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Vector/Enumeration
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An instance of the Vector class provides a list of any Object derived types. Vectors differ considrably from arrays. Arrays are of only one type and the number of elements cannot be changed. Vectors instead can hold a mix of class objects (they are, of course, all subclasses of Object.)

Vectors can also grow and shrink:

  Vector list = new Vector ();
  list.addElement (" a new string object");
  list.addElement (" another new string object");
  list.addElement (new Date ());
  list.addElement (new Date ());
  list.removeElementAt (3); // Remove the 3rd entry.
                            // The current 4th entry then becomes the 3rd entry

When an element of a Vector returns from a method, it is returned as an Object type. So it must be cast to the proper type:

   String str = (String)list.firstElement ();

If you cast the returned object to a class that it does not belong to, then an runtime ClassCastException occurs:

  String date = (String)list.lastElement ();
 ** Error: the object returned is of the Date type, not a String object. **

You can use the operator instanceof to check for what kind of object has been returned.

  Object o = list.lastElement ();
  if (o instanceof String)
      String date = (String)o;
  else if (o instanceof Date)
      Date aDate = (Date)o;

The Vector class has a number of other methods such as a search for the index number

  int i = list.indexOf (str);

The class ArrayList offers the same capabilities as Vector but is not synchronized so a thread-safe (e.g. single thread) situation for faster performance. See the Iterator and ArrayList section.

Enumeration

A Vector can also return an Enumeration. An Enumeration provides a one time scan through a list of Objects. . (We note that an Enumeration has no relationship at all with the enumerated type of J2SE 5.0.)

  Enumeration e = list.elements ();
  while (e.hasMoreElements ()) {
    System.out.println(e.nextElement ().toString ());
  }

After the hasMoreElements() returns false, the Enumeration cannot be used again.

With Java 1.2 came an alternative to Enumeration called Iterator. We will discuss Iterator after we discuss the Collections Framework.

References and Web Resources

• Chapter 10 : Java : Iterator and ArrayList
• Chapter 10: Java : Collections Framework
• Class specs:
  • java.util.Vector
  • java.util.Enumeration
  • java.util.Iterator

   

Latest update: Nov. 18, 2004

              Tech
ArbitaryPrecision
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Bit Handling
Exercises

           Physics
Data Gen&Analysis
  Demo 1
  Demo 2
Exercises