TheAlgorithms/Java · error · IllegalArgumentException

Character array and frequency array must have the same…

Error message

Character array and frequency array must have the same length

What it means

Thrown by Huffman.buildHuffmanTree when charArray.length != charFreq.length. Each symbol must have exactly one corresponding frequency, so a mismatch means the arrays are misaligned and the loop that reads charFreq[i] for each charArray[i] would read out of bounds or pair the wrong values. The check runs after null and empty checks.

Solutions

1. Build symbols and frequencies from a single source (e.g., a Map<Character,Integer>) so they cannot diverge.
2. After any mutation, re-derive both arrays together before the call.
3. Log both lengths to catch divergence early.

Example fix

// before
char[] syms = ...; int[] frq = ...; // built separately, may diverge
HuffmanNode root = Huffman.buildHuffmanTree(syms, frq);

// after
Map<Character,Integer> table = buildFreqTable(input);
char[] syms = new char[table.size()];
int[] frq = new int[table.size()];
int i=0; for (var e : table.entrySet()) { syms[i]=e.getKey(); frq[i]=e.getValue(); i++; }
HuffmanNode root = Huffman.buildHuffmanTree(syms, frq);

if (charArray.length != charFreq.length) {
    throw new IllegalStateException("symbols/frequencies length mismatch: " + charArray.length + " vs " + charFreq.length);
}

When it happens

Trigger: Building symbols and frequencies from two separate collections that drifted out of sync, removing an entry from one map but not the other, or a copy/transform that dropped an element from one array.

Common situations: Parallel collections maintained by hand, a partial update that modified one structure, or a deserialization that truncated one array.

Related errors

• Character array and frequency array cannot be empty
• Character array and frequency array cannot be null
• Frequencies must be non-negative
• Character '%c' (U+%04X) not found in Huffman dictionary.
• Edges cannot be null

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/6df778df0b57fcd8. Report an issue: GitHub.

Appendix: source

Thrown at src/main/java/com/thealgorithms/others/Huffman.java:88

    /**
     * Builds a Huffman tree from the given character array and their frequencies.
     *
     * @param charArray array of characters
     * @param charFreq  array of frequencies corresponding to the characters
     * @return root node of the Huffman tree
     * @throws IllegalArgumentException if arrays are null, empty, or have different
     *                                  lengths
     */
    public static HuffmanNode buildHuffmanTree(char[] charArray, int[] charFreq) {
        if (charArray == null || charFreq == null) {
            throw new IllegalArgumentException("Character array and frequency array cannot be null");
        }
        if (charArray.length == 0 || charFreq.length == 0) {
            throw new IllegalArgumentException("Character array and frequency array cannot be empty");
        }
        if (charArray.length != charFreq.length) {
            throw new IllegalArgumentException("Character array and frequency array must have the same length");
        }

        int n = charArray.length;
        PriorityQueue<HuffmanNode> priorityQueue = new PriorityQueue<>(n, new HuffmanComparator());

        // Create leaf nodes and add to priority queue
        for (int i = 0; i < n; i++) {
            if (charFreq[i] < 0) {
                throw new IllegalArgumentException("Frequencies must be non-negative");
            }
            HuffmanNode node = new HuffmanNode(charArray[i], charFreq[i]);
            priorityQueue.add(node);
        }

        // Build the Huffman tree
        while (priorityQueue.size() > 1) {
            HuffmanNode left = priorityQueue.poll();
            HuffmanNode right = priorityQueue.poll();

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