In this tutorial we’re going to implement own ResourceBundle to show how to provide locale-sensitive values dynamically.

ResourceBundle can be extended in a couple of ways. Here we extend it directly and supply Spanish language translations for a Map:

package com.farenda.java.util;

import java.util.*;

public class Words_es extends ResourceBundle {

    private static final Map<String, String> WORDS = new HashMap<>();
    static {
        // Load from some data source
        WORDS.put("book", "el libro");
        WORDS.put("date-format", "yyyy.MM.dd");
    }

    @Override
    protected Object handleGetObject(String key) {
        // Return null for unknown key to allow
        // check other Resource Bundles
        return WORDS.get(key);
    }

    @Override
    protected Set<String> handleKeySet() {
        // Faster implementation than default one
        return WORDS.keySet();
    }

    @Override
    public Enumeration<String> getKeys() {
        return Collections.enumeration(WORDS.keySet());
    }
}

Return null from handleGetObject(String key) if your ResourceBundle doesn’t know that key. This will make further searching of the key in more general bundles – like shown in Intro to ResourceBundles.

Overriding Set<String> handleKeySet() is optional and done only for performance reasons, because here we’ve got complete set of keys.

Default properties file Words.properties, located also in com.farenda.java.util path:

book=book
date-format=MM/dd/yyyy

The above ResourceBundle is located in package com.farenda.java.util, so it has to be loaded from that path:

public static void main(String[] args) {
    Locale.setDefault(Locale.ENGLISH);
    displayWord("book", Locale.getDefault());
    displayWord("book", new Locale("es", "ES"));

    displayDate(Locale.CANADA);
    displayDate(new Locale("es", "ES"));
}

private static final String BUNDLE_NAME
        = "com.farenda.java.util.Words";

private static void displayDate(Locale locale) {
    ResourceBundle bundle = getBundle(BUNDLE_NAME, locale);
    String localeDate = formatDate(bundle.getString("date-format"));
    System.out.printf("19th May 2015 in locale '%s': %s%n",
            locale, localeDate);
}

private static String formatDate(String dateFormat) {
    Date date = Date.from(Instant.parse("2015-05-19T00:00:00.00Z"));
    return new SimpleDateFormat(dateFormat).format(date);
}

private static void displayWord(String word, Locale locale) {
    ResourceBundle words = getBundle(BUNDLE_NAME, locale);
    System.out.printf("'%s' in locale '%s': %s%n",
            word, locale, words.getString(word));
}

We get locale version of “date format” string and format the date accordingly. Notice that we use Instant from Java 8 to parse date string and then the new Date.from(Instant) to create old-style java.util.Date from it.

Now, when we run the above program it will print:

'book' in locale 'en': book
'book' in locale 'es_ES': el libro
19th May 2015 in locale 'en_CA': 05/19/2015
19th May 2015 in locale 'es_ES': 2015.05.19

• Collection to Enumeration
• Java ResourceBundle – internationalized program