Core Java for the Impatient (2013)

Sometimes, you need to display just the names of weekdays and months, for example, in a calendar application. Call the getDisplayName method of the DayOfWeek and Month enumerations.

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for (Month m : Month.values())
System.out.println(m.getDisplayName(textStyle, locale) + " ");

Table 13–5 shows the text styles. The STANDALONE versions are for display outside a formatted date. For example, in Finnish, January is “tammikuuta” inside a date, but “tammikuu” standalone.

Table 13–5 Values of the java.time.format.TextStyle Enumeration

13.5 Collation and Normalization

Most programmers know how to compare strings with the compareTo method of the String class. Unfortunately, when interacting with human users, this method is not very useful. The compareTo method uses the values of the UTF-16 encoding of the string, which leads to absurd results, even in English. For example, the following five strings are ordered according to the compareTo method:

Athens
Zulu
able
zebra
Ångström

For dictionary ordering, you would want to consider upper case and lower case equivalent, and accents should not be significant. To an English speaker, the sample list of words should be ordered as

able
Ångström
Athens
zebra
Zulu

However, that order would not be acceptable to a Swedish user. In Swedish, the letter Å is different from the letter A, and it is collated after the letter Z! That is, a Swedish user would want the words to be sorted as

able
Athens
zebra
Zulu
Ångström

To obtain a locale-sensitive comparator, call the static Collator.getInstance method:

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Collator coll = Collator.getInstance(locale);
words.sort(coll);
// Collator implements Comparator<Object>

There are a couple of advanced settings for collators. You can set a collator’s strength to adjust how selective it should be. Character differences are classified as primary, secondary, or tertiary. For example, in English, the difference between e and f is considered primary, the difference between e and é is secondary, and between e and E is tertiary.

For example, when processing city names, you may not care about the differences between

San José
San Jose
SAN JOSE

In that case, configure the collator by calling

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coll.setStrength(Collator.PRIMARY);

A more technical setting is the decomposition mode which deals with the fact that a character or sequence of characters can sometimes be described in more than one way in Unicode. For example, an é (U+00E9) can also be expressed as a plain e (U+0065) followed by a ´ (combining acute accent U+0301). You probably don’t care about that difference, and by default, it is not significant. If you do care, you need to configure the collator as follows:

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coll.setStrength(Collator.IDENTICAL);
coll.setDecomposition(Collator.NO_DECOMPOSITION);

Conversely, if you want to be very lenient and consider the trade mark symbol ™ (U+2122) the same as the character combination TM, then set the decomposition mode to Collator.FULL_DECOMPOSITION.

You might want to convert strings into a normalized forms even when you don’t do collation—for example, for persistent storage or communication with another program. The Unicode standard defines four normalization forms (C, D, KC, and KD)—see www.unicode.org/unicode/reports/tr15/tr15–23.html. In the normalization form C, accented characters are always composed. For example, a sequence of e and a combining acute accent ´ is combined into a single character é. In form D, accented characters are always decomposed into their base letters and combining accents: é is turned into e followed by ´. Forms KC and KD also decompose characters such the trademark symbol ™. The W3C recommends that you use normalization form C for transferring data over the Internet.

The static normalize method of the java.text.Normalizer class carries out the normalization process. For example,

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String city = "San Jose\u0301";
String normalized = Normalizer.normalize(city, Normalizer.Form.NFC);

13.6 Message Formatting

When you internationalize a program, you often have messages with variable parts. The static format method of the MessageFormat class takes a template string with placeholders, followed by the placeholder values, like this:

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String template = "{0} has {1} messages"
String message = MessageFormat.format(template, "Pierre", 42);

Of course, instead of hardcoding the template, you should look up a locale-specific one, such as "Il y a {1} messages pour {0}" in French. You will see how to do that in Section 13.7, “Resource Bundles,” on p. 410.

Note that the ordering of the placeholders may differ among languages. In English, the message is “Pierre has 42 messages”, but in French, it is “Il y a 42 messages pour Pierre”. The placeholder {0} is the first argument after the template in the call to format, {1} is the next argument, and so on.

You can format numbers as currency amounts by adding a suffix number,currency, to the placeholder, like this:

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template="Your current total is {0,number,currency}."

In the United States, a value of 1023.95 is be formatted as $1,023.95. The same value is displayed as 1.023,95€ in Germany, using the local currency symbol and decimal separator convention.

The number indicator can be followed by currency, integer, percent, or a number format pattern of the DecimalFormat class, such as $,##0.

You can format values of the legacy java.util.Date class with an indicator date or time, followed by the format short, medium, long, or full, or a format pattern of the SimpleDateFormat such as yyyy-MM-dd.

Note that you need to convert java.time values; for example,

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String message = MessageFormat("It is now {0,time,short}.", Date.from(Instant.now()));

Finally, a choice formatter lets you generate messages such as

No files copied
1 file copied
42 files copied

depending on the placeholder value.

A choice format is a sequence of pairs, each containing a lower limit and a format string. The limit and format string are separated by a # character, and the pairs are separated by | characters.

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String template = "{0,choice,0#No files|1#1 file|2#{0} files} copied";

Note that {0} occurs twice in the template. When the message format applies the choice format to the {0} placeholder and the value is 42, the choice format returns "{0} files". That string is then formatted again, and the result is spliced into the message.