Interface Constants
- All Superinterfaces:
Localizable
,Localizable
,LocalizableResource
- All Known Subinterfaces:
ConstantsWithLookup
,CurrencyCodeMapConstants
,DateTimeConstantsImpl
,NumberConstantsImpl
,SimplePager.ImageButtonsConstants
,TimeZoneConstants
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
To use Constants
, begin by defining an interface that extends
it. Each interface method is referred to as a constant accessor, and
its corresponding localized value is loaded based on the key for that method.
The default key is simply the unqualified name of the method, but can be specified
directly with an @Key
annotation or a different generation method using
@GenerateKeys
. Also, the default value can be specified in an annotation
rather than a default properties file (and some key generators may require the value
to be given in the source file via annotations). For example,
expects to find properties namedpublic interface NumberFormatConstants extends Constants { /** * Returns the localized decimal separator. */ String decimalSeparator(); /** * Returns the localized thousands separator. */ String thousandsSeparator(); }
decimalSeparator
and
thousandsSeparator
in an associated properties file. For
example, the following properties would be used for a German locale:
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,
would match the names of the following properties: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
Constant accessors must be of the formT methodName()where
T
is one of the return types in the following table:
The property value is interpreted as... | Annotation to use for default value | |
---|---|---|
String |
A plain string value | @DefaultStringValue |
String[] |
A comma-separated array of strings; use '\\, ' to escape
commas |
@DefaultStringArrayValue |
int |
An int value, checked during compilation |
@DefaultIntValue |
float |
A float value, checked during compilation |
@DefaultFloatValue |
double |
A double value, checked during compilation |
@DefaultDoubleValue |
boolean |
A boolean value ("true" or "false"), checked during
compilation |
@DefaultBooleanValue |
Map |
A comma-separated list of property names, each of which is a key into a generated map; the value mapped to given key is the value of the property having named by that key | @DefaultStringMapValue |
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:
@DefaultLocale("en_US") 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
If an interfaceorg.example.foo.Intf
extends
Constants
and the following code is used to create an object
from Intf
as follows:
Intf constants = (Intf)GWT.create(Intf.class);then
constants
will be assigned an instance of a generated
class whose constant accessors are implemented by extracting values from a
set of matching properties files. Property values are sought using a
best-match algorithm, and candidate properties files are those which (1)
reside in the same package as the interface (org/example/foo/
),
(2) have a base filename matching the interface name (Intf
),
and (3) have a suffix that most closely matches the locale. Suffixes are
matched as follows:
locale is... |
The properties file that binds to
org.example.foo.Intf is... |
---|---|
unspecified | org/example/foo/Intf.properties |
x |
org/example/foo/Intf_x.properties if it exists and
defines the property being sought, otherwise treated as if
locale were unspecified |
x_Y |
org/example/foo/Intf_x_Y.properties if it exists and
defines the property being sought, otherwise treated as if
locale were x |
x
and Y
are language and locale
codes, as described in the documentation for
Localizable
. Note that default values
supplied in the source file in annotations take precedence over those in
the default properties file, if it is also present.
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
Modules that use this interface should inheritcom.google.gwt.i18n.I18N
.
<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 usingGWT.create(Class)
.-
Nested Class Summary
Modifier and TypeInterfaceDescriptionstatic @interface
Default boolean value to be used if no translation is found (and also used as the source for translation).static @interface
Default double value to be used if no translation is found (and also used as the source for translation).static @interface
Default float value to be used if no translation is found (and also used as the source for translation).static @interface
Default integer value to be used if no translation is found (and also used as the source for translation).static @interface
Default string array value to be used if no translation is found (and also used as the source for translation).static @interface
Default string map value to be used if no translation is found (and also used as the source for translation).static @interface
Default string value to be used if no translation is found (and also used as the source for translation).Nested classes/interfaces inherited from interface com.google.gwt.i18n.client.LocalizableResource
LocalizableResource.DefaultLocale, LocalizableResource.Description, LocalizableResource.Generate, LocalizableResource.GeneratedFrom, LocalizableResource.GenerateKeys, LocalizableResource.Key, LocalizableResource.Meaning