`CharAt`

This method returns a character in a string at a certain index. The index is the argument. A valid index (one found in the string) must be used.

Returns a value

CharAt returns a value, a character, that is similar to an int. Meanwhile substring() returns a new string. We find it is faster to use charAt when possible.

Example, `for`-loop

Often we want to loop over the characters in a string. In this example we use charAt inside a for-loop. We display each character to the console.

Tip For a three-character string, the valid indexes are 0, 1 and 2. The last index is equal to the length minus 1.

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

        String value = "cat";
        // Loop through all characters in the string.
        // ... Use charAt to get the values.
        for (int i = 0; i < value.length(); i++) {
            char c = value.charAt(i);
            System.out.println(c);
        }
    }
}c
a
t

First, last chars

We use the charAt method to get first and last characters in a string (and also ones at certain offsets). Here we get the first two and last two chars.

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

        String value = "intercept";
        // First two characters.
        System.out.println(value.charAt(0));
        System.out.println(value.charAt(1));

        // Last two characters.
        System.out.println(value.charAt(value.length() - 1));
        System.out.println(value.charAt(value.length() - 2));
    }
}i
n
t
p

Out of bounds exception

We must be careful when using charAt. If we pass a negative index or an index that is past the last character in the string, we get an exception.

Note In real programs, it is typically faster to check before accessing an index. Causing an exception is slow.

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

        String value = "abc";
        // Access a character out-of-range.
        char error = value.charAt(100);
        System.out.println(error); // Not reached.
    }
}
Exception in thread "main"
    java.lang.StringIndexOutOfBoundsException:
    String index out of range: 100
    at java.lang.String.charAt(Unknown Source)
    at Program.main(Program.java:6)

Benchmark, charAt versus substring. Sometimes we can choose between charAt and substring calls. In my testing, I found charAt is faster to call.

Version 1 This version of the code uses charAt to test a character in a string. It also uses an if-statement.

Version 2 Here we use substring: we get a 1-character substring from the string, and test it in an if-statement.

Result Using charAt to test a character is many times faster than taking a 1-character substring.

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

        // The example string.
        String value = "cat";
        // Test results of our methods.
        System.out.println(value.charAt(2));
        System.out.println(value.substring(2, 3));

        long t1 = System.currentTimeMillis();
        // Version 1: use charAt to test characters in a string.
        int count = 0;
        for (int i = 0; i < 10000000; i++) {
            if (value.charAt(2) == 't') {
                count++;
            }
        }

        long t2 = System.currentTimeMillis();
        // Version 2: use substring to test characters in a string.
        count = 0;
        for (int i = 0; i < 10000000; i++) {
            if (value.substring(2, 3) == "t") {
                count++;
            }
        }

        long t3 = System.currentTimeMillis();

        // ... Times.
        System.out.println(count);
        System.out.println(t2 - t1);
        System.out.println(t3 - t2);
    }
}t
t
0
  5 ms:  charAt
114 ms:  substring

A review

Often we need to access in strings. With charAt, we have a simple and fast way to do this. We must check that an index is within range for the string.