1. Proposed 1
Modify the count servlet in order to generate an answer showing the IP addresses of ALL computer that have accessed it
Use a vector or an array to store the Addresses of the computers

2. Using an Array to Keep all IPs
Starts empty every time the servlet is deployed
import java.io.*; import javax.servlet.*;
import javax.servlet.http.*;
public class AddressesServlet extends HttpServlet {
int nIP = 0;
String[] IPs = new String[1000];
public void doGet(HttpServletRequest request,
HttpServletResponse response) throws
IOException, ServletException {
String ip = request.getRemoteHost();
IPs[nIP++] = ip;
response.setContentType("text/html");
PrintWriter out = response.getWriter();
out.println("<h1> Han visitado este sitio </h1><br>");
for (int i = 0; i < nIP; i++)
out.println("<br> "+ IPs[i]);
out.close(); }

3. Get IP of client and store it in the array
import java.io.*; import javax.servlet.*;
import javax.servlet.http.*;
public class AddressesServlet extends HttpServlet {
int nIP = 0;
String[] IPs = new String[1000];
public void doGet(HttpServletRequest request,
HttpServletResponse response) throws
IOException, ServletException {
String ip = request.getRemoteHost();
IPs[nIP++] = ip;
response.setContentType("text/html");
PrintWriter out = response.getWriter();
out.println("<h1> Han visitado este sitio </h1><br>");
for (int i = 0; i < nIP; i++)
out.println("<br> "+ IPs[i]);
out.close(); }
Get IP of client and
store it in the array

4. Print every string to the browser separated with a horizontal bar
import java.io.*; import javax.servlet.*;
import javax.servlet.http.*;
public class AddressesServlet extends HttpServlet {
int nIP = 0;
String[] IPs = new String[1000];
public void doGet(HttpServletRequest request,
HttpServletResponse response) throws
IOException, ServletException {
String ip = request.getRemoteHost();
IPs[nIP++] = ip;
response.setContentType("text/html");
PrintWriter out = response.getWriter();
out.println("<h1> Han visitado este sitio </h1><br>");
for (int i = 0; i < nIP; i++)
out.println("<hr> "+ IPs[i]);
out.close(); }
Print every string to the
browser separated with
a horizontal bar

5. The Vector class in Java
With an array we are limited to have a fixed number of elements
If the number used to index an array exceeds the number of declares positions the program crashes and throws an ArrayIndexOutOfBoundsException
A vector contains objects of any kind but it has a viarable number of elements
New elements can be inserted, appended
Existing elements can be deleted
Most important methods of the class are
Vector v = new Vector() //creates a new (empty) Vector
v.add(“hola”) //adds the string “hola” at the end
v.add( i, “hola) //adds “hola” at the i-th place
String s = v.elementAt(i) //retrieves the element at the i-th place //(starting from 0)
v.remove(i) //removes the element in the i-th place

6. Get IP and add it to the end
Import util package
import java.io.*; import javax.servlet.*;
import javax.servlet.http.*; import java.util.*;
public class AddressesServlet extends HttpServlet {
Vector IPs = new Vector();
public void doGet(HttpServletRequest request,
HttpServletResponse response) throws
IOException, ServletException {
String ip = request.getRemoteHost();
IPs.add(ip);
response.setContentType("text/html");
PrintWriter out = response.getWriter();
out.println("<h1> Han visitado este sitio </h1><br>");
for (int i = 0; i < IPs.size(); i++)
out.println("<hr> "+ IPs.elementAt(i));
out.close(); }
Create empty vector
Get IP and add it to the end
Print every string to the
browser separated with
a horizontal bar

7. Using a File import java.io.*; import javax.servlet.*;
import javax.servlet.http.*;
public class AddressesServlet extends HttpServlet {
public void doGet(HttpServletRequest request,
HttpServletResponse response) throws
IOException, ServletException {
String ip = request.getRemoteHost();
PrintWriter file = new PrintWriter(
new FileWriter(“ips.txt”,true));
file.println(ip); file.close();
response.setContentType("text/html");
PrintWriter out = response.getWriter();
out.println("<h1> Han visitado este sitio </h1><br>");
BufferedReade infile = new BufferedReader(
new FileReader(“ips.txt”));
for (int i = 0; i < nIP; i++){
String linea = infile.readLine();
out.println("<br> "+ IPs[i]); }
out.close();

8. Proposed 2
Modify the count servlet in order to generate an answer showing all the IPs of the clients and number of times that computer has contacted the servlet
Solution 1: use a vector containing object from a class which has a String (for storing the IP number) and an integer (for storing the number of times the client was here)
public class Node {
int count; String ip;
Node(String x, int y) {
ip = x; count = y; }

9. public class ShowCountsServlet extends HttpServlet {
Vector v = new Vector();
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws IOException, ServletException {
response.setContentType("text/html");
PrintWriter out = response.getWriter();
String ip = request.getRemoteHost();
Node aux; int p =0;
while(p < v.size()) {
aux = (Node)v.elementAt(p);
if (ip.equals(aux.ip)) break;
p++; }
if (p == v.size()) {
aux = new Node(ip,0);
v.add(aux);
out.println(“added");
aux.count++;
out.println("<h1> Following clients visited this site </h1><br>");
for (int i = 0; i < v.size(); i++) {
aux = (Node)v.elementAt(i);
out.print("<hr> "+ aux.ip+" has been "+aux.count+" times here");
out.close();

