throw and throws

throw and throws look similar but do entirely different jobs. throw is a statement used inside a method body to actually raise an exception at a specific point in the code. throws is a declaration used in a method's signature to announce that the method might propagate a particular checked exception to its caller, without handling it itself.

Why it matters

throw and throws are one letter apart and do different jobs: one raises an exception, the other warns callers that a method may raise one. You use throw to reject invalid input at the edge of a method, and throws to pass a problem to code that is better placed to deal with it.

The throw Statement

throw is followed by a single exception object, and it immediately transfers control out of the current method, just like a return statement, except it hands control to the nearest matching catch block instead of the caller's next line. You can throw a newly created exception (throw new IllegalArgumentException("invalid input")) or re-throw one you already caught. Execution after a throw statement in the same block is unreachable and will not compile if it consists of ordinary statements.

The throws Declaration

throws appears after a method's parameter list and lists one or more checked exception types the method does not handle internally, forcing any caller to either catch them or declare them further up the call chain. This is how the compiler enforces that checked exceptions are never silently dropped anywhere in a call chain. Unchecked exceptions (RuntimeException and its subclasses) never need to be declared with throws, although you are allowed to document them that way for clarity.

• throw: a statement, appears in a method body, throws exactly one exception instance.
• throws: a declaration, appears in a method signature, lists one or more exception types.
• A method that calls another method declaring throws must catch that exception or declare it too.

Example

import java.io.IOException;

public class ThrowThrowsDemo {

    static void readConfig(boolean fileMissing) throws IOException {
        if (fileMissing) {
            throw new IOException("config.txt not found");
        }
        System.out.println("Config loaded successfully.");
    }

    public static void main(String[] args) {
        try {
            readConfig(true);
        } catch (IOException e) {
            System.out.println("Handled: " + e.getMessage());
        }
    }
}

Handled: config.txt not found

Exercise

Make setAge reject negative values by throwing an IllegalArgumentException with the message Age cannot be negative. In main, catch the exception and print its message.

Expected output: Age set to 25 then Age cannot be negative

public class AgeCheck {

    static void setAge(int age) {
        // TODO: throw IllegalArgumentException for a negative age
        System.out.println("Age set to " + age);
    }

    public static void main(String[] args) {
        setAge(25);
        // TODO: call setAge(-3) and print the message of the exception it throws
    }
}

public class AgeCheck {

    static void setAge(int age) {
        if (age < 0) {
            throw new IllegalArgumentException("Age cannot be negative");
        }
        System.out.println("Age set to " + age);
    }

    public static void main(String[] args) {
        setAge(25);
        try {
            setAge(-3);
        } catch (IllegalArgumentException e) {
            System.out.println(e.getMessage());
        }
    }
}

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