Hash of a String object is 0

Question

Scenario: There is a message sender and a message receiver. The contents of the message are converted into a xom.nu document and passed to the receiver. Something funny happens here. There is an attribute (language) for one of the tags, i.e text, whose value is a string "en".

The sender's "en" has a hash value but the receiver's hash is shown to be zero. What causes the hash of a string be zero?

Sender:

Receiver:

Suresh is right I think, try calling `hashCode()`. Also, hashCode on different platforms is not guaranteed to be the same. — Marcelo, Sep 11 '13 at 15:50
@Marcelo The hash algorithm is defined in the documentation. So it's platform independent. — Denys Séguret, Sep 11 '13 at 15:52

score 10 · Accepted Answer · answered Sep 11 '13 at 15:50

You should not inspect a variable only by its internals.

In this case your problem is that the field hash is acting as a cache. It only contains a value if hashCode() has ever been called on this instance.

Try watching yourVariable.hashCode() and you'll notice that the hash field will change as well.

score 5 · Answer 2 · edited May 23 '17 at 12:09

Here's the source of the hashCode method :

1493    public int hashCode() {
1494        int h = hash;
1495        if (h == 0) {
1496            int off = offset;
1497            char val[] = value;
1498            int len = count;
1499
1500            for (int i = 0; i < len; i++) {
1501                h = 31*h + val[off++];
1502            }
1503            hash = h;
1504        }
1505        return h;
1506    }

You see that the value is computed only when you (first) call hashCode. Which doesn't matter as hash is private : you can only get it normally using the hashCode method.

To answer your explicit question : the hash of a string (as returned by hashCode) is 0 for empty strings (but not only for empty strings).

The hashCode() value is not only zero for empty strings. – Stephen C Sep 11 '13 at 16:03 — Stephen C, Sep 11 '13 at 16:03

Hash of a String object is 0

2 Answers2