`LastIndexOf`

A string in Java may contain the letters "abc." But this occurs twice in the string. With lastIndexOf we search from the rightmost index of a String.

With an index argument, we begin searching from a certain position in the string. In a while-loop, we can scan for all occurrences of a string from the end of the source string.

Method example

This simple example uses the lastIndexOf method with char arguments. It locates first the final instance of the char.

Info As with indexOf, lastIndexOf can accept a String and a starting index as arguments. It mirrors indexOf in these ways.

Here We find the last "s" in the string "spins." But the character "x" is not present in the string.

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

        String line = "spins";

        // Get index of last "s."
        int result1 = line.lastIndexOf('s');
        System.out.println(result1);

        // There is no "x" in the string.
        // ... This returns negative 1.
        int result2 = line.lastIndexOf('x');
        System.out.println(result2);
    }
}4
-1

While, search strings

Here we call lastIndexOf with a string argument in a while-loop. The lastIndexOf call locates the last "cat" string at a starting position.

Argument 1 This is the string (or character) we are searching for in the source string.

Argument 2 This is the starting position. With lastIndexOf we begin searching from this index to the left.

Tip At the end of the while-loop we must decrement the index variable "i" to continue searching the string to the left.

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

        // The string we are searching.
        String line = "cat cat;cat";
        // Search from this index (the last index).
        int i = line.length() - 1;
        // Continue calling lastIndexOf from the last start position.
        while ((i = line.lastIndexOf("cat", i)) >= 0) {
            // The string "cat" was found.
            // ... It starts at this position.
            System.out.println(line.substring(i));
            // Move to previous index.
            i--;
        }
    }
}cat
cat;cat
cat cat;cat

Benchmark, `lastIndexOf`

Sometimes we can use lastIndexOf as an optimization. If a substring occurs 0 or 1 times in a String, indexOf and lastIndexOf return the same result.

Version 1 This code calls the indexOf method, so it searches for the char 8 from the start of the string.

Version 2 This version of the code uses lastIndexOf, so it searches for the char from the string send instead.

Result If, based on our data, we find that the value occurs near the end more often, lastIndexOf is a faster yet equal method.

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

        String input = "0123456789";
        long t1 = System.currentTimeMillis();

        // Version 1: use indexOf.
        for (int i = 0; i < 10000000; i++) {
            int index = input.indexOf('8');
            if (index < 0) {
                System.out.println(false);
            }
        }

        long t2 = System.currentTimeMillis();

        // Version 2: use lastIndexOf.
        for (int i = 0; i < 10000000; i++) {
            int index = input.lastIndexOf('8');
            if (index < 0) {
                System.out.println(false);
            }
        }

        long t3 = System.currentTimeMillis();

        // ... Times.
        System.out.println(t2 - t1);
        System.out.println(t3 - t2);
    }
}41 ms, indexOf
14 ms, lastIndexOf

The lastIndexOf method searches from the right to the left. It is essentially the same method as indexOf but with an inverted order of searching.

LastIndexOf

Method example

While, search strings

Benchmark, lastIndexOf

`LastIndexOf`

Benchmark, `lastIndexOf`