Custom String Comparison in Java

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A hidden gem in the standard Java library is the RuleBasedCollator class. It allows you to define custom collation — a set of rules for comparison between different characters in a string — in a flexible way. Your use cases might be one of the following:

• you need to sort strings with uppercase characters with higher or lower priority than lowercase characters;
• you want to set higher or lower precedence for characters with an accent (like à) or any other non-Latin characters;
• you need to specify any other non-standard rules for character precedence that would be too complicated to define in a Comparator implementation.

The RuleBasedCollator class allows you to set the rules in a convenient and flexible manner, just by writing them as a string expression. For instance, if you sort the following list in a usual order, you get the expected result (I’m using the Stream.toList() method from Java 16 here — in earlier versions of Java you can just use .collect(Collectors.toList())):

If you now want letter b to rank higher than letter a but keep all the other comparison rules the same, you could write such rule as follows: < b < a. Here it is in practice:

There are more complicated examples of usage of the RuleBasedCollator class in the JavaDocs— check them out to know more.