karatelabs/karate · error · RuntimeException

toBean() needs two arguments: object and class name

Error message

toBean() needs two arguments: object and class name

What it means

karate.toBean() converts a JS/Map object to an instance of a named Java class by serializing it to JSON and deserializing with Json.fromJson into the target class. The library requires exactly this pair — the object and the class name string — so it throws when fewer than two arguments are supplied. Without both, conversion is impossible since neither the data nor the target type is known.

Solutions

1. Supply both arguments: karate.toBean(obj, 'com.example.MyClass') with the fully qualified class name as a string.
2. Ensure the first argument is the object (Map) and the second is the class name string — order matters.
3. If the object may be null, still pass it explicitly so the call has two arguments.

Example fix

// before
var user = karate.toBean(response)
// after
var user = karate.toBean(response, 'com.example.User')

if (obj == null || typeof className !== 'string' || className.indexOf('.') < 0) { throw new Error('toBean(obj, fullyQualifiedClassName) requires an object and a class name string') }

function canToBean(obj, className) { return obj != null && typeof className === 'string' && className.length > 0 }

try { var bean = karate.toBean(obj, 'com.example.User') } catch (e) { if (('' + e).indexOf('needs two arguments') >= 0) { karate.logger.warn('toBean misused: need object + class name'); bean = null } throw e }

When it happens

Trigger: karate.toBean() with zero or one argument, e.g. karate.toBean(someMap) forgetting the class name, or karate.toBean('com.example.User') forgetting the object.

Common situations: Bridging JS objects to Java POJOs for Java interop in tests; copying old call signatures where the class name was previously inferred; typos after refactoring the helper's arity.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

• read() needs at least one argument
• sysenv() needs the environment-variable name
• sysprop() needs the property name
• readAsBytes() needs at least one argument
• get() needs at least one argument

AI-assisted analysis of karatelabs/karate@a22eb90246 (2026-09-12). Data as JSON: /api/errors/bc6768a44ec48073. Report an issue: GitHub.

Appendix: source

Thrown at karate-core/src/main/java/io/karatelabs/core/KarateJsUtils.java:524

    @SuppressWarnings("unchecked")
    private static int compareKeys(Object a, Object b) {
        if (a == null || b == null) {
            return a == b ? 0 : (a == null ? -1 : 1);
        }
        if (a instanceof Number && b instanceof Number) {
            return Double.compare(((Number) a).doubleValue(), ((Number) b).doubleValue());
        }
        if (a instanceof Comparable && a.getClass().isInstance(b)) {
            return ((Comparable<Object>) a).compareTo(b);
        }
        // last resort for mixed or non-comparable keys, at least the order is stable
        return String.valueOf(a).compareTo(String.valueOf(b));
    }

    static JavaInvokable toBean() {
        return args -> {
            if (args.length < 2) {
                throw new RuntimeException("toBean() needs two arguments: object and class name");
            }
            Object obj = args[0];
            String className = args[1].toString();
            // Convert to JSON string and deserialize to the target class
            String jsonString = Json.of(obj).toString();
            return Json.fromJson(jsonString, className);
        };
    }

    /**
     * Convert a list of maps to CSV string.
     * Usage: karate.toCsv([{a:1,b:2},{a:3,b:4}]) => "a,b\n1,2\n3,4\n"
     */
    @SuppressWarnings("unchecked")
    static JavaInvokable toCsv() {
        return args -> {
            if (args.length == 0 || args[0] == null) {
                return "";

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