TheAlgorithms/Java · error · IllegalArgumentException

Key must not be null

Error message

Key must not be null

What it means

Thrown by LIFOCache.get(K) when the key is null. The cache is HashMap-backed and null keys would collide with the absent-entry contract; the guard runs before lock acquisition and before any eviction-strategy or statistics updates. LIFOCache is the LIFO (stack-based) sibling of FIFOCache and shares the same null-key policy.

Source

Thrown at src/main/java/com/thealgorithms/datastructures/caches/LIFOCache.java:120

        this.evictionListener = builder.evictionListener;
        this.evictionStrategy = builder.evictionStrategy;
    }

    /**
     * Retrieves the value associated with the specified key from the cache.
     *
     * <p>If the key is not present or the corresponding entry has expired, this method
     * returns {@code null}. If an expired entry is found, it will be removed and the
     * eviction listener (if any) will be notified. Cache hit-and-miss statistics are
     * also updated accordingly.
     *
     * @param key the key whose associated value is to be returned; must not be {@code null}
     * @return the cached value associated with the key, or {@code null} if not present or expired
     * @throws IllegalArgumentException if {@code key} is {@code null}
     */
    public V get(K key) {
        if (key == null) {
            throw new IllegalArgumentException("Key must not be null");
        }

        lock.lock();
        try {
            evictionStrategy.onAccess(this);

            final CacheEntry<V> entry = cache.get(key);
            if (entry == null || entry.isExpired()) {
                if (entry != null) {
                    cache.remove(key);
                    keys.remove(key);
                    notifyEviction(key, entry.value);
                }
                misses++;
                return null;
            }
            hits++;
            return entry.value;

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Null-check the key before lookup, returning a default upstream.
2. Use Optional.ofNullable(key).map(cache::get).orElse(null).
3. Validate at the service boundary.

Example fix

// before
V v = cache.get(request.getKey());
// after
K k = request.getKey();
if (k == null) return defaultValue;
return cache.get(k);

if (key == null) return defaultValue;
V value = cache.get(key);

static <K> boolean isLookupKey(K key) {
    return key != null;
}

When it happens

Trigger: cache.get(null); cache.get(map.get(absentKey)) yielding null; lookups driven by unvalidated user input.

Common situations: Request parameters mapped straight to cache keys; optional deserialized fields; migrating from a tolerant cache implementation.

Related errors

• Key and value must not be null
• Key cannot be null
• Listener must not be null
• Eviction strategy must not be null
• Key must not be null

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/caaf4ef3efb8d86a. Report an issue: GitHub.