieved with {for loop}, {while
	loop} or {repeat loop} constructs.

	In other languages these constructs may be synthesised with a
	{jump} ({assembly language}) or a {GOTO} (early Fortran or
	BASIC).

	(1999-05-06)

unrolling and
common-subexpression refactoring

<object-oriented, programming> Improving a computer {program}
	by reorganising its internal structure without altering its
	external behaviour.

	When software developers add new features to a program, the
	code degrades because the original program was not designed
	with the extra features in mind.

	This problem could be solved by either rewriting the existing
	code or working around the problems which arise when adding
	the new features.  Redesigning a program is extra work, but
	not doing so would create a program which is more complicated
	than it needs to be.  Refactoring is a collection of
	techniques which have been designed to provide an alternative
	to the two situations mentioned above.

	The techniques enable programmers to restructure code so
	that the design of a program is clearer.  It also allows
	programmers to extract {reusable components}, streamline a
	program, and make additions to the program easier to
	implement.

	Refactoring is usually done by renaming {methods}, moving
	{fields} from one {class} to another, and moving code into a
	separate method.

	Although it is done using small and simple steps,
	refactoring a program will vastly improve its design and
	structure, making it easier to maintain and leading to
	more robust code.

	{"Refactoring, Reuse & Reality" by Bill Opdyke
	(http://st-www.cs.uiuc.edu/users/opdyke/wfo.990201.refac.html)}.

	{"Refactoring, a first example" by Martin Fowler
	(http://www.aw.com/cseng/titles/0-201-89542-0/vidrefact/vidrefact.html)}.

	(2001-05-02)

are things it does that I can imagine
affecting String allocation, much less interning.

I suspect that most programs facing memory problems won't find interning to
be much of an issue or much help. I'm also pretty sure that there are times
when it'll. And there always come those times when every last drop counts.

My concern is that the real

1. Not simulated.  Often used as a specific antonym to
	{virtual} in any of its jargon senses.

	2. <mathematics> {real number}.

	[{Jargon File}]

	(1997-03-12)

impact on performance of different idioms is so
very often counter-intuitive.

--
Lew

From: Roedy Green

Date: Friday, October 26, 2007

wrote in message, quoted
or indirectly quoted someone who said :

String str = new String("ABC")
is equivalent to
String str = a.intern();

nope. The very opposite. See http://mindprod.com/jgloss/intern.html
--
Roedy Green

A language proposed by Cii {Honeywell-Bull} to meet the DoD
	{Ironman} requirements which led to {Ada}.  This language won
	in 1979.

	["On the GREEN Language Submitted to the DoD", E.W. Dijkstra,
	SIGPLAN Notices 13(10):16-21 (Oct 1978)].

	(1994-12-02)

Canadian Mind Products
The JAVA Glossary
http://mindprod.com

From: Owen Jacobson

Date: Friday, October 26, 2007

wrote in message:

Hi,
>From 1.5 and on, does JAVA automatically intern strings?
During a recent team discussion, we thought we read somewhere that
this was added to 1.5 as default behaviour, but we weren't sure. By
automatically interning I mean that if you've String a =3D new
String("ABC") then
String str =3D new String("ABC")
is equivalent to
String str =3D a.intern();
Thus far, we've found nothing to say that this was added.

It wasn't.

You've to read between the lines a bit, but the JLS forbids this
optimization if its effects'd be detectable. Follow along, if you
will:

1=2E 15.21.3 states that "At run

<operating system, programming> The process of carrying out
	the {instructions} in a computer program by a computer.

	See also {dry run}.

	(1996-05-13)

time, the result of =3D=3D is true if the
operand values are both null

<programming> A special value used in several languages to
	represent the thing referred to by an uninitialised pointer.

	<database> A special value that may be stored in some database
	columns to represent an unknown, missing, not applicable, or
	undefined value.  Nulls are treated completely differently
	from ordinary values when evaluating SQL expressions and there
	are several SQL constructs for dealing with nulls.

	(2003-06-17)

or both refer to the same object or
array; otherwise, the result is false."

