Interface Messages

public interface Messages
extends Localizable
A tag interface that facilitates locale-sensitive, compile-time binding of messages supplied from properties files. Using GWT.create(class) to "instantiate" an interface that extends Messages returns an instance of an automatically generated subclass that is implemented using message templates from a property file selected based on locale. Message templates are based on a subset of the format used by MessageFormat.

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

Extending Messages

To use Messages, begin by defining an interface that extends it. Each interface method is referred to as a message accessor, and the name of each message accessor is assumed to match the name of a property defined in a properties file. For example, 
public interface GameStatusMessages extends Messages {
  /**
   * @param username the name of a player
   * @param numTurns the number of turns remaining
   * @return a message specifying the remaining turns for a player
   */
  String turnsLeft(String username, int numTurns);

  /**
   * @param numPoints the number of points
   * @return a message describing the current score for the current player
   */
  String currentScore(int numPoints);
}
expects to find properties named turnsLeft and currentScore in an associated properties file, formatted as message templates taking two arguments and one argument, respectively. For example, the following properties would correctly bind to the GameStatusMessages interface: 
turnsLeft = Turns left for player ''{0}'': {1}
currentScore = Current score: {0}

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

public void beginNewGameRound(String username) {
  GameStatusMessages messages = (GameStatusMessages) GWT.create(GameStatusMessages.class);

  // Tell the new player how many turns he or she has left.
  int turnsLeft = computeTurnsLeftForPlayer(username);
  showMessage(messages.turnsLeft(username, turnsLeft));

  // Tell the current player his or her score.
  int currentScore = computeScore(username);
  setCurrentPlayer(username);
  showMessage(messages.currentScore(currentScore));
}

It is possible to change the property name bound to a message accessor using the gwt.key doc comment, exactly as is done for constant accessors. See Constants for an example.

Defining Message Accessors

Message accessors must be of the form 
String methodName(optional-params)
and parameters may be of any type. Arguments are converted into strings at runtime using Java string concatenation syntax (the '+' operator), which uniformly handles primitives, null, and invoking toString() to format objects.

Compile-time checks are performed to ensure that the number of placeholders in a message template (e.g. {0}) matches the number of parameters supplied.

Binding to Properties Files

Interfaces extending Messages are bound to properties file using the same algorithm as interfaces extending Constants. See the documentation for Constants for a description of the algorithm.

Required Module

Modules that use this interface should inherit com.google.gwt.i18n.I18N. 
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<module>
  <!-- other inherited modules, such as com.google.gwt.user.User -->
  <inherits name="com.google.gwt.i18n.I18N"/>
  <!-- additional module settings -->
</module>

Note

You should not directly implement this interface or interfaces derived from it since an implementation is generated automatically when message interfaces are created using GWT.create(Class).