CompareTo

In sorting, one string will come before or after another. Sometimes two strings are equal. With compareTo we determine lexical ordering.

In Java, we call compareTo on a String instance. We pass one argument—this is the string we are comparing against the instance string.

First example

Let us test the compareTo method. The string "bird" should come before the string "cat." It is alphabetically earlier.

public class Program {
    public static void main(String[] args) {

        String value1 = "cat";
        String value2 = "bird";

        // Compare these strings.
        int result1 = value1.compareTo(value2);
        System.out.println("cat compareTo bird = " + result1);

        int result2 = value2.compareTo(value1);
        System.out.println("bird compareTo cat = " + result2);

        // Compare string against itself.
        // ... It is equal so the result is 0.
        int result3 = value1.compareTo(value1);
        System.out.println("cat compareTo cat = " + result3);
    }
}
cat compareTo bird = 1
bird compareTo cat = -1
cat compareTo cat = 0

CompareToIgnoreCase

With compareTo, strings are compared based on their ordinal values. In ASCII, uppercase letters have lower values than lowercase ones.

public class Program {
    public static void main(String[] args) {

        String value1 = "DOG";
        String value2 = "bird";

        // This is negative, so DOG comes before bird.
        int result1 = value1.compareTo(value2);
        System.out.println(result1);

        // Ignore case.
        // ... Now bird comes before dog (the result is positive).
        int result2 = value1.compareToIgnoreCase(value2);
        System.out.println(result2);
    }
}
-30
2

Result value

The key part about compareTo (and compareToIgnoreCase) is that the result int is negative, positive, or zero. These mean different sort orders.

A final note, sorting

With compareTo and compareToIgnoreCase, we often implement a sorting method. We can directly return the result of compareTo.