Collections

• Collections store multiple objects in Java.  For our purposes, let's consider ArrayLists and Vectors.
• They are kind of like Arrays, but they are more flexible.  With an Array, you have to declare the number of elements and you cannot exceed that maximum number.
• Vectors were first in Java.  They store all objects as class Object.
• You add objects to Vectors with the addElement(Object o); method.  This adds the object to the end of the Vector, and increases the size of the Vector, if necessary.
• Then, you can get an object out of a Vector with the elementAt(int index); method.  This returns the object at the specified index.
• You can often use this in a loop, where the index is the iteration of the loop.  If you had a for loop set up as: for (int i = 0; i < vtrMyVector.size(); i++), you might use i in the body of the loop to get out each element of the vector.
• And, as you can see above, you can use the size() method of the Vector object to find the number of elements in the Vector.
• ArrayLists came in Java 1.2.  They are a bit more efficient, because they are not thread safe.  Vectors are thread safe, which make them slower.
• ArrayLists operate on many of the same principles.  They have a size() method, like Vectors.  They do not have  addElement and elementAt methods, but they do have add(Object o) and get(int index) methods, which perform the same tasks.

Created by:  Brandan Jones January 4, 2002