TheAlgorithms/Java · error · Error

Please call solve() before checking for a solution.

Error message

Please call solve() before checking for a solution.

What it means

Thrown by `TwoSat.isSolutionExists` when `solve()` has not yet been called. The solver defers all SCC computation to `solve()`, so `hasSolution` is meaningless before then. This is thrown as `Error` (not `IllegalArgumentException`), signalling API misuse rather than bad data.

Source

Thrown at src/main/java/com/thealgorithms/datastructures/graphs/TwoSat.java:200

            int notI = negate(i);
            if (component[i] == component[notI]) {
                hasSolution = false;
                return;
            }
            // If SCC(i) > SCC(¬i), then variable i is true.
            variableAssignments[i] = component[i] > component[notI];
        }
    }

    /**
     * Returns whether the given boolean formula is satisfiable.
     *
     * @return {@code true} if a solution exists; {@code false} otherwise
     * @throws Error if called before {@link #solve()}
     */
    boolean isSolutionExists() {
        if (!isSolved) {
            throw new Error("Please call solve() before checking for a solution.");
        }
        return hasSolution;
    }

    /**
     * Returns one valid assignment of variables that satisfies the boolean formula.
     *
     * @return a boolean array where {@code result[i]} represents the truth value of
     *         variable {@code xᵢ}
     * @throws Error if called before {@link #solve()} or if no solution exists
     */
    boolean[] getSolutions() {
        if (!isSolved) {
            throw new Error("Please call solve() before fetching the solution.");
        }
        if (!hasSolution) {
            throw new Error("No satisfying assignment exists for the given expression.");
        }

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Call `solve()` before `isSolutionExists()`
2. Wrap the build->solve->query sequence in a helper so the order cannot be skipped
3. Add a unit test that asserts the sequence

Example fix

// before
if (ts.isSolutionExists()) { ... } // throws Error
// after
ts.solve();
if (ts.isSolutionExists()) { ... }

ts.solve(); // always solve before querying satisfiability

try {
    ts.isSolutionExists();
} catch (Error e) {
    // solve() was not called first
}

When it happens

Trigger: Calling `isSolutionExists()` before `solve()` — forgetting the `solve()` call or reordering the two statements.

Common situations: Copy-paste omission of `solve()`; refactoring that moved the solve call into a branch that was not taken; assuming the constructor solves.

Related errors

• Please call solve() before fetching the solution.
• Number of variables cannot be negative.
• Variable number must be between 1 and {numberOfVariables}
• No satisfying assignment exists for the given expression.

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/00331f4146758b5a. Report an issue: GitHub.