What causes "comparator must not be null" in Blankj/AndroidUtilCode?

For natural ordering, use MapUtils.newTreeMap(Comparator.naturalOrder(), pairs) or construct new TreeMap<>() directly.

Blankj/AndroidUtilCode · error · IllegalArgumentException

comparator must not be null

Error message

comparator must not be null

What it means

MapUtils.newTreeMap() creates a TreeMap populated with key-value pairs, requiring a non-null Comparator to define ordering. Passing a null comparator throws IllegalArgumentException immediately because the method signature promises ordering via the comparator — there is no natural-ordering fallback in this overload.

Source

Thrown at lib/utilcode/src/main/java/com/blankj/utilcode/util/MapUtils.java:69

    @SafeVarargs
    public static <K, V> LinkedHashMap<K, V> newLinkedHashMap(final Pair<K, V>... pairs) {
        LinkedHashMap<K, V> map = new LinkedHashMap<>();
        if (pairs == null || pairs.length == 0) {
            return map;
        }
        for (Pair<K, V> pair : pairs) {
            if (pair == null) continue;
            map.put(pair.first, pair.second);
        }
        return map;
    }

    @SafeVarargs
    public static <K, V> TreeMap<K, V> newTreeMap(final Comparator<K> comparator,
                                                  final Pair<K, V>... pairs) {
        if (comparator == null) {
            throw new IllegalArgumentException("comparator must not be null");
        }
        TreeMap<K, V> map = new TreeMap<>(comparator);
        if (pairs == null || pairs.length == 0) {
            return map;
        }
        for (Pair<K, V> pair : pairs) {
            if (pair == null) continue;
            map.put(pair.first, pair.second);
        }
        return map;
    }

    @SafeVarargs
    public static <K, V> Hashtable<K, V> newHashTable(final Pair<K, V>... pairs) {
        Hashtable<K, V> map = new Hashtable<>();
        if (pairs == null || pairs.length == 0) {
            return map;
        }

View on GitHub (pinned to 7b4caf9e54)

Solutions

For natural ordering, use MapUtils.newTreeMap(Comparator.naturalOrder(), pairs) or construct new TreeMap<>() directly.
Ensure the comparator is non-null before the call — validate or provide a default: comparator = comparator != null ? comparator : Comparator.naturalOrder().
If you need a TreeMap without a custom comparator, don't use this overload — use the standard TreeMap constructor.

Example fix

// before
TreeMap<String, Integer> map = MapUtils.newTreeMap(null, Pair.create("a", 1));

// after
TreeMap<String, Integer> map = MapUtils.newTreeMap(
    Comparator.naturalOrder(), Pair.create("a", 1));

Defensive patterns

Strategy: validation

Validate before calling

// Ensure comparator is non-null before calling
Comparator<String> cmp = getCmp();
if (cmp == null) cmp = Comparator.naturalOrder();
TreeMap<String, Integer> map = MapUtils.newTreeMap(cmp, Pair.create("a", 1));

Type guard

// N/A — this is a null-value check, not a type check
static <K> Comparator<K> safeComparator(Comparator<K> cmp) {
    return cmp != null ? cmp : Comparator.naturalOrder();
}

Try / catch

try {
    map = MapUtils.newTreeMap(comparator, pairs);
} catch (IllegalArgumentException e) {
    // comparator was null — retry with natural ordering
    map = MapUtils.newTreeMap(Comparator.naturalOrder(), pairs);
}

Prevention

Always pass Comparator.naturalOrder() instead of null when you want natural ordering.
Validate comparators before passing: use Objects.requireNonNullElse(cmp, Comparator.naturalOrder()).
For null-safe TreeMap construction, use new TreeMap<>() directly and putAll().
Document which overloads require non-null arguments.

When it happens

Trigger: Calling MapUtils.newTreeMap(null, pair1, pair2) or passing a comparator variable that resolved to null (e.g., a conditionally-assigned field or a method return that produced null).

Common situations: Developer passes null expecting natural ordering (unaware this overload requires a comparator); a comparator obtained from configuration or a map lookup returns null; refactor that changed a non-null comparator to a nullable one.

Related errors

AI-assisted analysis of Blankj/AndroidUtilCode@7b4caf9e54 (2026-08-14). Data as JSON: /api/errors/9801f346bb9fc998. Report an issue: GitHub.