Then if you try to set bit 1024 or greater without first setting bit 1023, you’ll throw an exception in Java 1...

0. Fortunately, this is fixed in Java 1.1, but avoid using the BitSet if you write code for Java 1.0.
Stack
A Stack is sometimes referred to as a “last-in, first-out” (LIFO) collection. That is, whatever you “push” on the Stack last is the first item you can “pop” out. Like all of the other collections in Java, what you push and pop are Objects, so you must cast what you pop.
What’s rather odd is that instead of using a Vector as a building block to create a Stack, Stack is inherited from Vector. So it has all of the characteristics and behaviors of a Vector plus some extra Stack behaviors. It’s difficult to know whether the designers explicitly decided that this was an especially useful way to do things, or whether it was just a naïve design.
Here’s a simple demonstration of Stack that reads each line from an array and pushes it as a String:
//: Stacks.java
// Demonstration of Stack Class
import java.util.*;
public class Stacks {
static String[] months = {
"January", "February", "March", "April",
"May", "June", "July", "August", "September",
"October", "November", "December" };
public static void main(String[] args) {
Stack stk = new Stack();
for(int i = 0; i < months.length; i++)
stk.push(months[i] + " ");
System.out.println("stk = " + stk);
// Treating a stack as a Vector:
stk.addElement("The last line");
System.out.println(
"element 5 = " + stk.elementAt(5));
System.out.println("popping elements:");
while(!stk.empty())
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System.out.println(stk.pop());
}
} ///:~
Each line in the months array is inserted into the Stack with push( ), and later fetched from the top of the stack with a pop( ). To make a point, Vector operations are also performed on the Stack object. This is possible because, by virtue of inheritance, a Stack is a Vector. Thus, all operations that can be performed on a Vector can also be performed on a Stack, such as elementAt( ).
Hashtable
A Vector allows you to select from a sequence of objects using a number, so in a sense it associates numbers to objects. But what if you’d like to select from a sequence of objects using some other criterion? A Stack is an example: its selection criterion is “the last thing pushed on the stack.” A powerful twist on this idea of “selecting from a sequence” is alternately termed a map, a dictionary, or an associative array. Conceptually, it seems like a vector, but instead of looking up objects using a number, you look them up using another object! This is often a key process in a program.
The concept shows up in Java as the abstract class Dictionary. The interface for this class is straightforward: size( ) tells you how many elements are within, isEmpty( ) is true if there are no elements, put(Object key, Object value) adds a value (the thing you want), and associates it with a key (the thing you look it up with). get(Object key) produces the value given the corresponding key, and remove(Object key) removes the key-value pair from the list. There are enumerations: keys( ) produces an Enumeration of the keys, and elements( ) produces an Enumeration of all the values. That’s all there is to a Dictionary.
A Dictionary isn’t terribly difficult to implement. Here’s a simple approach, which uses two Vectors, one for keys and one for values:
//: AssocArray.java
// Simple version of a Dictionary
import java.util.*;
public class AssocArray extends Dictionary {
private Vector keys = new Vector();
private Vector values = new Vector();
public int size() { return keys.size(); }
public boolean isEmpty() {
return keys.isEmpty();
}
public Object put(Object key, Object value) {
keys.addElement(key);
values.addElement(value);
return key;
}
public Object get(Object key) {
int index = keys.indexOf(key);
// indexOf() Returns -1 if key not found:
if(index == -1) return null;
return values.elementAt(index);
}
public Object remove(Object key) {
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int index = keys.indexOf(key);
if(index == -1) return null;
keys.removeElementAt(index);
Object returnval = values.elementAt(index);
values.removeElementAt(index);
return returnval;
}
public Enumeration keys() {
return keys.elements();
}
public Enumeration elements() {
return values.elements();
}
// Test it:
public static void main(String[] args) {
AssocArray aa = new AssocArray();
for(char c = 'a'; c <= 'z'; c++)
aa.put(String.valueOf(c),
String.valueOf(c)
.toUpperCase());
char[] ca = { 'a', 'e', 'i', 'o', 'u' };
for(int i = 0; i < ca.length; i++)
System.out.println("Uppercase: " +
aa.get(String.valueOf(ca[i])));
}
} ///:~