In Java, I want to take a double value and convert it to a BigDecimal
and print out its String value to a certain precision.
import java.math.BigDecimal;
public class Main {
public static void main(String[] args) {
double d=-.00012;
System.out.println(d+""); //This prints -1.2E-4
double c=47.48000;
BigDecimal b = new BigDecimal(c);
System.out.println(b.toString());
//This prints 47.47999999999999687361196265555918216705322265625
}
}
It prints this huge thing:
47.47999999999999687361196265555918216705322265625
and not
47.48
The reason I'm doing the BigDecimal
conversion is sometimes the double value will contain a lot of decimal places (i.e. -.000012
) and the when converting the double to a String will produce scientific notation -1.2E-4
. I want to store the String value in non-scientific notation.
I want to have BigDecimal always have two units of precision like this: "47.48". Can BigDecimal restrict precision on conversion to string?
This question is related to
java
double
bigdecimal
You want to try String.format("%f", d)
, which will print your double in decimal notation. Don't use BigDecimal
at all.
Regarding the precision issue: You are first storing 47.48
in the double c
, then making a new BigDecimal
from that double
. The loss of precision is in assigning to c
. You could do
BigDecimal b = new BigDecimal("47.48")
to avoid losing any precision.
The reason of such behaviour is that the string that is printed is the exact value - probably not what you expected, but that's the real value stored in memory - it's just a limitation of floating point representation.
According to javadoc, BigDecimal(double val) constructor behaviour can be unexpected if you don't take into consideration this limitation:
The results of this constructor can be somewhat unpredictable. One might assume that writing new BigDecimal(0.1) in Java creates a BigDecimal which is exactly equal to 0.1 (an unscaled value of 1, with a scale of 1), but it is actually equal to 0.1000000000000000055511151231257827021181583404541015625. This is because 0.1 cannot be represented exactly as a double (or, for that matter, as a binary fraction of any finite length). Thus, the value that is being passed in to the constructor is not exactly equal to 0.1, appearances notwithstanding.
So in your case, instead of using
double val = 77.48;
new BigDecimal(val);
use
BigDecimal.valueOf(val);
Value that is returned by BigDecimal.valueOf is equal to that resulting from invocation of Double.toString(double)
.
BigDecimal b = new BigDecimal(c).setScale(2,BigDecimal.ROUND_HALF_UP);
The String.format syntax helps us convert doubles and BigDecimals to strings of whatever precision.
This java code:
double dennis = 0.00000008880000d;
System.out.println(dennis);
System.out.println(String.format("%.7f", dennis));
System.out.println(String.format("%.9f", new BigDecimal(dennis)));
System.out.println(String.format("%.19f", new BigDecimal(dennis)));
Prints:
8.88E-8
0.0000001
0.000000089
0.0000000888000000000
In Java 9 the following is deprecated:
BigDecimal.valueOf(d).
setScale(2, BigDecimal.
ROUND_HALF_UP);
instead use:
BigDecimal.valueOf(d).setScale(2, RoundingMode.HALF_UP);
Example:
double d = 47.48111;
System.out.println(BigDecimal.valueOf(d)); //Prints: 47.48111
BigDecimal bigDecimal = BigDecimal.valueOf(d).setScale(2, RoundingMode.HALF_UP);
System.out.println(bigDecimal); //Prints: 47.48
Why not :
b = b.setScale(2, RoundingMode.HALF_UP);
It's printing out the actual, exact value of the double
.
Double.toString()
, which converts double
s to String
s, does not print the exact decimal value of the input -- if x
is your double value, it prints out exactly enough digits that x
is the closest double
to the value it printed.
The point is that there is no such double
as 47.48 exactly. Doubles store values as binary fractions, not as decimals, so it can't store exact decimal values. (That's what BigDecimal
is for!)
Source: Stackoverflow.com