Message Formatting in Java

The Java library has a MessageFormat class that formats text with variable parts. It is similar to formatting with the printf method, but it supports locales and formats for numbers and dates. We will examine that mechanism in the following sections.

1. Formatting Numbers and Dates

Here is a typical message format string:

“On {2}, a {0} destroyed {1} houses and caused {3} of damage.”

The numbers in braces are placeholders for actual names and values. The static method MessageFormat.format lets you substitute values for the variables. It is a “varargs” method, so you can simply supply the parameters as follows:

String msg

= MessageFormat.format(“On {2}, a {0} destroyed {1} houses and caused {3} of damage.”,

“hurricane”, 99, new GregorianCalendar(1999, 0, 1).getTime(), 10.0E8);

In this example, the placeholder {0} is replaced with “hurricane”, {1} is replaced with 99, and so on.

The result of our example is the string

On 1/1/99 12:00 AM, a hurricane destroyed 99 houses and caused 100,000,000 of damage.

That is a start, but it is not perfect. We don’t want to display the time “12:00 AM,” and we want the damage amount printed as a currency value. The way we do this is by supplying an optional format for some of the placeholders:

“On {2,date,long}, a {0} destroyed {1} houses and caused {3,number,currency} of damage.”

This example code prints:

On January 1, 1999, a hurricane destroyed 99 houses and caused $100,000,000 of damage.

In general, the placeholder index can be followed by a type and a style. Separate the index, type, and style by commas. The type can be any of





If the type is number, then the style can be




or it can be a number format pattern such as $,##0. (See the documentation of the DecimalFormat class for more information about the possible formats.)

If the type is either time or date, then the style can be





or a date format pattern such as yyyy-MM-dd. (See the documentation of the SimpleDateFormat class for more information about the possible formats.)

2. Choice Formats

Let’s look closer at the pattern of the preceding section:

“On {2}, a {0} destroyed {1} houses and caused {3} of damage,”

If we replace the disaster placeholder {0} with “earthquake”, the sentence is not grammatically correct in English:

On January 1, 1999, a earthquake destroyed , , ,

What we really want to do is integrate the article “a” into the placeholder:

“On {2}, {0} destroyed {1} houses and caused {3} of damage,”

The {0} would then be replaced with “a hurricane” or “an earthquake”. That is espe­cially appropriate if this message needs to be translated into a language where the gender of a word affects the article. For example, in German, the pattern would be

“{0} zerstorte am {2} {1} Hauser und richtete einen Schaden von {3} an.”

The placeholder would then be replaced with the grammatically correct combination of article and noun, such as “Ein Wirbetsturm” or “Eine Naturkatastrophe”.

Now let us turn to the {1} parameter. If the disaster wasn’t all that catastrophic, {1} might be replaced with the number 1, and the message would read:

On January 1, 1999, a mudslide destroyed 1 houses and . . .

Ideally, we would like the message to vary according to the placeholder value, so it would read

no houses

one house

2 houses

depending on the placeholder value. The choice formatting option was designed for this purpose.

A choice format is a sequence of pairs, each containing

  • A lower limit
  • A format string

The lower limit and format string are separated by a # character, and the pairs are separated by | characters.

For example,

{1,choice,0#no houses|1#one house|2#{1} houses}

Table 7.8 shows the effect of this format string for various values of {1}.

Why do we use {1} twice in the format string? When the message format ap­plies the choice format to the {1} placeholder and the value is 2, the choice format returns “{1} houses”. That string is then formatted again by the message format, and the answer is spliced into the result.

You can use the < symbol to denote that a choice should be selected if the lower bound is strictly less than the value.

You can also use the < symbol (expressed as the Unicode character code \u2264) as a synonym for #. If you like, you can even specify a lower bound of -∞ as -\u221E for the first value.

For example,

-∞<no houses|0<one house|2<{1} houses

or, using Unicode escapes,

-\u221E<no houses|0<one house|2\u2264{1} houses

Let’s finish our natural disaster scenario. If we put the choice string inside the original message string, we get the following format instruction:

String pattern = “On {2,date,long}, {0} destroyed {1,choice,0#no houses|1#one house|2#{1} houses}” + “and caused {3,number,currency} of damage.”;

Or, in German,

String pattern

= “{0} zerstorte am {2,date,long} {1,choice,0#kein Haus|1#ein Haus|2#{1} Hauser}”

+ “und richtete einen Schaden von {3,number,currency} an.”;

Note that the ordering of the words is different in German, but the array of objects you pass to the format method is the same. The order of the placeholders in the format string takes care of the changes in the word ordering.

Source: Horstmann Cay S. (2019), Core Java. Volume II – Advanced Features, Pearson; 11th edition.

Leave a Reply

Your email address will not be published. Required fields are marked *