2=2E 4.3.1 states that "An object is a class

1. <programming> The prototype for an {object} in an
	{object-oriented language}; analogous to a {derived type} in a
	{procedural language}.  A class may also be considered to be a
	set of objects which share a common structure and behaviour.
	The structure of a class is determined by the {class
	variables} which represent the {state} of an object of that
	class and the behaviour is given by a set of {methods}
	associated with the class.

	Classes are related in a {class hierarchy}.  One class may be
	a specialisation (a "{subclass}") of another (one of its
	"{superclasses}") or it may be composed of other classes or it
	may use other classes in a {client-server} relationship.  A
	class may be an {abstract class} or a {concrete class}.

	See also {signature}.

	2. <programming> See {type class}.

	3. <networking> One of three types of {Internet addresses}
	distinguished by their most significant bits.

	3. <language> A language developed by the {Andrew Project}.
	It was one of the first attempts to add {object-oriented}
	features to {C}.

	(1995-05-01)

instance or an array." and
that "A class instance is explicitly created by a class instance
creation expression." The reference

1. <programming> An {address}, from the point of view of a
	programming language.  A pointer may be typed, with its {type}
	indicating the type of data to which it points.

	The terms "pointer" and "reference" are generally
	interchangable although particular programming languages often
	differentiate these two in subtle ways.  For example, {Perl}
	always calls them references, never pointers.  Conversely, in
	C, "pointer" is used, although "a reference" is often used to
	denote the concept that a pointer implements.

	{Anthony Hoare} once said:

	Pointers are like jumps, leading wildly from one part of the
	data structure to another.  Their introduction into high-level
	languages has been a step backward from which we may never
	recover.

	[C.A.R.Hoare "Hints on Programming Language Design", 1973,
	Prentice-Hall collection of essays and papers by Tony Hoare].

	2. <operating system> (Or "mouse pointer") An {icon}, usually
	a small arrow, that moves on the screen in response to
	movement of a {pointing device}, typically a {mouse}.  The
	pointer shows the user which object on the screen will be
	selected etc. when a mouse button is clicked.

	(1999-07-07)

for that second statement covers
expressions of the form 'new Type (Arguments)', which implicitly
includes 'new String ("Some literal")'.

Those two statements combine to imply that

String a =3D new String ("foo");
String b =3D new String ("foo");

boolean same =3D (a =3D=3D b);

will always result in 'same' containing false, eg., that the two
objects aren't the same instance.

3=2E 3.10.5 states, "String literals-or, more generally, strings that
are the values of constant expressions (=A715.28)-are "interned" so as
to share unique instances, using the method String.intern," which
does not cover new expressions, and the documentation

<programming> The multiple kilograms of macerated, pounded,
	steamed, bleached, and pressed trees that accompany most
	modern software or hardware products (see also {tree-killer}).
	Hackers seldom read paper documentation and (too) often resist
	writing it; they prefer theirs to be terse and {on-line}.  A
	common comment on this predilection is "You can't {grep} dead
	trees".

	See {drool-proof paper}, {verbiage}, {treeware}.

	[{Jargon File}]

	(2003-10-25)

for String.intern
follows this with

"for any two strings s and t, s.intern() =3D=3D t.intern() is true if
and only
if s.equals(t) is true."

which implies

<logic> (=> or a thin right arrow) A binary {Boolean} function
	and {logical connective}.  A => B is true unless A is true and
	B is false.  The {truth table} is

		A B | A => B
		----+-------
		F F |   T
		F T |   T
		T F |   F
		T T |   T

	It is surprising at first that A => B is always true if A is
	false, but if X => Y then we would expect that (X & Z) => Y
	for any Z.

	(1995-09-30)

that were strings automatically interned such that 'new
String ("foo")' evaluated to an interned instance of "foo", then

String a =3D new String ("foo");
String b =3D new String ("foo");

boolean same =3D (a.intern () =3D=3D b.intern ());

would result in 'same' being set to true, since a.equals(b) and the
strings would've been interned.

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