TheAlgorithms/Java · error · IllegalArgumentException

Capacity matrix must not be null or empty

Error message

Capacity matrix must not be null or empty

What it means

GomoryHuTree.validateCapacityMatrix throws this IllegalArgumentException when the capacity matrix passed to the public API is null or has zero rows. It is the first check in the private validator invoked before any tree construction.

Source

Thrown at src/main/java/com/thealgorithms/graph/GomoryHuTree.java:56

            for (int v = 0; v < n; v++) {
                if (v != s && parent[v] == t && res.reachable[v]) {
                    parent[v] = s;
                }
            }

            if (t != 0 && res.reachable[parent[t]]) {
                parent[s] = parent[t];
                parent[t] = s;
                weight[s] = weight[t];
                weight[t] = f;
            }
        }
        return new int[][] {parent, weight};
    }

    private static void validateCapacityMatrix(int[][] cap) {
        if (cap == null || cap.length == 0) {
            throw new IllegalArgumentException("Capacity matrix must not be null or empty");
        }
        final int n = cap.length;
        for (int i = 0; i < n; i++) {
            if (cap[i] == null || cap[i].length != n) {
                throw new IllegalArgumentException("Capacity matrix must be square");
            }
            for (int j = 0; j < n; j++) {
                if (cap[i][j] < 0) {
                    throw new IllegalArgumentException("Capacities must be non-negative");
                }
            }
        }
    }

    private static final class MaxFlowResult {
        final int flow;
        final boolean[] reachable;
        MaxFlowResult(int flow, boolean[] reachable) {

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Null/empty-check the matrix before invoking the Gomory-Hu method and return an empty result.
2. Ensure the graph build step produces at least one vertex.
3. Log when the matrix comes back empty from the loader to catch upstream bugs.

Example fix

// before
int[][] tree = GomoryHuTree.build(cap); // cap may be null

// after
if (cap == null || cap.length == 0) {
    return new int[0][];
}
int[][] tree = GomoryHuTree.build(cap);

if (cap == null || cap.length == 0) {
    return new int[0][]; // empty tree
}

boolean hasMatrix(int[][] cap) { return cap != null && cap.length > 0; }

When it happens

Trigger: Calling the Gomory-Hu tree constructor/method with a null capacity array or an empty int[][] (length 0).

Common situations: Graph parsed from input that yielded no vertices. Matrix left uninitialized after a failed load. Guard removed during a refactor assuming upstream always provides data.

Related errors

• Capacity matrix must not be null or empty
• Capacity matrix must be square
• Capacities must be non-negative
• Cost matrix must not be null or empty
• successors must not be null

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/f54232c2b0340e21. Report an issue: GitHub.