How to prevent JSONObject from json.jar converts decimal numbers string into double

Unrelated to the JSON library you use, you should not use the BigDecimal constructor taking a double parameter, since that uses the exact decimal representation of the double's binary floating-point value

Instead, use the static valueOf method since that uses the string value of the double and thus rounds correctly.

When a double must be used as a source for a BigDecimal, note that this constructor provides an exact conversion; it does not give the same result as converting the double to a String using the Double.toString(double) method and then using the BigDecimal(String) constructor. To get that result, use the static valueOf(double) method.

However, for very large or small numbers the parsing to a double might already have introduced rounding errors, in that case the only solution would be to use a different JSON library that supports parsing numbers as BigDecimals.

In org.json.JSONObject class comment out lines in method stringToValue(String string)

    /*     * If it might be a number, try converting it. If a number cannot be     * produced, then the value will just be a string.     */    char initial = string.charAt(0);    if ((initial >= '0' && initial <= '9') || initial == '-') {        try {            // if we want full Big Number support this block can be replaced with:            // return stringToNumber(string);            if (isDecimalNotation(string)) {                Double d = Double.valueOf(string);                if (!d.isInfinite() && !d.isNaN()) {                    return d;                }            } else {                Long myLong = Long.valueOf(string);                if (string.equals(myLong.toString())) {                    if (myLong.longValue() == myLong.intValue()) {                        return Integer.valueOf(myLong.intValue());                    }                    return myLong;                }            }        } catch (Exception ignore) {        }    }