why is the Double.parseDouble making 9999999999999999
to 10000000000000000
?
For Example :
Double d =Double.parseDouble("9999999999999999");
String b= new DecimalFormat("#.##").format(d);
System.out.println(b);
IS Printing
10000000000000000
instead it has to show 9999999999999999
or 9999999999999999.00
Any sort of help is greatly appreciated.
The parseDouble() method of Java Double class is a built in method in Java that returns a new double initialized to the value represented by the specified String, as done by the valueOf method of class Double. Parameters: It accepts a single mandatory parameter s which specifies the string to be parsed.
valueOf() creates a Double object which is often not needed. parseDouble() does not. With autoboxing it's valueOf(String) which is no longer needed, but is therefore backward compatibility.
The number 9999999999999999
is just above the precision limit of double-precision floating-point. In other words, the 53-bit mantissa is not able to hold 9999999999999999
.
So the result is that it is rounded to the nearest double-precision value - which is 10000000000000000
.
9999999999999999 = 0x2386f26fc0ffff // 54 significant bits needed
10000000000000000 = 0x2386f26fc10000 // 38 significant bits needed
double
only has 15/16 digits of accuracy and when you give it a number it can't represent (which is most of the time, even 0.1 is not accurate) it takes the closest representable number.
If you want to represent 9999999999999999
exactly, you need to use BigDecimal.
BigDecimal bd = new BigDecimal("9999999999999999");
System.out.println(new DecimalFormat("#.##").format(bd));
prints
9999999999999999
Very few real world problems need this accuracy because you can't measure anything this accurately anyway. i.e. to an error of 1 part per quintillion.
You can find the largest representable integer with
// search all the powers of 2 until (x + 1) - x != 1
for (long l = 1; l > 0; l <<= 1) {
double d0 = l;
double d1 = l + 1;
if (d1 - d0 != 1) {
System.out.println("Cannot represent " + (l + 1) + " was " + d1);
break;
}
}
prints
Cannot represent 9007199254740993 was 9.007199254740992E15
The largest representable integer is 9007199254740992 as it needs one less bit (as its even)
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