1. Data structure
in java

2. What is Data Structure
Datastructures have been around since the structured programming era. A definition from that era: a datastructure is a set of types, a designated type from that type set, a set of functions, and a set of axioms. That definition implies that a datastructure is a type with implementation. In our object-oriented programming era, type with implementation means class.

3. What is Data Structure
The datastructure is a class definition is too broad because it embraces Employee,Vehicle, Account, and many other real-world entity-specific classes as datastructures. Although those classes structure various data items, they do so to describe real-world entities (in the form of objects) instead of describing container objects for other entity (and possibly container) objects. 

4. Overview
The data structures provided by the Java utility package are very powerful and perform a wide range of functions. These data structures consist of the following interface and classes:
Enumeration
BitSet
Vector
Stack
Dictionary
Hashtable
Properties

5. Overview
All these classes are now legacy and Java-2 has introduced a new framework called Collections Framework

6. The Enumeration
The Enumeration interface isn't itself a data structure, but it is very important within the context of other data structures. The Enumeration interface defines a means to retrieve successive elements from a data structure.
For example, Enumeration defines a method called nextElement that is used to get the next element in a data structure that contains multiple elements.

7. Stages of the life cycle
Following are the stages of the life cycle :
New − A new thread begins its life cycle in the new state. It remains in this state until the program starts the thread. It is also referred to as aborn thread.
Runnable − After a newly born thread is started, the thread becomes runnable. A thread in this state is considered to be executing its task.

8. Stages of the life cycle
Waiting − Sometimes, a thread transitions to the waiting state while the thread waits for another thread to perform a task. A thread transitions back to the runnable state only when another thread signals the waiting thread to continue executing.

9. The BitSet
The BitSet class implements a group of bits or flags that can be set and cleared individually.
This class is very useful in cases where you need to keep up with a set of Boolean values; you just assign a bit to each value and set or clear it as appropriate.

10. The Vector
The Vector class is similar to a traditional Java array, except that it can grow as necessary to accommodate new elements.Like an array, elements of a Vector object can be accessed via an index into the vector.
The nice thing about using the Vector class is that you don't have to worry about setting it to a specific size upon creation; it shrinks and grows automatically when necessary.

11. The Stack
The Stack class implements a last-in-first-out (LIFO) stack of elements.You can think of a stack literally as a vertical stack of objects; when you add a new element, it gets stacked on top of the others.
When you pull an element off the stack, it comes off the top. In other words, the last element you added to the stack is the first one to come back off.

12. The Dictionary
The Dictionary class is an abstract class that defines a data structure for mapping keys to values.
This is useful in cases where you want to be able to access data via a particular key rather than an integer index.
Since the Dictionary class is abstract, it provides only the framework for a key-mapped data structure rather than a specific implementation.

13. The Hashtable
The Hashtable class provides a means of organizing data based on some user-defined key structure.
For example, in an address list hash table you could store and sort data based on a key such as ZIP code rather than on a person's name.
The specific meaning of keys with regard to hash tables is totally dependent on the usage of the hash table and the data it contains.

14. The Properties
Properties is a subclass of Hashtable. It is used to maintain lists of values in which the key is a String and the value is also a String.
The Properties class is used by many other Java classes. For example, it is the type of object returned by System.getProperties( ) when obtaining environmental values.

15. Example
Following example demonstrates how to add first n natural numbers by using the concept of stack .

16. Example import java.io.IOException; public class AdditionStack {
static int num; static int ans; static Stack theStack;
public static void main(String[] args) throws IOException {
num = 50;
stackAddition();
System.out.println("Sum=" + ans); }
public static void stackAddition() {
theStack = new Stack(10000);
ans = 0;
while (num > 0) {
theStack.push(num); --num; }
while (!theStack.isEmpty()) {
int newN = theStack.pop(); ans += newN;
} } }