Java .compareTo()

The .compareTo() method is a built-in Java method compares two strings lexicographically by evaluating the Unicode value of each character.

This method is defined in the java.lang.String class and implements the Comparable<String> interface.

Syntax

string1.compareTo(string2);

Parameters:

• string1: The string on which .compareTo() is called.
• string2: The string to compare with string1.

Return value:

• Returns 0 if both strings are equal.
• Returns a positive number if string1 is lexicographically greater than string2.
• Returns a negative number if string1 is lexicographically less than string2.

A way to think about this lexicographical evaluation is noting the Unicode values for these character sets:

Example 1: Comparing Equal Strings

This example uses .compareTo() to compare "Codecademy" to "Codecademy":

class CompareStringsLexicographically {
  public static void main(String[] args) {
    String word1 = "Codecademy";
    String word2 = "Codecademy";

    System.out.println(word1.compareTo(word2));
  }
}

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Here is the output:

0

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Example 2: First String is Lexicographically Less

This example uses .compareTo() to compare "Codecademy" to "codecademy":

class CompareStringsLexicographically {
  public static void main(String[] args) {
    String word1 = "Codecademy";
    String word2 = "codecademy";

    System.out.println(word1.compareTo(word2));
  }
}

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Here is the output:

-32

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Example 3: First String is Lexicographically Greater

This example uses .compareTo() to compare "codecademy" to "Codecademy":

class CompareLexicographically {
  public static void main(String[] args) {
    String word1 = "codecademy";
    String word2 = "Codecademy";

    System.out.println(word1.compareTo(word2));
  }
}

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Here is the output:

32

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Frequently Asked Questions

1. Is .compareTo() case-sensitive?

Yes, it is. Uppercase letters have lower Unicode values than lowercase letters. For example, "Apple".compareTo("apple") returns a negative number.

2. How does .compareToIgnoreCase() differ?

The .compareToIgnoreCase() method compares two strings lexicographically but ignores case differences. For example:

"Java".compareToIgnoreCase("java") // Output: 0

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3. Can .compareTo() be used for sorting strings?

Yes. You can use .compareTo() in sorting algorithms or with data structures like TreeSet or Collections.sort() to sort strings alphabetically.