What causes "inputUnit must be different from outputUnit." in TheAlgorithms/Java?

Short-circuit in the caller: if inputUnit.equals(outputUnit), return the value unchanged instead of calling convert.

TheAlgorithms/Java · error · IllegalArgumentException

inputUnit must be different from outputUnit.

Error message

inputUnit must be different from outputUnit.

What it means

Thrown by UnitsConverter.convert when inputUnit.equals(outputUnit) — the library refuses no-op conversions and requires distinct units. This check fires before any lookup, so even units not in the converter set will trip this guard if both strings are equal.

Source

Thrown at src/main/java/com/thealgorithms/conversions/UnitsConverter.java:133

     */
    public UnitsConverter(final Map<Pair<String, String>, AffineConverter> basicConversions) {
        conversions = computeAllConversions(basicConversions);
        units = extractUnits(conversions);
    }

    /**
     * Converts a value from one unit to another.
     *
     * @param inputUnit the unit of the input value.
     * @param outputUnit the unit to convert the value into.
     * @param value the value to convert.
     * @return the converted value in the target unit.
     * @throws IllegalArgumentException if inputUnit equals outputUnit.
     * @throws NoSuchElementException if no conversion exists between the units.
     */
    public double convert(final String inputUnit, final String outputUnit, final double value) {
        if (inputUnit.equals(outputUnit)) {
            throw new IllegalArgumentException("inputUnit must be different from outputUnit.");
        }
        final var conversionKey = Pair.of(inputUnit, outputUnit);
        return conversions.computeIfAbsent(conversionKey, k -> { throw new NoSuchElementException("No converter for: " + k); }).convert(value);
    }

    /**
     * Retrieves the set of all units supported by this converter.
     *
     * @return a set of available units.
     */
    public Set<String> availableUnits() {
        return units;
    }
}

View on GitHub (pinned to fdfb9a395b)

Solutions

Short-circuit in the caller: if inputUnit.equals(outputUnit), return the value unchanged instead of calling convert.
Validate that the two unit parameters are distinct before invoking convert.
If the same-unit case is valid in your domain, handle it explicitly before the call.

Example fix

// before
return converter.convert(unit, unit, value);
// after
if (inputUnit.equals(outputUnit)) {
    return value;
}
return converter.convert(inputUnit, outputUnit, value);

Defensive patterns

Strategy: validation

Validate before calling

static double safeConvert(UnitsConverter c, String in, String out, double v) {
    if (in == null || out == null) throw new IllegalArgumentException("units must not be null");
    if (in.equals(out)) return v; // no-op short-circuit
    return c.convert(in, out, v);
}

Type guard

static boolean isDistinctNonNull(String a, String b) {
    return a != null && b != null && !a.equals(b);
}

Try / catch

try {
    return converter.convert(inputUnit, outputUnit, value);
} catch (IllegalArgumentException e) {
    if (inputUnit.equals(outputUnit)) return value; // legitimate no-op
    throw e;
}

Prevention

Short-circuit same-unit conversions in the caller before invoking convert.
When source and target units come from independent selectors, validate distinctness on selection change.
Document that same-unit calls are rejected by design.

When it happens

Trigger: Calling converter.convert("Celsius", "Celsius", 100), or passing the same variable for both arguments (e.g., convert(unit, unit, val)).

Common situations: Building generic pipelines where source and target unit come from the same config field or are user-selected independently; refactoring that accidentally passes the same variable twice.

Related errors

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/83eba6b71811f35b. Report an issue: GitHub.