// null key
    InternalEntry<Object, Object, ?> nullEntry = segment.newEntryForTesting(null, hash, entry);
    Object nullValue = new Object();
    WeakValueReference<Object, Object, ?> nullValueRef =
        segment.newWeakValueReferenceForTesting(nullEntry, nullValue);
    segment.setWeakValueReferenceForTesting(nullEntry, nullValueRef);
    segment.setTableEntryForTesting(index, nullEntry);
    // skip the null key

```

### Type Conversion
```java
import org.codelibs.core.convert.*;

// Safe type conversions with null handling
Integer value = IntegerConversionUtil.toInteger("123");        // Returns 123
Integer nullValue = IntegerConversionUtil.toInteger(null);     // Returns null
Boolean flag = BooleanConversionUtil.toBoolean("true");        // Returns true
Date date = DateConversionUtil.toDate("2023-12-25", "yyyy-MM-dd");

        () -> ImmutableMap.ofEntries((Entry<Integer, Integer>) nullKey));
    Entry<@Nullable Integer, @Nullable Integer> nullValue = entry(23, null);
    assertThrows(
        NullPointerException.class,
        () -> ImmutableMap.ofEntries((Entry<Integer, Integer>) nullValue));
  }

  private static <T> Map<T, T> map(T... keysAndValues) {
    assertThat(keysAndValues.length % 2).isEqualTo(0);