Unlike for other collections, there don't seem to be any fundamental tests for IdentityHashMap. Most basically, there should be tests to ensure that membership in the map is identity-based and not value-based.

For example, consider what should happen if the following (erroneous!) patch is applied to IdentityHashMap.

--- a/src/java.base/share/classes/java/util/IdentityHashMap.java
+++ b/src/java.base/share/classes/java/util/IdentityHashMap.java
@@ -296,7 +296,7 @@ public class IdentityHashMap<K,V>
      * Returns index for Object x.
      */
     private static int hash(Object x, int length) {
- int h = System.identityHashCode(x);
+ int h = x.hashCode();
         // Multiply by -254 to use the hash LSB and to ensure index is even
         return ((h << 1) - (h << 8)) & (length - 1);
     }
@@ -333,10 +333,10 @@ public class IdentityHashMap<K,V>
         int i = hash(k, len);
         while (true) {
             Object item = tab[i];
- if (item == k)
- return (V) tab[i + 1];
             if (item == null)
                 return null;
+ if (item.equals(k))
+ return (V) tab[i + 1];
             i = nextKeyIndex(i, len);
         }
     }

This patch essentially changes the get() method to look up by value instead of by identity. Normally IDHM works like this:

    jshell> record Box(int i) {}
    jshell> var box1 = new Box(0)
    box1 ==> Box[i=0]
    jshell> var box2 = new Box(0)
    box2 ==> Box[i=0]
    jshell> box1 == box2
    $79 ==> false
    jshell> box1.equals(box2)
    $80 ==> true
    jshell> var map = new IdentityHashMap<Box, String>()
    map ==> {}
    jshell> map.put(box1, "a")
    jshell> map.put(box2, "b")
    jshell> map
    map ==> {Box[i=0]=b, Box[i=0]=a}
    jshell> map.get(box1)
    $86 ==> "a"
    jshell> map.get(box2)
    $87 ==> "b"
    jshell> map.get(new Box(0))
    $88 ==> null
    
However, with this patch in place, the behavior is as follows:

    jshell> record Box(int i) {}
    jshell> var box1 = new Box(0)
    box1 ==> Box[i=0]
    jshell> var box2 = new Box(0)
    box2 ==> Box[i=0]
    jshell> box1 == box2
    $4 ==> false
    jshell> box1.equals(box2)
    $5 ==> true
    jshell> var map = new IdentityHashMap<Box, String>()
    map ==> {}
    jshell> map.put(box1, "a")
    jshell> map.put(box2, "b")
    jshell> map
    map ==> {Box[i=0]=a, Box[i=0]=b}
    jshell> map.get(box1)
    $10 ==> "a"
    jshell> map.get(box2)
    $11 ==> "a"
    jshell> map.get(new Box(0))
    $12 ==> "a"

This is of course completely incorrect. However, with this patch applied, tiers 1, 2, and 3 of the regression suite don't report any failures at all! Failures do occur in a variety of java/beans/XMLEncoder tests (tier 4) because they apparently do rely on the identity semantics of IdentityHashMap.

Some fundamental tests for IdentityHashMap should be added to test/jdk/java/util/IdentityHashMap, which will put them into the jdk_collections_core test group, which is in tier 1.