10. Vector containing the “nodes”
public class ShowCountsServlet extends HttpServlet {
Vector v = new Vector();
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws IOException, ServletException {
response.setContentType("text/html");
PrintWriter out = response.getWriter();
String ip = request.getRemoteHost();
Node aux; int p =0;
while(p < v.size()) {
aux = (Node)v.elementAt(p);
if (ip.equals(aux.ip)) break;
p++; }
if (p == v.size()) {
aux = new Node(ip,0);
v.add(aux);
out.println("agregado");
aux.count++;
out.println("<h1> Following clients visited this site </h1><br>");
for (int i = 0; i < v.size(); i++) {
aux = (Node)v.elementAt(i);
out.print("<hr> "+ aux.ip+" has been "+aux.count+" times here");
out.close();
Vector containing the “nodes”

11. initialize response header get writer to browser get IP of client
public class ShowCountsServlet extends HttpServlet {
Vector v = new Vector();
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws IOException, ServletException {
response.setContentType("text/html");
PrintWriter out = response.getWriter();
String ip = request.getRemoteHost();
Node aux; int p =0;
while(p < v.size()) {
aux = (Node)v.elementAt(p);
if (ip.equals(aux.ip)) break;
p++; }
if (p == v.size()) {
aux = new Node(ip,0);
v.add(aux);
out.println("agregado");
aux.count++;
out.println("<h1> Following clients visited this site </h1><br>");
for (int i = 0; i < v.size(); i++) {
aux = (Node)v.elementAt(i);
out.print("<hr> "+ aux.ip+" has been "+aux.count+" times here");
out.close();
initialize response header
get writer to browser
get IP of client

12. Search for a Node in the vector containing a string with the ip
public class ShowCountsServlet extends HttpServlet {
Vector v = new Vector();
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws IOException, ServletException {
response.setContentType("text/html");
PrintWriter out = response.getWriter();
String ip = request.getRemoteHost();
Node aux; int p =0;
while(p < v.size()) {
aux = (Node)v.elementAt(p);
if (ip.equals(aux.ip)) break;
p++; }
if (p == v.size()) {
aux = new Node(ip,0);
v.add(aux);
out.println("agregado");
aux.count++;
out.println("<h1> Following clients visited this site </h1><br>");
for (int i = 0; i < v.size(); i++) {
aux = (Node)v.elementAt(i);
out.print("<hr> "+ aux.ip+" has been "+aux.count+" times here");
out.close();
Search for a Node in the vector
containing a string with the ip
of the client

13. If the ip was not there, create a new Node with that ip and a
public class ShowCountsServlet extends HttpServlet {
Vector v = new Vector();
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws IOException, ServletException {
response.setContentType("text/html");
PrintWriter out = response.getWriter();
String ip = request.getRemoteHost();
Node aux; int p =0;
while(p < v.size()) {
aux = (Node)v.elementAt(p);
if (ip.equals(aux.ip)) break;
p++; }
if (p == v.size()) {
aux = new Node(ip,0);
v.add(aux);
out.println("agregado");
aux.count++;
out.println("<h1> Following clients visited this site </h1><br>");
for (int i = 0; i < v.size(); i++) {
aux = (Node)v.elementAt(i);
out.print("<hr> "+ aux.ip+" has been "+aux.count+" times here");
out.close();
If the ip was not there, create
a new Node with that ip and a
count = 0

14. In any case, the Node containing the ip
public class ShowCountsServlet extends HttpServlet {
Vector v = new Vector();
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws IOException, ServletException {
response.setContentType("text/html");
PrintWriter out = response.getWriter();
String ip = request.getRemoteHost();
Node aux; int p =0;
while(p < v.size()) {
aux = (Node)v.elementAt(p);
if (ip.equals(aux.ip)) break;
p++; }
if (p == v.size()) {
aux = new Node(ip,0);
v.add(aux);
out.println("agregado");
aux.count++;
out.println("<h1> Following clients visited this site </h1><br>");
for (int i = 0; i < v.size(); i++) {
aux = (Node)v.elementAt(i);
out.print("<hr> "+ aux.ip+" has been "+aux.count+" times here");
out.close();
In any case, the Node containing the ip
of the client will be at position p.
We use it for retrieving the Node and
incrementing the count variable.

15. Fina Finally, retrieve each elemento of the
public class ShowCountsServlet extends HttpServlet {
Vector v = new Vector();
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws IOException, ServletException {
response.setContentType("text/html");
PrintWriter out = response.getWriter();
String ip = request.getRemoteHost();
Node aux; int p =0;
while(p < v.size()) {
aux = (Node)v.elementAt(p);
if (ip.equals(aux.ip)) break;
p++; }
if (p == v.size()) {
aux = new Node(ip,0);
v.add(aux);
out.println("agregado");
aux.count++;
out.println("<h1> