Why substring() End Index Is Exclusive

Direct Answer

substring(start, end) uses a half-open range: start is included and end is excluded, so the result length is end - start. This supports empty ranges and adjacent slices without overlap: s.substring(0, k) and s.substring(k) divide the string. Both indexes must satisfy 0 <= start <= end <= length().

Code Example

public class SubstringDemo {
    public static void main(String[] args) {
        String s = "hello";
        //           01234   — index positions

        System.out.println(s.substring(1, 3)); // el  — indices 1 and 2 only
        System.out.println(s.substring(0, 5)); // hello — all 5 chars
        System.out.println(s.substring(2));    // llo — from index 2 to end

        // Length formula: end - start = result length
        System.out.println(s.substring(1, 4).length()); // 3 (= 4 - 1)

        // Common mistake
        System.out.println(s.substring(0, 4)); // hell — NOT hello!
        // To get 'hello': substring(0, 5) or substring(0, s.length())

        // Practical use: extract domain from email
        String email = "alice@example.com";
        int at = email.indexOf('@');               // 5
        String domain = email.substring(at + 1);   // example.com
        System.out.println(domain);

        // Extract filename without extension
        String file = "report.pdf";
        int dot = file.lastIndexOf('.');
        String name = file.substring(0, dot);      // report
        System.out.println(name);
    }
}

Common Mistakes & Pitfalls

• Using the last character's index as end — "hello".substring(0, 4) returns "hell", not "hello". To include index 4 you need end = 5, one past the last character.
• Forgetting end can equal length() — substring(0, s.length()) is perfectly legal and returns the whole string. s.length() is the one index you can pass that doesn't point at a real character.
• Passing end > length() or start < 0 — both throw StringIndexOutOfBoundsException. end may equal the length but never exceed it.
• Swapping the arguments — start > end throws the same exception even when both numbers are individually in range.
• Assuming substring() is free — current JDK implementations copy the selected content; this storage detail is not part of the String API contract, so on a hot path that slices a huge string repeatedly you pay a real copy cost each call (older JDKs shared the parent array and leaked it instead).