The Collections Framework - Beginning Java - page 655

FIGURE 14-9
A stack in Java doesn't have a spring, but it does have all the facilities of a vector because the generic
Stack<> type is derived from the Vector<> type. Of course, you know the Vector<> class implements the
List<> interface so a Stack<> object is also a List<> .
The Stack<T> class adds five methods to those inherited from Vector<T> , two of which provide you
with the LIFO mechanism; the other three give you extra capabilities. These methods are:
• T push(T obj) : Pushes obj onto the top of the stack. It also returns the obj reference.
• T pop() : Pops the object off the top of the stack and returns it. This removes the reference from
the stack. If the stack contains no references when you call this method, an EmptyStackExcep-
tion is thrown.
• T peek() : Returns the object reference at the top of the stack without removing it. Like the previ-
ous method, this method can throw an EmptyStackException .
• int search(Object obj) : Returns a value that is the position of obj on the stack. The reference
at the top of the stack is at position 1, the next reference is at position 2, and so on. Note that this
is quite different from referencing elements in a Vector<> or an array, where indexes start at 0. If
the object isn't found on the stack, -1 is returned.
• boolean empty() : Returns true if the stack is empty and returns false otherwise.
The only constructor for a Stack<> object is the no-arg constructor. This calls the default constructor for
the base class, Vector<> , so you always get an initial capacity for 10 objects. Because it's basically a vector
a stack grows automatically in the same way.
One possible point of confusion is the relationship between the top of a Stack<> object and the elements
in the underlying Vector<> object. Intuitively, you might think that the top of the stack is going to corres-
pond to the first element in the vector, with index 0. If so, you would be totally wrong ! The push() method
for a Stack<> object is analogous to add() for a Vector<> , which adds an object to the end of the vector.
Thus, the top of the stack corresponds to the end of the vector.
Let's try a Stack<> object out in an example so you get a feel for how the methods are used.