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From: Eric Sosman <esosman@comcast-dot-net.invalid>
Newsgroups: comp.lang.java.programmer
Subject: Re: how actually string store in java machine
Date: Wed, 26 Mar 2014 13:54:28 -0400
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On 3/26/2014 11:32 AM, markspace wrote:
> On 3/25/2014 7:37 PM, Roedy Green wrote:
>> On Tue, 25 Mar 2014 20:31:50 +0100, Jan Burse <janburse@fastmail.fm>
>> wrote, quoted or indirectly quoted someone who said :
>>
>>>
>>> http://stackoverflow.com/questions/16690815/shared-constant-pool
>>
>> I was under the impression all string constants lived in the same pool
>> along with the interned strings. That would imply a shared constant
>> pool since day 1.  Have I had this wrong since Java 1.0?
>>
>
>
> Yes, I'm pretty sure you've been wrong if that's what you thought.  Each
> class has its own pool of strings.  If you have a class with a constant
> string:
>
>    String a = "a";
>    String b = "a";
>
> The constant "a" is only stored once, and a==b will be true.  But that's
> only for a single class file.  Outside of the class file, each string is
> unique and a different object.  I don't think any implementations of the
> JVM called String.intern() for each constant string read in each class
> file.

	// Foo.java:
	public class Foo {
	   public static String HELLO = "Hello";
	}

	// FooBar.java:
	public class FooBar {
	   public static void main(String[] unused) {
	      System.out.println("Foo.HELLO == \"Hello\": "
	         + (Foo.HELLO == "Hello"));
	   }
	}

... prints

	Foo.HELLO == "Hello": true

... on my Java 1.7.0_51 system.  You might also want to re-read
Section 3.10.5 of the Java Language Specification; for Java 7 and 8
it reads (in part)

	"Moreover, a string literal always refers to the /same/
	instance of class String. This is because string literals -
	or, more generally, strings that are the values of constant
	expressions (§15.28) - are "interned" so as to share unique
	instances, using the method String.intern."

Java 6 words it a bit differently:

	"String literals-or, more generally, strings that are the
	values of constant expressions (§15.28)-are "interned" so
	as to share unique instances, using the method String.intern."

... and goes on to give a multi-class, multi-package example.

> And certainly if you read a string from a file or the network, the VM
> does not call String.intern() for you.  That would be silly.

     That would be silly, yes.  But the handling of literals is
a different matter, and does not behave as you described.

-- 
Eric Sosman
esosman@comcast-dot-net.invalid