DecimalFormat 源码简单阅读

DecimalFormat 源码简单阅读

最终应该是使用 subformat 来进行格式化的。

因为我主要是想看一下，"0000" 这种格式，源码里是如何进行格式化的，所以，其它细节忽略。

关注到这几行代码：
        char zero = symbols.getZeroDigit();
        int zeroDelta = zero - '0'; // '0' is the DigitList representation of zero

        if (digitList.isZero()) {
            digitList.decimalAt = 0; // Normalize
        }

前缀 0 应该是这里添加的。
            if (!fractionPresent && result.length() == sizeBeforeIntegerPart) {
                result.append(zero);
            }


查看了一下 symbols.getZeroDigit()
写得真是复杂。。。

    private StringBuffer subformat(StringBuffer result, FieldDelegate delegate,
                                   boolean isNegative, boolean isInteger,
                                   int maxIntDigits, int minIntDigits,
                                   int maxFraDigits, int minFraDigits) {
        // NOTE: This isn't required anymore because DigitList takes care of this.
        //
        //  // The negative of the exponent represents the number of leading
        //  // zeros between the decimal and the first non-zero digit, for
        //  // a value < 0.1 (e.g., for 0.00123, -fExponent == 2).  If this
        //  // is more than the maximum fraction digits, then we have an underflow
        //  // for the printed representation.  We recognize this here and set
        //  // the DigitList representation to zero in this situation.
        //
        //  if (-digitList.decimalAt >= getMaximumFractionDigits())
        //  {
        //      digitList.count = 0;
        //  }

        char zero = symbols.getZeroDigit();
        int zeroDelta = zero - '0'; // '0' is the DigitList representation of zero
        char grouping = symbols.getGroupingSeparator();
        char decimal = isCurrencyFormat ?
            symbols.getMonetaryDecimalSeparator() :
            symbols.getDecimalSeparator();

        /* Per bug 4147706, DecimalFormat must respect the sign of numbers which
         * format as zero.  This allows sensible computations and preserves
         * relations such as signum(1/x) = signum(x), where x is +Infinity or
         * -Infinity.  Prior to this fix, we always formatted zero values as if
         * they were positive.  Liu 7/6/98.
         */
        if (digitList.isZero()) {
            digitList.decimalAt = 0; // Normalize
        }

        if (isNegative) {
            append(result, negativePrefix, delegate,
                   getNegativePrefixFieldPositions(), Field.SIGN);
        } else {
            append(result, positivePrefix, delegate,
                   getPositivePrefixFieldPositions(), Field.SIGN);
        }

        if (useExponentialNotation) {
            int iFieldStart = result.length();
            int iFieldEnd = -1;
            int fFieldStart = -1;

            // Minimum integer digits are handled in exponential format by
            // adjusting the exponent.  For example, 0.01234 with 3 minimum
            // integer digits is "123.4E-4".

            // Maximum integer digits are interpreted as indicating the
            // repeating range.  This is useful for engineering notation, in
            // which the exponent is restricted to a multiple of 3.  For
            // example, 0.01234 with 3 maximum integer digits is "12.34e-3".
            // If maximum integer digits are > 1 and are larger than
            // minimum integer digits, then minimum integer digits are
            // ignored.
            int exponent = digitList.decimalAt;
            int repeat = maxIntDigits;
            int minimumIntegerDigits = minIntDigits;
            if (repeat > 1 && repeat > minIntDigits) {
                // A repeating range is defined; adjust to it as follows.
                // If repeat == 3, we have 6,5,4=>3; 3,2,1=>0; 0,-1,-2=>-3;
                // -3,-4,-5=>-6, etc. This takes into account that the
                // exponent we have here is off by one from what we expect;
                // it is for the format 0.MMMMMx10^n.
                if (exponent >= 1) {
                    exponent = ((exponent - 1) / repeat) * repeat;
                } else {
                    // integer division rounds towards 0
                    exponent = ((exponent - repeat) / repeat) * repeat;
                }
                minimumIntegerDigits = 1;
            } else {
                // No repeating range is defined; use minimum integer digits.
                exponent -= minimumIntegerDigits;
            }

