Substring in Java

From an email address you want the username; from a date like "2026-11-12" you want the year; from a full name, the first name. Each time you need part of a string. substring cuts out that part for you. The only tricky detail is that the end position you give is not included, and once you see why, you'll rarely get it wrong.

Cutting Out a Piece

public class Emails {
    public static void main(String[] args) {
        String email = "navika.anamaneni@example.com";
        int at = email.indexOf('@');
 
        String user = email.substring(0, at);
        String domain = email.substring(at + 1);
        System.out.println(user);
        System.out.println(domain);
 
        String date = "2026-11-12";
        System.out.println("Year " + date.substring(0, 4)
            + ", month " + date.substring(5, 7));
    }
}

navika.anamaneni
example.com
Year 2026, month 11

How it works

1. indexOf('@') finds where the @ is: position 16.
2. substring(0, at) takes characters from position 0 up to but not including 16: everything before the @.
3. substring(at + 1) takes from just after the @ to the end of the string.
4. In the date, substring(0, 4) gives 4 characters (positions 0 to 3), and substring(5, 7) gives 2.

Why the End Is Excluded

Excluding the end looks odd at first, but it makes everyday code simpler:

The Errors

Positions must stay within 0 and length(), and begin can't be after end:

The [2, 9) in the message uses the mathematical way of writing "2 included, 9 excluded".

Common Mistakes

Off-by-one on the end. To get 4 characters from position 0, write substring(0, 4), not substring(0, 3).

Not checking indexOf for -1. Handle "not found" before cutting.

Expecting substring to change the original. Strings are immutable; use the returned value.

Practice

1. Easy: From "Srihitha Anamaneni", print the first name and the surname using indexOf(' ') and substring.
2. Harder: Write static String extension(String fileName) that returns the part after the last dot ("report.final.pdf" gives "pdf"), and returns "" when there's no dot.

Key Takeaways