When using NumberFormat.getInstance(), and print hashCode() like below there are always the same.

val twoFractionDigitsFormat: NumberFormat by lazy {
        NumberFormat.getInstance().apply {
            minimumFractionDigits = 2
            maximumFractionDigits = 2
            roundingMode = RoundingMode.HALF_UP
        }
    }

val fourFractionDigitsFormat: NumberFormat by lazy {
        NumberFormat.getInstance().apply {
            minimumFractionDigits = 4
            maximumFractionDigits = 4
            roundingMode = RoundingMode.HALF_UP
        }
    }

Then print

    println(NumberFormat.getInstance().hashCode())
    println(fourFractionDigitsFormat.hashCode())
    println(twoFractionDigitsFormat.hashCode())

The output are:

423132
423132
423132

If check the source code, the getInsstance() calls

DecimalFormat numberFormat = new DecimalFormat(numberPatterns[entry], symbols);to create a DecimalFormat every time. These should be not the same instance. Which also be provided by === returns false.

Check hashCode() method:

@Override
    public int hashCode() {
        return maximumIntegerDigits * 37 + maxFractionDigits;
        // just enough fields for a reasonable distribution
    }

This maximumIntegerDigits * 37 + maxFractionDigits; always return same value. And maximumFractionDigits is public, maxFractionDigits is private.

Here is the quotation:

/**

 * The maximum number of digits allowed in the fractional portion of a
 * number.  {@code maximumFractionDigits} must be greater than or equal to
 * {@code minimumFractionDigits}.
 * <p>
 * <strong>Note:</strong> This field exists only for serialization
 * compatibility with JDK 1.1.  In Java platform 2 v1.2 and higher, the new
 * {@code int} field {@code maximumFractionDigits} is used instead.
 * When writing to a stream, {@code maxFractionDigits} is set to
 * {@code maximumFractionDigits} or {@code Byte.MAX_VALUE},
 * whichever is smaller.  When reading from a stream, this field is used
 * only if {@code serialVersionOnStream} is less than 1.
 *
 * @serial
 * @see #getMaximumFractionDigits
 */
private byte    maxFractionDigits = 3;  

Then my question is the hashCode() is always the same(if you don’t change maximumIntegerDigits), then how === works, is it a bug?

Thanks!