What String.intern() actually improve

Question

Everyone is saying that calling String.intern() for comparison is a much better way to do than calling String.equals() method. I make a quick test to make sure about it before using in my program. However, it comes out as a surprise that by running a loop of 10 millions iterations, version of equals() spend 800 ms while vesion of intern() spend 3000 ms which is much longer.
Anyone has any idea why? Is .intern() really an improvement?
My code:
class testIntern {
public static void main(String[] args) {
String sTest = new String("Hello Everyone");
System.out.println(sTest.intern()=="Hello Everyone");
System.out.println("version".intern()=="version");
long x = System.currentTimeMillis();
int j = 0;
for (int i = 0; i < 10000000; i ++) {
sTest = new String("Hello Everyone");
if(sTest.intern() == "Hello Everyone") {
j++;
System.out.println(System.currentTimeMillis() - x);
System.out.println(j);
}

Answer 1

Grepping the JDK 1.5 source for the pattern "intern *(", I found 56 files. So far, I'm considering it an example of how not to use intern(). For example, in com.sun.org.apache.xml.internal.utils.NamespaceSupport2, we have this gem:
    void declarePrefix (String prefix, String uri)
                                // Lazy processing...
        if (!tablesDirty) {
            copyTables();
        if (declarations == null) {
            declarations = new Vector();
        prefix = prefix.intern();
        uri = uri.intern();
        if ("".equals(prefix)) {
            if ("".equals(uri)) {
                defaultNS = null;
            } else {
                defaultNS = uri;
        } else {
            prefixTable.put(prefix, uri);
            uriTable.put(uri, prefix); // may wipe out another prefix
        declarations.addElement(prefix);
    }So, they intern both the namespace prefix and URI, taking up space in the permgen. Then they use .equals() to compare those to a constant string, and put both values in standard Hashtables!
There are several cases where intern() is used to prevent the compiler from performing constant substitution:
    public final static String PREFIX_XMLNS = "xmlns".intern();It's a nice little trick, when your constants are likely to change: no need to recompile dependent classes. However, since this particular prefix is defined by the W3C namespace spec, the trick is of very dubious value here.
There are a few cases, such as in java.lang.Class, where intern() is used so that you can do a fast string search over a fixed array. I suspect these cases exist to avoid a dependency on Map. Whether or not a simple equals() would have been sufficient is an open question.