TheAlgorithms/Java · error · IllegalArgumentException

Listener must not be null

Error message

Listener must not be null

What it means

Thrown by LIFOCache.Builder.evictionListener(BiConsumer) when the listener is null. The listener fires on every eviction/overwrite; a null would NPE inside put/removeKey rather than at configuration. The guard surfaces the failure at build/configuration time.

Source

Thrown at src/main/java/com/thealgorithms/datastructures/caches/LIFOCache.java:533

         */
        public Builder<K, V> defaultTTL(long ttlMillis) {
            if (ttlMillis < 0) {
                throw new IllegalArgumentException("Default TTL must be >= 0");
            }
            this.defaultTTL = ttlMillis;
            return this;
        }

        /**
         * Sets an eviction listener to be notified when entries are evicted from the cache.
         *
         * @param listener a {@link BiConsumer} that accepts evicted keys and values; must not be {@code null}
         * @return this builder instance for chaining
         * @throws IllegalArgumentException if {@code listener} is {@code null}
         */
        public Builder<K, V> evictionListener(BiConsumer<K, V> listener) {
            if (listener == null) {
                throw new IllegalArgumentException("Listener must not be null");
            }
            this.evictionListener = listener;
            return this;
        }

        /**
         * Builds and returns a new {@link LIFOCache} instance with the configured parameters.
         *
         * @return a fully configured {@code LIFOCache} instance
         */
        public LIFOCache<K, V> build() {
            return new LIFOCache<>(this);
        }

        /**
         * Sets the eviction strategy used to determine when to clean up expired entries.
         *
         * @param strategy an {@link EvictionStrategy} implementation; must not be {@code null}

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Provide a no-op listener instead of null: (k,v) -> {}.
2. Guard the wiring: if (listener != null) builder.evictionListener(listener);
3. Make the listener a required DI dependency.

Example fix

// before
builder.evictionListener(maybeListener);
// after
builder.evictionListener(maybeListener != null ? maybeListener : (k, v) -> {});

BiConsumer<K, V> listener = maybeListener != null ? maybeListener : (k, v) -> {};
builder.evictionListener(listener);

static <K, V> boolean isUsableListener(BiConsumer<K, V> listener) {
    return listener != null;
}

When it happens

Trigger: builder.evictionListener(null); builder.evictionListener(maybeMetrics) where maybeMetrics is uninitialized; conditional wiring in some environments.

Common situations: Feature-flagged metrics off; DI injecting null; tests omitting the listener.

Related errors

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

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/98b3f5cb283ccfaa. Report an issue: GitHub.