Collection Classes - Beginning Java 2 SDK - page 511

With all these ways of removing elements from a Vector , there's a lot of potential for ending up with
an empty Vector . It's often handy to know whether a Vector contains elements or not, particularly if
there's been a lot of adding and deleting of elements. You can check whether or not a Vector contains
elements by using the isEmpty() method. This returns true if a Vector object has zero size , and
false otherwise.

Note that a Vector may contain only null references, but this doesn't mean the size() will be
zero or that isEmpty() will return true . To empty a Vector object you must actually remove
the elements, not just set the elements to null .

Searching a Vector

You can get the index position of an object stored in a Vector by passing the object as an argument to
the method indexOf() . For example, the statement

int position = transactions.indexOf(aTransaction);

will search the Vector from the beginning for the object aTransaction using the equals() method
for the argument, so your class needs to have a proper implementation of equals() for this to work.
The variable position will contain either the index of the first reference to the object in
transactions , or -1 if the object isn't found.

You have another version of the method indexOf() available that accepts a second argument that is
the index position where the search for the object should begin. The main use for this arises when an
object can be referenced more than once in a Vector . You can use the method in this situation to
recover all occurrences of any particular object, as follows:

int position = -1; // Search starting index
while(++position<transactions.size()) { // Search with a valid index
if(position = transactions.indexOf(aTransaction, position))<0) // Find next
break;
// Code to process the object in some way...
}

The while loop will continue as long as the method indexOf() returns a valid index value and the
index doesn't get incremented beyond the end of the Vector .

position =
transactions.indexOf(aTransaction,position)

Use to start next search

Starts at 0

index of object found

aTransaction aTransaction

Increment

++position