strings and memory allocation in java?

Question

one thing that i always wondered, if i have a method like this:

String replaceStuff (String plainText) {
        return plainText.replaceAll("&", "&amp;");
}

will it create new String objects all the time for the "&" and the "&" that gets destroyed by the GC and then recreated again by next call? E.g. would it in theory be better to do something like this

final String A ="&";
final String AMP ="&amp;";

    String replaceStuff (String plainText) {
            return plainText.replaceAll(A, AMP);
    }

i think this is probably a more theoretic question than a real life problem, I am just curious how the memory management is handled in this aspect.

Jon Skeet · Accepted Answer · 2011-08-08 09:23:46Z

4

No. String literals are interned. Even if you use an equal literal (or other constant) from elsewhere, you'll still refer to the same object:

Object x = "hello";
Object y = "he" + "llo";

System.out.println(x == y); // Guaranteed to print true.

EDIT: The JLS guarantees this in section 3.10.5

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.

Section 15.28 shows the + operator being included as an operation which can produce a new constant from two other constants.

edited Aug 8, 2011 at 9:23

answered Aug 8, 2011 at 9:15

Jon Skeet

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4 Comments

Sean Patrick Floyd Over a year ago

It's Guaranteed for Object y = "hello", but is the compiler optimization for "he" + "llo" also guaranteed? (I'm asking, not challenging :-))

Joachim Sauer Over a year ago

@Sean: it's also guaranteed. It's in this part of §3.10.5 in the JLS: "String literals-or, more generally, strings that are the values of constant expressions (§15.28)-are "interned" [...]"

Jon Skeet Over a year ago

@Sean: Guaranteed; I've added the reference.

Jon Skeet Over a year ago

@Joachim: Yup, I was adding the same reference :)

Michael Berry · Accepted Answer · 2011-08-08 09:16:44Z

3

Nope, they're literals and therefore automatically interned to the constant pool.

The only way you'd create new strings each time would be to do:

String replaceStuff (String plainText) {
   return plainText.replaceAll(new String("&"), new String("&amp;"));
}

answered Aug 8, 2011 at 9:16

Michael Berry

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Comments

Sathwick · Accepted Answer · 2011-08-08 09:18:48Z

1

Strings are handled little different than the normal objects by GC. For example if

String a = "aaa"; String a1 = "aaa";

Now both a and a1 will point to same String value in memory till any of the value changes. Hence there will be only 1 object in memory.

Also, if we change 'a' and 'a1' to point to any other string, still the value "aaa" is left in the string pool and will be used later by JVM if required. The string is not GC'd

answered Aug 8, 2011 at 9:18

Sathwick

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2 Comments

Joachim Sauer Over a year ago

Actually they are not handled different by the GC, but during construction.

Jon Skeet Over a year ago

It's worth noting this is for compile-time string constants. Strings created in other ways aren't automatically interned.

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