5.2 Vectors

The Vector class implements a variable-length array that can hold any kind of object. Like an array, the elements in a Vector are accessed with an integer index. However, unlike an array, the size of a Vector can grow and shrink as needed to accommodate a changing number of objects. Vector provides methods to add and remove elements, as well as ways to search for objects in a Vector and iterate through all of the objects.

You can create a Vector object using the constructor that takes no arguments.

Vector v = new Vector()

This constructor creates an empty Vector with an initial capacity of 10. The capacity of a Vector specifies how many objects it can contain before more space must be allocated. You can improve the performance of a Vector by setting its initial capacity to a more appropriate value when you create it. For example, if you know that you are going to be storing close to 100 objects in a Vector, you could set the initial capacity as follows:

Vector v = new Vector(100)

It can be time-consuming for a Vector to increase its capacity, so it is better to set the initial capacity based on a rough estimate of the number of objects a Vector will contain than to simply use the default capacity.

The capacity increment of a Vector specifies how much more space is allocated each time the Vector needs to increase its capacity. If you do not specify a capacity increment when you create a Vector, it uses the default value of 0, which causes the Vector to double in size every time it needs to increase its capacity. Doubling in size is a good way for a Vector to become large enough quickly when you have no idea what size it needs to be. However, if you do have a rough idea of the final size of a Vector, specifying a positive capacity increment is less wasteful of memory. For example, if you know that you will be putting 100 or so objects in a Vector, you could create it as follows:

Vector v = new Vector(110, 20)

Once you have created an empty Vector object, you can put object references in it using the addElement() and insertElementAt() methods. The addElement() method adds an element to the end of a Vector. The following code fragment shows the use of the addElement() method:

Vector v = new Vector();
v.addElement("abc");
v.addElement("jkl");
v.addElement("xyz");

The insertElementAt() method inserts a new element into a Vector before a given position, so it can be used to insert an element at any position in a Vector except the last. Like arrays, Vector objects are indexed starting at 0. Here's how to insert an object at the beginning of the Vector object created above:

v.insertElementAt("123", 0);

The size() method returns the number of elements in a Vector object.

After you have added some elements to a Vector object, you can retrieve elements with a number of different methods. For example, the elementAt() method fetches the object at the specified position in the Vector, while the firstElement() and lastElement() methods return the first and last objects in the Vector, respectively. Finally, the elements() method returns an Enumeration object that accesses the elements in the Vector object.

The setElementAt() method allows you to change the object stored at a specified position in the Vector, while the removeElementAt() method removes the object at a specified position from the Vector. The removeElement() method takes an object reference as an argument and removes the first element in the Vector that refers to the given object, if there is such an element. You can also remove all of the elements from the Vector using the removeAllElements() method.

The Vector class also provides some methods for searching the contents of a Vector object. For example, the contains() method returns true if a Vector contains a reference to a specified object. The indexOf() and lastIndexOf() methods return the positions of the first and last elements, respectively, in a Vector that match a specified object.