indexOf and substring Parsing

indexOf returns the position of a target char or substring (or -1 when absent). substring extracts a portion using start and end indices. Together they parse structured strings: "Jane Doe" splits into first and last name; "user@example.com" splits into local part and domain.

On the AP exam, this pair is the most common String manipulation pattern in parsing FRQs.

Email parsing

public String getDomain(String email) {
    int atIndex = email.indexOf('@');
    return email.substring(atIndex + 1);
}

public String getLocalPart(String email) {
    int atIndex = email.indexOf('@');
    return email.substring(0, atIndex);
}

For "jane@example.com": atIndex = 4. getDomain returns "example.com"; getLocalPart returns "jane".

substring index rules

• s.substring(start) — from start to end, inclusive of start.
• s.substring(start, end) — from start up to but NOT including end.
• Length of result = end - start.

Guard against missing delimiter

int atIndex = email.indexOf('@');
if (atIndex == -1) return "";   // no @ found

Without the guard, substring(-1 + 1) would return the entire String — silently producing a wrong "domain".