com.google.gwt.i18n.client
Interface Constants

All Superinterfaces:

Localizable, LocalizableResource

All Known Subinterfaces:

ConstantsWithLookup, CurrencyCodeMapConstants, DateTimeConstantsImpl, NumberConstantsImpl, SimplePager.ImageButtonsConstants, TimeZoneConstants

public interface Constants
extends LocalizableResource

A tag interface that facilitates locale-sensitive, compile-time binding of constant values supplied from properties files. Using GWT.create(class) to "instantiate" an interface that extends Constants returns an instance of an automatically generated subclass that is implemented using values from a property file selected based on locale.

Locale is specified at run time using a meta tag or query string as described for Localizable.

Extending Constants

public interface NumberFormatConstants extends Constants {
  /**
   * Returns the localized decimal separator.
   */
  String decimalSeparator();

  /**
   * Returns the localized thousands separator.
   */
  String thousandsSeparator();
}

decimalSeparator = ,
thousandsSeparator = .

The following example demonstrates how to use constant accessors defined in the interface above:

public void useNumberFormatConstants() {
  NumberFormatConstants constants = (NumberFormatConstants) GWT.create(NumberFormatConstants.class);
  String decimalSep = constants.decimalSeparator();
  String thousandsSep = constants.thousandsSeparator();
  String msg = "Decimals are separated using '" + decimalSep + "'";
  msg += ", and thousands are separated using '" + thousandsSep + "'";
  showMessage(msg);
}

Here is the same example using annotations to store the default values:

public interface NumberFormatConstantsAnnot extends Constants {
  /**
   * Returns the localized decimal separator.
   */
  @DefaultStringValue(".")
  String decimalSeparator();

  /**
   * Returns the localized thousands separator.
   */
  @DefaultStringValue(",")
  String thousandsSeparator();
}

It is also possible to change the property name bound to a constant accessor using the @Key annotation. For example,

public interface NumberFormatConstantsWithAltKey extends Constants {
  /**
   * Returns the localized decimal separator.
   */
  @Key("fmt.sep.decimal")
  String decimalSeparator();

  /**
   * Returns the localized thousands separator.
   */
  @Key("fmt.sep.thousands")
  String thousandsSeparator();
}

fmt.sep.decimal = .
fmt.sep.thousands = ,

Defining Constant Accessors

T methodName()

As an example of a Map, for the following property file:

 a = X
 b = Y
 c = Z
 someMap = a, b, c
 

the constant accessor someMap() would return a Map that maps "a" onto "X", "b" onto "Y", and "c" onto "Z". Iterating through this Map will return the keys or entries in declaration order.

The benefit of using annotations, aside from not having to switch to a different file to enter the default values, is that you can make use of compile-time constants and not worrying about quoting commas. For example:

public interface AnnotConstants extends Constants {
  int CURRENCY_DECIMALS = 2;

  @DefaultStringValue("Orange")
  @Meaning("the color")
  String orange();

  @DefaultStringValue("Red")
  String red();

  @DefaultIntValue(CURRENCY_DECIMALS)
  int currencyDecimals();
  
  @DefaultIntValue(CURRENCY_DECIMALS * 2)
  int extraCurrencyDecimals();
  
  @DefaultStringArrayValue({"zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine"})
  String[] numberNames();
  
  @DefaultDoubleValue(3.14159)
  double pi();
  
  @DefaultStringMapValue({"key1", "comma,value", "comma,key", "value2"})
  Map<String, String> map();
}

Binding to Properties Files

Intf constants = (Intf)GWT.create(Intf.class);

Note that the matching algorithm is applied independently for each constant accessor. It is therefore possible to create a hierarchy of related properties files such that an unlocalized properties file acts as a baseline, and locale-specific properties files may redefine a subset of those properties, relying on the matching algorithm to prefer localized properties while still finding unlocalized properties.

Required Module

<module>
  <!-- other inherited modules, such as com.google.gwt.user.User -->
  <inherits name="com.google.gwt.i18n.I18N"/>
  <!-- additional module settings -->
</module>

Note