NullPointerException in DeepEquals.getContainingDescription when object's custom equals() is inconsistent with field-level equality

[lipisak]

Affected versions: confirmed on 3.3.0, still present in 4.107.0 (method is byte-for-byte identical).

Body:

DeepEquals.deepEquals(a, b) throws an NPE while generating the breadcrumb diff (only on the not-equal path) when the object graph contains a type whose custom equals() returns false for two instances that are nevertheless deeply field-equal.

Root cause is in getContainingDescription:

private static String getContainingDescription(List<ItemsToCompare> path) {
    ListIterator<ItemsToCompare> it = path.listIterator(path.size());
    String a = it.previous().difference.getDescription();   // <-- last node's `difference` not null-checked

    if (it.hasPrevious()) {
        Difference diff = it.previous().difference;
        if (diff != null) {                                 // parent node IS null-checked
            String b = diff.getDescription();
            if (b != null) {
                return b;
            }
        }
    }
    return a;
}

The last node's difference is dereferenced without a null-check, unlike the parent node just below it. It becomes null via the custom-equals branch in the recursive comparison: when key1.equals(key2) is false but the recursive field-by-field deep comparison finds them equal (diff_item == null), nothing is pushed onto the stack, so the root node (new ItemsToCompare(a, b), difference == null) stays on top → NPE.

Minimal reproduction:

class IdentityEquals {
    final String value = "x";
    @Override public boolean equals(Object o) { return this == o; }
    @Override public int hashCode() { return System.identityHashCode(this); }
}
DeepEquals.deepEquals(new IdentityEquals(), new IdentityEquals()); // throws NPE

Suggested fix: null-guard the last node the same way the parent node is guarded, e.g. String a = last.difference != null ? last.difference.getDescription() : null; (and fall back gracefully when both are null).