Core Java: How to sort strings in a language-specific order

When sorting a list of strings in alphabetical order you shouldn’t use the comparison methods of the String class (especially when writing internationalized programs). The String.compareTo method performs a binary comparison of the Unicode characters within the two strings, which is ineffective when sorting in most languages, because the Unicode values do not correspond to the relative order of the characters. Instead, you should use the java.text.Collator class to sort strings in language-specific order.

Let’s assume we have to sort the following list of German words:

über,
zahlen,
können,
kreativ,
Äther,
Österreich

Using the String.compareTo method (see code below), this will result in a “sorted” list of:

kreativ,
können,
zahlen,
Äther,
Österreich,
über

According to the collation rules of the German language, the preceding list is obviously in a wrong order. In German, the proper sorting is as follows:

Äther,
können,
kreativ,
Österreich,
über,
zahlen

Here’s a quick example that demonstrates the differences:

import java.text.Collator;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import java.util.Locale;

public class CollatorDemo {

	public static void main(String[] args) {

		List words = Arrays.asList("über", "zahlen", "können",
				"kreativ", "Äther", "Österreich");

		//
		// this won't sort the words properly, in German language
		//
		Collections.sort(words);

		System.out.println("wrong sorting: " + words);

		//
		// Define a collator for German language
		//
		Collator collator = Collator.getInstance(Locale.GERMAN);

		//
		// Sort the list using Collator
		//
		Collections.sort(words, collator);

		System.out.println("proper sorting with Collator: " + words);

	}
}

In conclusion, the Collator class takes into account language-sensitive sorting issues and doesn’t just try to sort words based upon their Unicode character values. You can also set the Collator’s strength property to determine the level of difference considered significant in comparisons. Four strengths are provided: PRIMARY, SECONDARY, TERTIARY, and IDENTICAL. What actually happens with each is dependent on the locale. Typically what happens is as follows: IDENTICAL strength means that the characters must be identical for them to be treated the same, TERTIARY typically is for ignoring case differences, SECONDARY is for ignoring diacritical marks, PRIMARY is like IDENTICAL for base letter differences, but has some differences when handling control characters and accents. See the Collator javadoc for more information.

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