Is String intern is shared between run?

Question

Everytime I run this program that I want to display the memory address, it always output same result:

public static void main(String[] args) throws InterruptedException {
    String foo = "a";
    String foo2 = "a";
    System.out.println(Integer.toHexString(foo.hashCode()));
    System.out.println(Integer.toHexString(foo2.hashCode()));

    foo = "a";
    foo2 = "a";
    System.out.println(System.identityHashCode(foo));
    System.out.println(System.identityHashCode(foo2));

    Thread.sleep(1000000);
}

The purpose of this is I am doing some performance test between Streams and Imperative style. So I would like to know if this value is cache, and determines if I should restart my pc or manually run garbage collection via JConsole / JvisualVm.

No. And `String` intern (and hash code) **do not** return a memory address. — Elliott Frisch, Feb 09 '17 at 03:45
@ElliottFrisch please correct my question, how can I display the memory address of string? — richersoon, Feb 09 '17 at 03:46
By using a language that allows you address memory. C++ or C maybe. — Elliott Frisch, Feb 09 '17 at 03:46
What do you want to display it for? And why is it a problem that it shows the same? i.e. what do you expect? — weston, Feb 09 '17 at 03:48
Please also explain the `.hashcode()` calls, what relevance do they have to the question? — weston, Feb 09 '17 at 03:55
@weston I would like to play or experiment, if String cache in String poll / intern when each run of main method. This is crucial for performance test, to determine if should I restart my pc or clear the garbage collection in each run, to uncache it from String poll / intern. — richersoon, Feb 09 '17 at 04:43
But it isn't cached. It's a new JVM. You can't 'clear the garbage collection in each run'. The test is pointless. Restarting your PC is pointless. It's all pointless. — user207421, Feb 09 '17 at 06:04
Every time the JVM terminates, it's all gone. Consider it the ultimate garbage collection. — Erich Kitzmueller, Feb 09 '17 at 06:29
@ammoQ what do you mean by JVM terminates? do you mean after program runs? — richersoon, Feb 09 '17 at 06:47

score -1 · Answer 1 · edited May 23 '17 at 12:09

-1

String foo2 = "a"; may reuse an instance from the string constant pool if one is available.
this can be proved by

System.out.println(foo==foo2);

== on two reference types is a reference identity comparison. Two objects that are equals are not necessarily==

it output true means foo and foo2 reference the same memory.

Related issues
How do I compare strings in Java?
What is the difference between “text” and new String(“text”)?

edited May 23 '17 at 12:09

Community

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answered Feb 09 '17 at 06:05

nail fei

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Doesn't answer the question that was asked. – user207421 Feb 09 '17 at 06:13

score -1 · Answer 2 · edited May 23 '17 at 12:33

-1

If you want to find out the memory location of object, you can use sun.misc.Unsafe. But i wouldn't use this approach in real life. Just out of the curiosity :)

Here, the explanation of how you can do this: How can I get the memory location of a object in java?

BTW. Object.hashCode() returns the random number. By default it will use Lehmer random number generator.

edited May 23 '17 at 12:33

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answered Feb 09 '17 at 07:13

i.merkurev

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Doesn't answer the question that was asked. By default, ['as much as is reasonably practical, the `hashCode` method defined by class `Object` does return distinct integers for distinct objects'](https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html#hashCode--). Nothing specified about Lehmer random number generators. – user207421 Feb 09 '17 at 09:08
Actually the algorithm of generating identity hashCode can be altered by -XX:hashCode JVM option. There are the following options: -XX:hashCode=0 - global Park-Miller RNG (Lehmer random number generator. default until Java 7); -XX:hashCode=1 - function(obj_address, global_state); -XX:hashCode=2 - constant 1. All objects will have the same hashCode. Just for testing.; -XX:hashCode=3 - incremental counter.; -XX:hashCode=4 - lower 32 bits of the object address in the Heap; -XX:hashCode=5 - thread-local Marsaglia's Xor-shift RNG (default since Java 8); – i.merkurev Feb 09 '17 at 09:29
Exactly, so 'by default' what it does isn't specified. – user207421 Feb 09 '17 at 10:28
You can edit the answer and put the detail in there about random etc. Please link a reference, it's interesting. – weston Feb 09 '17 at 12:10
OK, well that doesn't mention the options. I guess you got it from here: http://stackoverflow.com/a/26049632/360211 or same place as that user did. – weston Feb 12 '17 at 21:41
@weston Perhaps, you didn't read the article yet. Right after a chapter **The Actual Identity Hash Generation** you will find these options. – i.merkurev Feb 12 '17 at 21:59
I saw it. No mention of the JVM flags at all on that page though. – weston Feb 12 '17 at 22:02

Is String intern is shared between run?

2 Answers2