1. Putting Values into a Suitable Form for Display
Formatting
Putting Values into a Suitable Form for Display
Copyright © Curt Hill

2. The Problem Java uses types int and double to represent numeric values
These have a binary internal representation
People want to see a character representation in a report or in a window
There are many such representations
How do we display these?
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3. What do we want?
Although the basic digits are the same we may want to see other information
Commas separating groups of digits
Leading/trailing zero suppression
Currency symbols such as $
Handling of the sign other than a leading + / -
Such as CR or DB
We call this editing
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4. If that is not bad enough
Java is an international language
The problem is complicated by different locales
Do you want:
341, – Common in America
,78 – Common in Europe
,78 – Also used in Europe
Just as most Operating Systems keep track of this, Java does as well
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5. The Problem Revisited
The Java compiler is notoriously poor at guessing the intent of the programmer
So the default formatting is usually insufficient
We will need to specify more information to get what we want
This will involve new classes and new methods
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6. NumberFormat
We will use a NumberFormat object to format numbers into strings
It has several methods that will be of interest
The format method will either take a long int or a double
An int will automatically cast into a long as also a float into a double
This method will return a string which is the formatted number
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7. NumberFormat NumberFormat is an abstract base class
It is the ancestor of several classes that will be important
The actual class will determine the formatting
As an abstract base class it cannot be instantiated
What does that mean?
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8. Abstract Base Classes NumberFormat is an abstract base class
Does that mean we cannot: NumberFormat nf; No
It means that we cannot: nf = new NumberFormat(…);
The variable nf may hold any descendent of NumberFormat
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9. NumberFormat Descendents
The currency classes are simple but not too simple
DecimalFormat is the most general and complicated
Before dealing with the currency descendents and instantiating an abstract base class we need to consider a Locale object
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10. Locale A Locale is an object that represents a region
This captures both geography and language
Used to modify how things are displayed
Specifically here currency symbols and format of numbers
Create a Locale object if you do not want your own locale
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11. Locale Constructors
The constructor may specify a language, nation or system
Locale loc = new Locale(“en”);
English, default country
Locale loc = new Locale(“en”,”GB”);
English of Great Britain
Locale loc = new Locale(“en”,”US”,”MAC”);
English, USA for a MacIntosh system
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12. Alternative Often it is easier to use static Locales
Locale loc = Locale.UK;
instead of previous
Locale loc = Locale.CANADA;
English for Canadians
Locale loc = Locale.CANADA_FRENCH;
French for Canadians
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13. Abstract Base Classes Again
Suppose NumberFormat nf;
We cannot use: nf = new NumberFormat(…);
How then do we?
We use the NumberFormat method getCurrencyInstance to make a descendent of NumberFormat that is instantiable
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14. Currencies
There are several descendents of NumberFormat that represent the currencies of various locales
The NumberFormat static method getCurrencyInstance is used to obtain
If no parameter then get the Locale for this machine: nf = NumberFormat getCurrencyInstance();
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15. Formatting for Currency
We now know the process:
Declare a NumberFormat object
Initialize with getCurrencyInstance
Use format method to create the string
Now look at the example code:
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16. Example
double d; … // load d with value NumberFormat nf; nf = nf.getCurrencyInstance(); String s = nf.format(d); … // display s somewhere
The nf before getCurrencyInstance could have been spelled out as NumberFormat, since this is a static method
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17. getCurrencyInstance Has two signatures One without parameters
One with a Locale object as a parameter
Use the latter to display currencies different than the local one
See example
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18. Example double d; … // load d with value NumberFormat nf;
Locale loc = Locale.UK;
nf = nf.getCurrencyInstance(loc);
String s = nf.format(d);
… // display s somewhere
Now a pound symbol will be displayed instead of a dollar sign
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19. Not yet enough
It is one thing to put us in correct locale, it is another thing to use the right currency symbol
In London they are just as likely to accept a Euro or Dollar as a Pound
How is formatting for Euros in London different than formatting Euros in France?
The formatting is based on Locale, but other currency symbols can be used
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20. Currency
A class that describes what symbol to use in currency formatting
We will use a static method to create, somewhat like NumberFormat
We can create one with a three character abbreviation Currency newCurrency = Currency.getInstance("EUR");
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21. Currency continued There are several abbreviations that are usable:
USD = US Dollar
HKD = Hong Kong Dollar
EUR = Euro
Use setCurrency to establish that in a NumberFormat object
See the next example
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22. Formatting Euros in England
Currency newCurrency = Currency.getInstance("EUR"); NumberFormat nf;
Locale loc = Locale.UK;
nf = nf.getCurrencyInstance(loc);
nf.setCurrency(newCurrency);
String s = nf.format(d);
… // display s somewhere
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23. There is always more
Formatting a number as currency is an important task
There are many other ways to format a number
How do we do this?
Use the DecimalFormat object
This generalizes most any type of formatting
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24. DecimalFormat Descendent of NumberFormat Will use the format method
Uses a pattern to edit the text
This pattern is similar to many other numeric editing patterns
The PIC clause in COBOL
Numerous others
The pattern is just a string of characters
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25. Pattern Format Just a string
Each character in string is either an editing character or will appear as-is
Editing characters have a special meaning for the formatting
The semicolon divides the pattern into two pieces
First is how to handle if number is positive
The second is for negatives
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26. Pattern Characters # means a digit, with leading zero suppression
0 means a digit that always displays
. Decimal separator
- Minus sign
, Grouping separator
E Separates mantissa and exponent in scientific notation
% Multiply by 100 and show as percentage
' Used to quote special characters in a prefix or suffix
"'#'#" formats 123 to "#123“
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27. Suppression Only certain values may exist within the number
Such as the comma
These will also be suppressed just like a digit
The result string may be shorter than the pattern
Any characters may be outside the number representation string
They will display as themselves
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28. Rounding When a float or double is cast as an int truncation occurs
All digits to right of decimal are lost
Formatting always rounds
Not to the nearest integer
To nearest displayable number
Thus using “###.###” the number will become 1.235
This is true of all descendents of NumberFormat
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29. Imports Several new import statements are needed for this
import java.text.NumberFormat;
import java.text.DecimalFormat;
import java.util.Locale;
import java.util.Currency;
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30. Conclusion
There are simple and more complicated ways to make the number look the way we would like
NumberFormat with getCurrencyInstance is the simplest way for currency displays
DecimalFormat is the most general and should be able to do any display
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