fix: raise clear error for on-disk (splayed) table reference

javakdb/src/main/java/com/kx/c.java

@@ -634,9 +634,19 @@ public static class Flip{
      * Create a Flip (KDB+ table) from the values stored in a Dict.
      * @param dict Values stored in the dict should be an array of Strings for the column names (keys), with an
      * array of arrays for the column values
+     * @throws IllegalArgumentException if the dict values are not an array of column data (Object[]). This can be
+     * the case when a splayed/partitioned table stored on disk is received (kdb+ transmits a mapping of column names
+     * to an on-disk path symbol rather than the column data), or when the Dict was constructed with values that are
+     * not column arrays.
+     * @see <a href="https://code.kx.com/q/kb/splayed-tables/">Splayed tables</a>
      */
     public Flip(Dict dict){
       x=(String[])dict.x;
+      if(!(dict.y instanceof Object[]))
+        throw new IllegalArgumentException("A Flip requires the dictionary values to be an array of column data"
+          +" (Object[]) but received: "+dict.y+". This can occur when a splayed/partitioned table stored on disk is"
+          +" received (kdb+ transmits an on-disk path reference rather than the columns), or when the Dict was"
+          +" constructed with values that are not column arrays.");
       y=(Object[])dict.y;
     }
     /**

javakdb/src/test/java/com/kx/CTest.java

@@ -931,6 +931,41 @@ public void testDeserializeEmptyTable()
         }
     }
 
+    @Test
+    public void testDeserializeSplayedTableReference()
+    {
+        // response from executing 'get `:test/ set ([] a:til 10)'
+        // A splayed (on-disk) table is transmitted as table(98) -> dict(99) whose value is the
+        // file-path symbol `:test/ (type -11) rather than the column data, so it cannot be
+        // represented as an in-memory Flip. This should surface as a clear IllegalArgumentException
+        // rather than an opaque ClassCastException (issue #51).
+        byte[] buff = {(byte)0x01, (byte)0x02, (byte)0x00, (byte)0x00, (byte)0x1b, (byte)0x00, (byte)0x00, (byte)0x00, (byte)0x62, (byte)0x00, (byte)0x63, (byte)0x0b, (byte)0x00, (byte)0x01, (byte)0x00, (byte)0x00, (byte)0x00, (byte)0x61, (byte)0x00, (byte)0xf5, (byte)0x3a, (byte)0x74, (byte)0x65, (byte)0x73, (byte)0x74, (byte)0x2f, (byte)0x00};
+        com.kx.c c=new com.kx.c();
+        try {
+            c.deserialize(buff);
+            Assert.fail("Expected an IllegalArgumentException to be thrown");
+        } catch (IllegalArgumentException e) {
+            assertTrue(e.getMessage().contains(":test/"));
+        } catch (Exception e) {
+            Assert.fail(e.toString());
+        }
+    }
+
+    @Test
+    public void testFlipFromDictWithNonColumnValues()
+    {
+        // The same guard also protects against a Dict that was simply constructed with values that are
+        // not column arrays (i.e. a caller bug, not a splayed table); the message describes the actual
+        // condition rather than assuming a specific cause.
+        c.Dict dict = new c.Dict(new String[]{"a"}, "not-a-column-list");
+        try {
+            new c.Flip(dict);
+            Assert.fail("Expected an IllegalArgumentException to be thrown");
+        } catch (IllegalArgumentException e) {
+            assertTrue(e.getMessage().contains("Object[]"));
+        }
+    }
+
     @Test
     public void testMonthToString()
     {