@Override public SlotAssignmentResult ensure(long key1, long key2) {
        assert key1 != unassignedSentinel : "ensure() called with key1 == nullKey1 (" + unassignedSentinel + ')';
        return super.ensure0(key1, key2);
    }
@Test
    public void testPut() {
        final long key1 = randomKey();
        final long key2 = randomKey();
        SlotAssignmentResult slot = hsa.ensure(key1, key2);
        assertTrue(slot.isNew());
        final long valueAddress = slot.address();
        assertNotEquals(NULL_ADDRESS, valueAddress);

        slot = hsa.ensure(key1, key2);
        assertFalse(slot.isNew());
        assertEquals(valueAddress, slot.address());
    }