带有double数组的Java System.out.format

Question

I am trying to output information with System.out.format using a double[] array as the argument. This does not work:

out.format("New dimensions:\n" +
        "Length: %f\n" +
        "Width: %f\n\n",
        doubleArray);

This, however, does:

out.format("New dimensions:\n" +
        "Length: %f\n" +
        "Width: %f\n\n",
        doubleArray[0], doubleArray[1]);

Why doesn't the first format work? It supposedly works with strings just fine.

Answer 1

Java will autobox your double to a Double , but it won't autobox your double[] to a Double[] , so it doesn't match Object[] . As a result, instead of being unpacked into the Object... varargs, your array is being treated as the array itself -- which, obviously, can't be formatted as a double.

If you declare your array as Double[] instead of double[] , the call to format works.

Answer 2

It doesn't work because an array isn't a double(in Java an array is a class, so it's like a general pointer here). You need to specify exactly what will be outputted, and you did - it's the %f format specifier . ArraySomething[] doesn't match..

See here for more on Java's Formatting and here - How does array class work in Java? , for Java arrays.