TheAlgorithms/Java · error · IllegalArgumentException

Capacity matrix must be square

Error message

Capacity matrix must be square

What it means

PushRelabel.validate throws this IllegalArgumentException when any row is null or its length differs from n (the matrix dimension). The push-relabel algorithm indexes capacity[u][v] symmetrically, requiring a square n×n matrix.

Source

Thrown at src/main/java/com/thealgorithms/graph/PushRelabel.java:150

        int minHeight = Integer.MAX_VALUE;
        for (int v = 0; v < n; v++) {
            if (residual[u][v] > 0) {
                minHeight = Math.min(minHeight, height[v]);
            }
        }
        if (minHeight < Integer.MAX_VALUE) {
            height[u] = minHeight + 1;
        }
    }

    private static void validate(int[][] capacity, int source, int sink) {
        if (capacity == null || capacity.length == 0) {
            throw new IllegalArgumentException("Capacity matrix must not be null or empty");
        }
        int n = capacity.length;
        for (int i = 0; i < n; i++) {
            if (capacity[i] == null || capacity[i].length != n) {
                throw new IllegalArgumentException("Capacity matrix must be square");
            }
            for (int j = 0; j < n; j++) {
                if (capacity[i][j] < 0) {
                    throw new IllegalArgumentException("Capacities must be non-negative");
                }
            }
        }
        if (source < 0 || sink < 0 || source >= n || sink >= n) {
            throw new IllegalArgumentException("Source and sink must be valid vertex indices");
        }
    }
}

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Allocate new int[n][n] and fill capacity[u][v] per edge.
2. Verify all rows are non-null and length n.
3. Replace null rows with new int[n] before calling.

Example fix

// before
int[][] cap = new int[n][]; // uninitialized rows

// after
int[][] cap = new int[n][n];

for (int[] row : capacity) {
    if (row == null || row.length != capacity.length) {
        throw new IllegalArgumentException("Non-square matrix");
    }
}

boolean isSquare(int[][] cap) {
    if (cap == null) return false;
    for (int[] row : cap) if (row == null || row.length != cap.length) return false;
    return true;
}

When it happens

Trigger: Passing a ragged array, a non-square rectangular matrix, or a matrix with a null row.

Common situations: Allocating new int[n][] and forgetting to initialize rows. Mixing edge-list-derived sizing with vertex-count sizing. Null row after partial fill.

Related errors

• Capacity matrix must be square
• Capacity matrix must not be null or empty
• Capacities must be non-negative
• Source and sink must be valid vertex indices
• Source and sink must be valid vertex indices

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/53e5a756ad470a2c. Report an issue: GitHub.