Java: String.format not working 'f != [F'

July 29, 2022

I tried using String.format to format some floats.

If I do this:

public class TestClass {
    public static void main(String[] args) {
        System.out.println(String.format("%5.2f %5.2f", 2.35792f, 8.9479f));
    }
}

Then it seems to work and prints out 2.36 8.95.

However, if I extract this to a method:

public class TestClass {
    public static void main(String[] args) {
        System.out.println(formatFloats(2.35792f, 8.9479f));
    }

    private static String formatFloats(float...fs) {
        StringBuilder formatStr = new StringBuilder();
        for (float v: fs) {
            formatStr.append("%5.2f ");
        }
        return String.format(formatStr.toString().trim(), fs);
    }
}

then it raises an exception:

Exception in thread "main" java.util.IllegalFormatConversionException: f != [F
    at java.base/java.util.Formatter$FormatSpecifier.failConversion(Formatter.java:4426)
    at java.base/java.util.Formatter$FormatSpecifier.printFloat(Formatter.java:2951)
    at java.base/java.util.Formatter$FormatSpecifier.print(Formatter.java:2898)
    at java.base/java.util.Formatter.format(Formatter.java:2673)
    at java.base/java.util.Formatter.format(Formatter.java:2609)
    at java.base/java.lang.String.format(String.java:3292)
    at demo.vec3.TestClass.formatFloats(TestClass.java:12)
    at demo.vec3.TestClass.main(TestClass.java:4)

I tried to print formatStr.toString().trim(), but it does output %5.2f %5.2f.

The error message is cryptic, and I don’t understand what it means at all. I believe it has something to do with how I am using float...fs.

(I am using Eclipse IDE to write and run my application.)

Remark: It turns out that if I change my function code slightly to this:

private static String formatFloats(Object...fs) {
    StringBuilder formatStr = new StringBuilder();
    for (Object v: fs) {
        formatStr.append("%5.2f ");
    }
    return String.format(formatStr.toString().trim(), fs);
}

then it works, outputting 2.36 8.95 again.

>Solution :

You’re correct. When you pass in a float[], Java thinks you are trying to print one argument, a float[], rather than all the individual float elements of the array. Only an Object[] is compatible with the varargs used by String.format, allowing each individual float to be interpreted as a separate format argument.

Note that Float[] will also work, since Float[] can be upcast to Object[], but float[] cannot.