            // We now output a minimum number of digits, and more if there
            // are more digits, up to the maximum number of digits.  We
            // place the decimal point after the "integer" digits, which
            // are the first (decimalAt - exponent) digits.
            int minimumDigits = minIntDigits + minFraDigits;
            if (minimumDigits < 0) {	// overflow?
                minimumDigits = Integer.MAX_VALUE;
            }

            // The number of integer digits is handled specially if the number
            // is zero, since then there may be no digits.
            int integerDigits = digitList.isZero() ? minimumIntegerDigits :
                    digitList.decimalAt - exponent;
            if (minimumDigits < integerDigits) {
                minimumDigits = integerDigits;
            }
            int totalDigits = digitList.count;
            if (minimumDigits > totalDigits) {
                totalDigits = minimumDigits;
            }
            boolean addedDecimalSeparator = false;

            for (int i=0; i<totalDigits; ++i) {
                if (i == integerDigits) {
                    // Record field information for caller.
                    iFieldEnd = result.length();

                    result.append(decimal);
                    addedDecimalSeparator = true;

                    // Record field information for caller.
                    fFieldStart = result.length();
                }
                result.append((i < digitList.count) ?
                              (char)(digitList.digits[i] + zeroDelta) :
                              zero);
            }

            if (decimalSeparatorAlwaysShown && totalDigits == integerDigits) {
                // Record field information for caller.
                iFieldEnd = result.length();

                result.append(decimal);
                addedDecimalSeparator = true;

                // Record field information for caller.
                fFieldStart = result.length();
            }

            // Record field information
            if (iFieldEnd == -1) {
                iFieldEnd = result.length();
            }
            delegate.formatted(INTEGER_FIELD, Field.INTEGER, Field.INTEGER,
                               iFieldStart, iFieldEnd, result);
            if (addedDecimalSeparator) {
                delegate.formatted(Field.DECIMAL_SEPARATOR,
                                   Field.DECIMAL_SEPARATOR,
                                   iFieldEnd, fFieldStart, result);
            }
            if (fFieldStart == -1) {
                fFieldStart = result.length();
            }
            delegate.formatted(FRACTION_FIELD, Field.FRACTION, Field.FRACTION,
                               fFieldStart, result.length(), result);

            // The exponent is output using the pattern-specified minimum
            // exponent digits.  There is no maximum limit to the exponent
            // digits, since truncating the exponent would result in an
            // unacceptable inaccuracy.
            int fieldStart = result.length();

            result.append(symbols.getExponentSeparator());

            delegate.formatted(Field.EXPONENT_SYMBOL, Field.EXPONENT_SYMBOL,
                               fieldStart, result.length(), result);

            // For zero values, we force the exponent to zero.  We
            // must do this here, and not earlier, because the value
            // is used to determine integer digit count above.
            if (digitList.isZero()) {
                exponent = 0;
            }

            boolean negativeExponent = exponent < 0;
            if (negativeExponent) {
                exponent = -exponent;
                fieldStart = result.length();
                result.append(symbols.getMinusSign());
                delegate.formatted(Field.EXPONENT_SIGN, Field.EXPONENT_SIGN,
                                   fieldStart, result.length(), result);
            }
            digitList.set(negativeExponent, exponent);

            int eFieldStart = result.length();

            for (int i=digitList.decimalAt; i<minExponentDigits; ++i) {
                result.append(zero);
            }
            for (int i=0; i<digitList.decimalAt; ++i) {
                result.append((i < digitList.count) ?
                          (char)(digitList.digits[i] + zeroDelta) : zero);
            }
            delegate.formatted(Field.EXPONENT, Field.EXPONENT, eFieldStart,
                               result.length(), result);
        } else {
            int iFieldStart = result.length();

