data type to represent a big decimal in java

Question

Which data type is apt to represent a decimal number like "10364055.81".

If tried using double:

double d = 10364055.81;

But when I try to print the number, its displaying as "1.036405581E7", which I don't want.

Should I use BigDecimal? But its displaying as 10364055.81000000052154064178466796875. Is there any datatype that displays the values as it is? Also the number may be bigger than the one taken as example.

BTW, will using BigDecimal effect the performance of the application?? I might use this in almost all my DTOs.

Wenger

Answer 1

You should use BigDecimal - but use the String constructor, e.g.:

new BigDecimal("10364055.81");

If you pass a double to BigDecimal, Java must create that double first - and since doubles cannot represent most decimal fractions accurately, it does create the value as 10364055.81000000052154064178466796875 and then passes it to the BigDecimal constructor. In this case BigDecimal has no way of knowing that you actually meant the rounder version.

Generally speaking, using non-String constructors of BigDecimal should be considered a warning that you're not getting the full benefit of the class.

Edit - based on rereading exactly what you wanted to do, my initial claim is probably too strong. BigDecimal is a good choice when you need to represent decimal values exactly (money handling being the obvious choice, you don't want 5.99 * one million to be 5990016.45 for example.

But if you're not worried about the number being stored internally as a very slightly different value to the decimal literal you entered, and just want to print it out again in the same format, then as others have said, an instance of NumberFormat (in this case, new DecimalFormat("########.##")) will do the trick to output the double nicely, or String.format can do much the same thing.

As for performance - BigDecimals will naturally be slower than using primitives. Typically, though, unless the vast majority of your program involves mathematical manipulations, you're unlikely to actually notice any speed difference. That's not to say you should use BigDecimals all over; but rather, that if you can get a real benefit from their features that would be difficult or impossible to realise with plain doubles, then don't sweat the miniscule performance difference they theoretically introduce.

Absorber

Well, I think you're right. But a double data type must be enough for that number (only two decimals). Using a BigDecimal where a double is enough can cause a low performing when doing some operations. – Draghound

Answer 2

How a number is displayed is distinct from how the number is stored.

Take a look at DecimalFormat for controlling how you can display your numbers when a double (or float etc.).

Note that choosing BigDecimal over double (or vice versa) has pros/cons, and will depend on your requirements. See here for more info. From the summary:

In summary, if raw performance and space are the most important factors, primitive floating-point types are appropriate. If decimal values need to be represented exactly, high-precision computation is needed, or fine control of rounding is desired, only BigDecimal has the needed capabilities.

Conard

Answer 3

A double would be enough in order to save this number. If your problem is you don't like the format when printing or putting it into a String, you might use NumberFormat: http://java.sun.com/javase/6/docs/api/java/text/NumberFormat.html

Draghound

Answer 4

you can use double and display if with System.out.printf().

double d = 100003.81;
System.out.printf("%.10f", d);

.10f - means a double with precision of 10

Centrality