5.4. Argument Checking

Java

void method(double d, int i, Foo foo) {
    if (d < 0 || d > 1.0)
        throw new IllegalArgumentException();
    if (i < 0 || i > 10)
        throw new IllegalArgumentException();
    if (foo == null)
        throw new IllegalArgumentException();
    // ...
}

Vala: reference type parameters are implicitly checked for null unless they are marked nullable with ?, so you don't have to check them manually. Methods may have preconditions:

void method (double d, int i, Foo foo)
     requires (d >= 0.0 && d <= 1.0)
     requires (i >= 0 && i <= 10)
{
    // ...
}

Vala additionally supports postconditions, for example for checking the return value:

int square (int i)
    ensures (result >= 0)
{
    return i * i;
}

result is a special variable representing the return value.

Use exceptions (errors) for recoverable runtime errors (database errors, I/O errors), use preconditions and assertions (assert (...)) for programming errors such as illegal arguments.