Immutability of String in JAVA

Question

Consider the code :

public class Stringer {

    public static void main(String[] args) {

        String s1 = "SomeLine";
        System.out.println(s1);  // prints SomeLine
        s1 = "AnotherLine";
        System.out.println(s1);  // prints AnotherLine
    }

}

When I change s1 from SomeLine to AnotherLine , since Strings are Immutable , does this mean that SomeLine is lost and is eligible for GC ?

Much appreciated

This has nothing to do with immutability. It has to do with changing references. — Sotirios Delimanolis, Mar 25 '14 at 03:46
Related http://stackoverflow.com/questions/8798403/string-is-immutable-what-exactly-is-the-meaning. — Dave Swartz, Mar 25 '14 at 03:50

score 5 · Accepted Answer · edited May 23 '17 at 11:57

When I change s1 from SomeLine to AnotherLine, since Strings are Immutable, does this mean that SomeLine is lost and is eligible for GC?

The fact that String is immutable is irrelevant to your question. The fact that you are reassigning the variable and therefore losing the other reference is what determines if the object is a candidate for GC or not.

To answer the question, it would be if it wasn't a String literal.

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.

That String object won't be garbage collected because the ClassLoader and the corresponding class have a reference to it.

Related reading:

What is the difference between "text" and new String("text")?

this is not that trivial. Interned Strings could be easily reclaimed by GC. http://www.codeinstructions.com/2009/01/busting-javalangstringintern-myths.html — Eugene, Mar 25 '14 at 09:27
@Eugene Right. I wanted to emphasize the concept of `String` literal rather than `String` interning. — Sotirios Delimanolis, Mar 25 '14 at 14:33

score 1 · Answer 2 · edited May 23 '17 at 11:49

1

Yes you're right, it does become eligible for garbage collection. This is because you are no longer referencing the original string with your variable.

Here is a related question on SO:

Aren't String objects in Java immutable?

edited May 23 '17 at 11:49

Community

1
1

answered Mar 25 '14 at 03:50

Andy Niles

36
3

Eugene · Answer 3 · 2014-03-25T09:28:22.613

It is not that trivial when talking about Strings in Java. Even if the String is interned it can still be garbage collected. In java 6, even if it is allocated in the permgen, it can be garbage collected, in Java 7 they are allocated on the Heap and are still eligible for GC.

Usually interned Strings are not eligible for garbage collection because the class has a reference to it. But what if that class was loaded dynamically (Class.forName)? It could easily be the case when that Class could be unloaded, thus an interned string could be reclaimed by GC.

See an example here that shows that interned strings could be garbage collected.

Immutability of String in JAVA

3 Answers3