Difference Between Throw and Throws

throw and throws differ by one letter and are constantly mixed up, especially in interviews. The simplest way to remember them: throw is an action ("throw this exception now"), and throws is a warning label ("this method might throw these"). One happens inside the method body; the other is written in the method's header.

Both in One Method

public class Withdraw {
    static class InsufficientFundsException extends Exception {
        InsufficientFundsException(String message) { super(message); }
    }
 
    static long balance = 2000;
 
    static void withdraw(long amount) throws InsufficientFundsException {
        if (amount > balance) {
            throw new InsufficientFundsException(
                "Navika needs Rs. " + (amount - balance) + " more");
        }
        balance -= amount;
        System.out.println("Withdrew " + amount + ", left " + balance);
    }
 
    public static void main(String[] args) {
        try {
            withdraw(500);
            withdraw(5000);
        } catch (InsufficientFundsException e) {
            System.out.println("Refused: " + e.getMessage());
        }
    }
}

Withdrew 500, left 1500
Refused: Navika needs Rs. 3500 more

How it works

1. throws InsufficientFundsException in the header warns callers that this checked exception may come out.
2. throw new InsufficientFundsException(...) in the body actually raises it when the balance is too low.
3. main catches it, as the warning label requires.

Side by Side

Common Mistakes

Writing throws new X() or throw X. throw takes an object; throws takes type names.

Adding throws but never throwing. The declaration then misleads callers.

Forgetting throws for a checked exception you throw. The compiler reports "unreported exception".

Interview Corner

Practice

1. Easy: For each line, say whether it's valid: throw new IOException("x");, void a() throws IOException, throws new IOException();, void b() throw IOException.
2. Harder: Write static int score(String text) throws InvalidScoreException with your own checked exception, throwing it for values outside 0 to 100.

Key Takeaways