IdentityHashMap in Google Web Toolkit (GWT): String Identity

Max Rohde

May 27, 2011 — 1 min read

Problem

IdentityHashMap in Google Web Toolkit (2.2.0) JavaScript applications does not seem to work correctly, if String objects are used as key.

Source Code:

final IdentityHashMap<Object, List> idsFromObjects = new IdentityHashMap<Object, List>();

final String node1 = "Node 1";

final String id = "value";

final ArrayList ids = new ArrayList();

ids.add(id);

idsFromObjects.put(node1, ids);

final String node1b = new String("Node 1");

GWT.log("Values: "+idsFromObjects.get(node1b));

GWT.log("Contains key?: "+idsFromObjects.containsKey(node1b));

GWT.log("Equals?: "+String.valueOf(node1b == node1));

Output (Hosted Mode):

Values: null

Contains Key?: false

Equals?: false

Output (Compiled JavaScript):

Values: [value]

Contains Key?: true

Equals?: true

Note: This problem does not exist in hosted mode.

Solution

It seems there is no direct solution. Since the identity test node1b == node1 shows that the two Strings are actually identical for JavaScript, this might be a feature of JavaScript. Maybe JavaScript is keeping track of all the strings created, and does not allocate new memory, when an identical string is created; unlike Java when using the constructor new String(…).

It also appears the Google Web Toolkit JRE emulation, keeps a kind of cache for String hash codes (see the class String.HashCache). A cache is even for their identity hash maps – see System.identityHashCode() in this commit). This might be a result on JavaScripts handling of strings.

However, it is unfortunate that in this scenario running code in GWT's hosted mode leads to different results than running the compiled JavaScript.

A possible workaround is to wrap strings in a simple proxy.

public class StringNode {

private final String value;

public StringNode(String value) {

this.value = value;

}

Note: This class is not serializable for GWT RPC. See this post for possible workarounds for serialization for immutable objects.