In JDK9b151
Vector::setSize(negative) throws ArrayIndexOutOfBoundsException on the first attempt but it also sets the size to the passed negative value.

Consider the following code

=========
public class NegativeSize {
    public static void main(String[] args) {
        java.util.Vector vector = new java.util.Vector<>();
        setNegativeSize(vector);
        setNegativeSize(vector);
    }

    private static void setNegativeSize(java.util.Vector vector) {
        try {
            System.err.println("trying Vector::setSize(-1)");
            vector.setSize(-1);
            System.err.println("No exception thrown");
        } catch (ArrayIndexOutOfBoundsException e) {
            System.err.println("OK. AIOOBE as expected.");;
        }
        System.err.println("and Vector::size() = " + vector.size());
    }
}
=========

For JDK9b150:

trying Vector::setSize(-1)
OK. AIOOBE as expected.
and Vector::size() = 0
trying Vector::setSize(-1)
OK. AIOOBE as expected.
and Vector::size() = 0

For JDK9b151:

trying Vector::setSize(-1)
OK. AIOOBE as expected.
and Vector::size() = -1
trying Vector::setSize(-1)
No exception thrown
and Vector::size() = -1