String Comparison in Java

Question

What is compare two strings lexicographically means?

Answer 1

If you check which string would come first in a lexicon, you've done a lexicographical comparison of the strings!

Some links:

• Wikipedia - String (computer science) Lexicographical ordering
• Note on comparisons: lexicographic comparison between strings

Stolen from the latter link:

A string s precedes a string t in lexicographic order if

• s is a prefix of t, or
• if c and d are respectively the first character of s and t in which s and t differ, then c precedes d in character order.

Note: For the characters that are alphabetical letters, the character order coincides with the alphabetical order. Digits precede letters, and uppercase letters precede lowercase ones.

Example:

• house precedes household
• Household precedes house
• composer precedes computer
• H2O precedes HOTEL

Answer 2

The String.compareTo(..) method performs lexicographical comparison. Lexicographically == alphebetically.

Answer 3

Comparing sequencially the letters that have the same position against each other.. more like how you order words in a dictionary

Answer 4

The wording "comparison" is mildly misleading. You are not comparing for strict equality but for which string comes first in the dictionary (lexicon).

This is the feature that allows collections of strings to be sortable.

Note that this is very dependent on the active locale. For instance, here in Denmark we have a character "å" which used to be spelled as "aa" and is very distinct from two single a's. Hence Danish sorting rules treat two consequtive a's identically to an "å", which means that it goes after z. This also means that Danish dictionaries are sorted differently than English or Swedish ones.

Answer 5

Leading from answers from @Bozho and @aioobe, lexicographic comparisons are similar to the ordering that one might find in a dictionary.

The Java String class provides the .compareTo () method in order to lexicographically compare Strings. It is used like this "apple".compareTo ("banana").

The return of this method is an int which can be interpreted as follows:

• returns < 0 then the String calling the method is lexicographically first (comes first in a dictionary)
• returns == 0 then the two strings are lexicographically equivalent
• returns > 0 then the parameter passed to the compareTo method is lexicographically first.

More specifically, the method provides the first non-zero difference in ASCII values.

Thus "computer".compareTo ("comparison") will return a value of (int) 'u' - (int) 'a' (21). Since this is a positive result, the parameter ("comparison") is lexicographically first.

There is also a variant .compareToIgnoreCase () which will return 0 for "a".compareToIgnoreCase ("A"); for example.