            // Output the integer portion.  Here 'count' is the total
            // number of integer digits we will display, including both
            // leading zeros required to satisfy getMinimumIntegerDigits,
            // and actual digits present in the number.
            int count = minIntDigits;
            int digitIndex = 0; // Index into digitList.fDigits[]
            if (digitList.decimalAt > 0 && count < digitList.decimalAt) {
                count = digitList.decimalAt;
            }

            // Handle the case where getMaximumIntegerDigits() is smaller
            // than the real number of integer digits.  If this is so, we
            // output the least significant max integer digits.  For example,
            // the value 1997 printed with 2 max integer digits is just "97".
            if (count > maxIntDigits) {
                count = maxIntDigits;
                digitIndex = digitList.decimalAt - count;
            }

            int sizeBeforeIntegerPart = result.length();
            for (int i=count-1; i>=0; --i) {
                if (i < digitList.decimalAt && digitIndex < digitList.count) {
                    // Output a real digit
                    result.append((char)(digitList.digits[digitIndex++] + zeroDelta));
                } else {
                    // Output a leading zero
                    result.append(zero);
                }

                // Output grouping separator if necessary.  Don't output a
                // grouping separator if i==0 though; that's at the end of
                // the integer part.
                if (isGroupingUsed() && i>0 && (groupingSize != 0) &&
                    (i % groupingSize == 0)) {
                    int gStart = result.length();
                    result.append(grouping);
                    delegate.formatted(Field.GROUPING_SEPARATOR,
                                       Field.GROUPING_SEPARATOR, gStart,
                                       result.length(), result);
                }
            }

            // Determine whether or not there are any printable fractional
            // digits.  If we've used up the digits we know there aren't.
            boolean fractionPresent = (minFraDigits > 0) ||
                (!isInteger && digitIndex < digitList.count);

            // If there is no fraction present, and we haven't printed any
            // integer digits, then print a zero.  Otherwise we won't print
            // _any_ digits, and we won't be able to parse this string.
            if (!fractionPresent && result.length() == sizeBeforeIntegerPart) {
                result.append(zero);
            }

            delegate.formatted(INTEGER_FIELD, Field.INTEGER, Field.INTEGER,
                               iFieldStart, result.length(), result);

            // Output the decimal separator if we always do so.
            int sStart = result.length();
            if (decimalSeparatorAlwaysShown || fractionPresent) {
                result.append(decimal);
            }

            if (sStart != result.length()) {
                delegate.formatted(Field.DECIMAL_SEPARATOR,
                                   Field.DECIMAL_SEPARATOR,
                                   sStart, result.length(), result);
            }
            int fFieldStart = result.length();

            for (int i=0; i < maxFraDigits; ++i) {
                // Here is where we escape from the loop.  We escape if we've
                // output the maximum fraction digits (specified in the for
                // expression above).
                // We also stop when we've output the minimum digits and either:
                // we have an integer, so there is no fractional stuff to
                // display, or we're out of significant digits.
                if (i >= minFraDigits &&
                    (isInteger || digitIndex >= digitList.count)) {
                    break;
                }

                // Output leading fractional zeros. These are zeros that come
                // after the decimal but before any significant digits. These
                // are only output if abs(number being formatted) < 1.0.
                if (-1-i > (digitList.decimalAt-1)) {
                    result.append(zero);
                    continue;
                }

                // Output a digit, if we have any precision left, or a
                // zero if we don't.  We don't want to output noise digits.
                if (!isInteger && digitIndex < digitList.count) {
                    result.append((char)(digitList.digits[digitIndex++] + zeroDelta));
                } else {
                    result.append(zero);
                }
            }

            // Record field information for caller.
            delegate.formatted(FRACTION_FIELD, Field.FRACTION, Field.FRACTION,
                               fFieldStart, result.length(), result);
        }

        if (isNegative) {
            append(result, negativeSuffix, delegate,
                   getNegativeSuffixFieldPositions(), Field.SIGN);
        }
        else {
            append(result, positiveSuffix, delegate,
                   getPositiveSuffixFieldPositions(), Field.SIGN);
        }

        return result;
    }