RegionMatches

This method tests a region of one string against another. We provide a start index for both strings. We also provide a length argument (the number of chars to test on both).

With regionMatches, we can see if a substring patches another substring. We can avoid creating substrings. This saves allocations and improves performance.

An example

Let us begin with this example. We have a string literal with four concatenated words in it. We match each part of this literal.

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

        String codes = "OneTwoThreeFour";

        // Check for "Two" at index 3.
        // ... Length is 3 chars.
        if (codes.regionMatches(3, "Two", 0, 3)) {
            System.out.println(true);
        }

        // Check for "Three" at index 6.
        // ... Length is 5 chars.
        if (codes.regionMatches(6, "Three", 0, 5)) {
            System.out.println(true);
        }

        // Check for "FOUR" from "IS_FOUR" at index 11.
        // ... Length is 4 chars.
        if (codes.regionMatches(true, 11, "IS_FOUR", 3, 4)) {
            System.out.println(true);
        }
    }
}
true
true
true

Benchmark

Here is a benchmark of regionMatches. We can use regionMatches for some significant performance wins in real-world Java programs.

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

        String part1 = "CatDogBird";
        String part2 = "DogBirdCat";

        // Test two versions.
        if (part1.regionMatches(3, part2, 0, 3)) {
            System.out.println("True 1");
        }

        if (part1.substring(3, 6).equals(part2.substring(0, 3))) {
            System.out.println("True 2");
        }

        int count = 0;
        long t1 = System.currentTimeMillis();

        // Version 1: use regionMatches to compare two substrings.
        for (int i = 0; i < 10000000; i++) {
            if (part1.regionMatches(3, part2, 0, 3)) {
                count++;
            }
        }

        long t2 = System.currentTimeMillis();

        // Version 2: use substring to compare two substrings.
        for (int i = 0; i < 10000000; i++) {
            if (part1.substring(3, 6).equals(part2.substring(0, 3))) {
                count++;
            }
        }

        long t3 = System.currentTimeMillis();

        // ... Times.
        System.out.println(count);
        System.out.println(t2 - t1);
        System.out.println(t3 - t2);
    }
}
True 1
True 2
20000000
15  ms:    regionMatches
172 ms:    substring, equals

Notes, indexes

When using methods like regionMatches and substring(), using correct indexes and lengths is important. The final argument on regionMatches is a length.

Performance

Avoiding allocations is one of the best performance optimizations. With regionMatches, we can achieve a big performance win if we can reduce substring allocations.

The arguments to regionMatches can be hard to get right at first. But the performance and code size improvements with regionMatches makes it worth the effort.