TheAlgorithms/Java · error · IllegalArgumentException

Capacity must be greater than zero.

Error message

Capacity must be greater than zero.

What it means

Thrown by the LFUCache(int capacity) constructor when capacity <= 0. LFU eviction depends on a frequency-ordered linked list whose invariants assume at least one slot; a zero capacity would make the cache unusable and every put an immediate eviction. The no-arg constructor delegates with DEFAULT_CAPACITY, so this only fires on an explicit bad capacity.

Source

Thrown at src/main/java/com/thealgorithms/datastructures/caches/LFUCache.java:75

    private final int capacity;
    private static final int DEFAULT_CAPACITY = 100;

    /**
     * Constructs an LFU cache with the default capacity.
     */
    public LFUCache() {
        this(DEFAULT_CAPACITY);
    }

    /**
     * Constructs an LFU cache with the specified capacity.
     *
     * @param capacity The maximum number of items that the cache can hold.
     * @throws IllegalArgumentException if the specified capacity is less than or equal to zero.
     */
    public LFUCache(int capacity) {
        if (capacity <= 0) {
            throw new IllegalArgumentException("Capacity must be greater than zero.");
        }
        this.capacity = capacity;
        this.cache = new HashMap<>();
    }

    /**
     * Retrieves the value associated with the given key from the cache.
     * If the key exists, the node's frequency is incremented, and the node is repositioned
     * in the linked list based on its updated frequency.
     *
     * @param key The key whose associated value is to be returned.
     * @return The value associated with the key, or {@code null} if the key is not present in the cache.
     */
    public V get(K key) {
        Node node = cache.get(key);
        if (node == null) {
            return null;
        }

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Default to a positive capacity when config is absent.
2. Validate config at startup: if (cap <= 0) throw with a clear message.
3. Use the no-arg constructor new LFUCache<>() to get the default capacity.

Example fix

// before
new LFUCache<>(config.getMaxEntries())
// after
int cap = config.getMaxEntries();
if (cap <= 0) cap = 1000;
new LFUCache<>(cap)

int cap = configuredCapacity;
if (cap <= 0) cap = DEFAULT_CAPACITY;
new LFUCache<>(cap);

static boolean isValidCapacity(int capacity) {
    return capacity > 0;
}

When it happens

Trigger: new LFUCache<>(0); new LFUCache<>(-1); capacity sourced from config that defaulted to 0 or was computed as size/batch with a 0 divisor.

Common situations: Optional config defaulting to 0; capacity derived from memory math that underflows; tests asserting behavior at capacity 0.

Related errors

• Capacity must be > 0
• Capacity must be > 0
• Key must not be null
• Key and value must not be null
• TTL must be >= 0

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/08788461e44abf71. Report an issue: GitHub.