IndexOf

A String in Java possibly contains a matching value. With indexOf, we can search it, from the start, for a match. This works with char and String arguments.

When no value is found, indexOf will return -1. To search from the end of a string, please use lastIndexOf. With indexOf we search from start to end.

An example

This program uses indexOf with character arguments. The 3 indexOf calls in this program locate indexes of the individual chars "bdz."

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

        String line = "abcd";

        // Part 1: locate char in string.
        int b = line.indexOf('b');
        System.out.println(b);

        // Part 2: locate another char at different index.
        int d = line.indexOf('d');
        System.out.println(d);

        // Part 3: not found, error code.
        int z = line.indexOf('z');
        System.out.println(z);
    }
}
1
3
-1

IndexOf, string

Finding a char within a String is useful. But finding a substring or multiple-character sequence is often more helpful. IndexOf (and lastIndexOf) supports this.

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

        String line = "one two three";

        // Get index of substring two.
        int index = line.indexOf("two");
        System.out.println(index);
    }
}
4

While-loop

We often use indexOf within a loop. Consider this program. It continues call indexOf as it advances the position. It finds all instances of the two-char string.

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

        String data = "abcabcabc";

        // Keep looping until -1 encountered.
        int pos = 0;
        while ((pos = data.indexOf("ab", pos)) != -1) {
            // Print position and the substring starting at it.
            System.out.println(pos);
            System.out.println(data.substring(pos));
            pos++;
        }
    }
}
0
abcabcabc
3
abcabc
6
abc

Null argument error

We must be careful with indexOf when we might have null Strings. If we pass it a null argument, it will throw a NullPointerException.

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

        String data = "welcome";
        int j = data.indexOf(null); // Cannot do this.
        System.out.println(j);
    }
}
Exception in thread "main" java.lang.NullPointerException
    at java.lang.String.indexOf(Unknown Source)
    at java.lang.String.indexOf(Unknown Source)
    at program.Program.main(Program.java:7)

Contains

How can we search one String for another? With contains(), we pass in a String we want to find. The method returns true, if the String is found. It returns false if not.

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

        String name = "SOCRATES";
        String value1 = "SOC";
        String value2 = "RATE";
        String value3 = "PLATO";

        // The name contains this value.
        if (name.contains(value1)) {
            System.out.println(value1);
        }

        // The name also contains this value.
        if (name.contains(value2)) {
            System.out.println(value2);
        }

        // The name does not contain this value.
        if (!name.contains(value3)) {
            System.out.println("no: " + value3);
        }
    }
}
SOC
RATE
no: PLATO

Benchmark, indexOf

In this test, we find that a call to indexOf is faster than a for-loop with calls to charAt. When possible, indexOf is preferable to for-loops. It locates chars faster.

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

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

        // Version 1: use indexOf to locate character.
        for (int i = 0; i < 10000000; i++) {
            int index = input.indexOf('m');
            if (index < 0) {
                System.out.println("Error");
            }
        }

        long t2 = System.currentTimeMillis();

        // Version 2: use for, charAt to locate character.
        for (int i = 0; i < 10000000; i++) {
            int index = -1;
            for (int x = 0; x < input.length(); x++) {
                if (input.charAt(x) == 'm') {
                    index = x;
                    break;
                }
            }
            if (index < 0) {
                System.out.println("Error");
            }
        }

        long t3 = System.currentTimeMillis();

        // ... Times.
        System.out.println(t2 - t1);
        System.out.println(t3 - t2);
    }
}
 61 ms, indexOf
172 ms, for loop, charAt

A review

With charAt, and a loop, we can search Strings without indexOf. But indexOf, a declarative method call, uses fewer statements. It may be easier to read, debug and test.