The key
word you are missing is 'defaults'. " Vector defaults to doubling the size of its array, while the ArrayList increases its array size by 50 percent." So in the absence of a capacityIncrement, Vector doubles its size. In the JavaDocs it tells us that if the capacityIncrement is <=0 then the behavior is to double in size. We also see that the constructors that don't have capacityIncrement parameters will set the increment to 0 (meaning double).
I disagree with the statement about the default ArrayList size increment - according to the Docs, the behavior is not defined, so it may increase by 50 percent under some conditions, or in some versions of
Java, but may increment in some other way on other versions/conditions.