String.format work strangely in Kotlin and Java

Question

I tried to move codebase to Kotlin from Java. But I found strange behavior in String.format.

I have both same codes (and feature, also) in Kotlin and Java.

fun callScriptMethod(methodName: String, vararg args: Any): String {
        var format = methodName
        if (!format.contains("javascript:")) {
            format = String.format("javascript:%s", format)
        }

        val objects = mutableListOf<Any>()
        for (arg in args) objects.add(arg)

        if (!objects.isEmpty()) {
            format += "("
            var i = 0
            val icnt = objects.size
            while (i < icnt) {
                format += "\'%s\'"
                if (i != icnt - 1) {
                    format += ", "
                }
                i++
            }

            format += ")"
        } else {
            format += "()"
        }

        val message = String.format(Locale.getDefault(), format, args)
        return message
    }

public static String callScriptMethod(String methodName, Object... args) {
        String format = methodName;
        if (!format.contains("javascript:")) {
            format = String.format("javascript:%s", format);
        }

        List<Object> objects = Arrays.asList(args);
        if (!objects.isEmpty()) {
            format += "(";
            for (int i = 0, icnt = objects.size(); i < icnt; i++) {
                format += "\'%s\'";
                if (i != icnt - 1) {
                    format += ", ";
                }
            }

            format += ")";
        } else {
            format += "()";
        }

        String message = String.format(format, args);
        return message;
    }

and some test code.

fun main() {
    val result = Java.callScriptMethod("nativeCallback", "1", "d8d8441n24n134n",
        "dasqhjidhkdhaskjdfhawoiudnqwaidnqwioldjnqawskld:djoashdojashdlkjasdjhas", "0")
    println(result)

    val result2 = Kotlin.callScriptMethod("nativeCallback", "1", "d8d8441n24n134n",
        "dasqhjidhkdhaskjdfhawoiudnqwaidnqwioldjnqawskld:djoashdojashdlkjasdjhas", "0")
    println(result2)
}

I can expect result is javascript:nativeCallback('1', 'd8d8441n24n134n', 'dasqhjidhkdhaskjdfhawoiudnqwaidnqwioldjnqawskld:djoashdojashdlkjasdjhas', '0').

But the version of Kotlin has exception MissingFormatArgumentException.

So, I tried to debug these codes to know the format is generated successfully.

Java: javascript:nativeCallback('%s', '%s', '%s', '%s')

Kotlin: javascript:nativeCallback('%s', '%s', '%s', '%s')

Both are the same result and have same args but has a different result.

javascript:nativeCallback('1', 'd8d8441n24n134n', 'dasqhjidhkdhaskjdfhawoiudnqwaidnqwioldjnqawskld:djoashdojashdlkjasdjhas', '0')
Exception in thread "main" java.util.MissingFormatArgumentException: Format specifier '%s'
    at java.util.Formatter.format(Formatter.java:2519)
    at java.util.Formatter.format(Formatter.java:2455)
    at java.lang.String.format(String.java:2981)
    at Kotlin.callScriptMethod(Kotlin.kt:31)
    at TestKt.main(test.kt:11)
    at TestKt.main(test.kt)

So, I want to know what is the problem. How can i do?

Answer 1

As vararg becomes an array once entering the function body, you have to use the spread operator in order to pass it as vararg. https://kotlinlang.org/docs/reference/functions.html#variable-number-of-arguments-varargs

When we call a vararg-function, we can pass arguments one-by-one, e.g. asList(1, 2, 3), or, if we already have an array and want to pass its contents to the function, we use the spread operator (prefix the array with *):

val message = String.format( format, *args)

The difference with Java is that Java actually allows passing an array as vararg directly, see this SO post: Can I pass an array as arguments to a method with variable arguments in Java?

i.e. Object... in Java is technically identical to Object[], there are no "real" vararg things in Java, while vararg is a real different thing in Kotlin.