The Vector Class

The Vector is a built-in Java class in the java.util package. It is an implementation of an array of Objects, with the added feature that it can resize itself automatically, so we don't have to.

The Vector class provides methods named get and set that are similar to the getCargo and setCargo methods we wrote for the Tree class. You should review the other Vector operations by consulting the online documentation.

Before using the Vector class, you should understand a few concepts. Every Vector has a capacity, which is the amount of space that has been allocated to store values, and a size, which is the number of values that are actually in the vector.

The following figure is a simple diagram of a Vector that contains three elements, but it has a capacity of seven.

In general, it is the responsibility of the client code to make sure that the vector has sufficient size before invoking set or get. If you try to access an element that does not exist (in this case the elements with indices 3 through 6), you will get an ArrayIndexOutOfBounds exception.

The Vector methods use the add and insert automatically increase the size of the Vector, but set does not. The resize method adds null elements to the end of the Vector to get to the given size.

Most of the time the client doesn't have to worry about capacity. Whenever the size of the Vector changes, the capacity is updated automatically. For performance reasons, some applications might want to take control of this function, which is why there are additional methods for increasing and decreasing capacity.

Because the client code has no access to the implementation of a vector, it is not clear how we should traverse one. Of course, one possibility is to use a loop variable as an index into the vector:

There's nothing wrong with that, but there is another way that serves to demonstrate the Iterator class. Vectors provide a method named iterator that returns an Iterator object that makes it possible to traverse the vector.