HashMap implementations and Integer specializations

[Martin Buchholz]

JavaOne 2008 technical session PDFs are now available

http://developers.sun.com/learning/javaoneonline/j1online.jsp?track=javase&yr=2008

I was surprised to discover a discussion of
HashMap optimization at the JavaOne talk
http://developers.sun.com/learning/javaoneonline/2008/pdf/TS-6434.pdf

Did I miss an announcement of that?

A comment on that talk:

It is easy to make the mistake of thinking that HashMap does not
have to store the actual Integer key in the call to put.
Unfortunately, Java's evil semantics (everything is an Object,
everything is a Lock, everything has an Identity) require
that the exact same objects inserted into a HashMap can be
removed later.  This means that specialized submaps will have
a harder time optimizing for footprint.  (I do think the
algorithm presented in the talk is correct, since the front cache
is likely only used for get(), and not, e.g. for entrySet().iterator())

I intend to check in a test case modification that will ensure
that future optimizations do not violate the current compatibility guarantees.

diff --git a/test/java/util/Collection/MOAT.java
b/test/java/util/Collection/MOAT.java
--- a/test/java/util/Collection/MOAT.java
+++ b/test/java/util/Collection/MOAT.java
@@ -544,6 +544,27 @@ public class MOAT {
         check(m.size() != 0 ^ m.isEmpty());
     }

+    private static void testPutPreversesObjectIdentity(Map<?,?> m,
+                                                       Object key,
+                                                       Object val) {
+        try {
+            Map<Object,Object> m2 = m.getClass().newInstance();
+            m2.put(key, val);
+            Map.Entry<Object,Object> e = m2.entrySet().iterator().next();
+            check(e.getKey()   == key);
+            check(e.getValue() == val);
+            check(m2.keySet().iterator().next() == key);
+            check(m2.values().iterator().next() == val);
+        } catch (Throwable t) { unexpected(t); }
+    }
+
+    private static void testPutPreversesObjectIdentity(Map<?,?> m) {
+        testPutPreversesObjectIdentity(m, new Integer(42),  new Integer(43));
+        testPutPreversesObjectIdentity(m, new Long(42L),    new Long(43L));
+        testPutPreversesObjectIdentity(m, new Double(42.0), new Double(43.0));
+        testPutPreversesObjectIdentity(m, new String("42"), new String("43"));
+    }
+
     private static void testMap(Map<Integer,Integer> m) {
         System.out.println("\n==> " + m.getClass().getName());

@@ -572,6 +593,7 @@ public class MOAT {
         }

         if (supportsPut(m)) {
+            testPutPreversesObjectIdentity(m);
             try {
                 check(m.put(3333, 77777) == null);
                 check(m.put(9134, 74982) == null);