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.

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.

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.

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.

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

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.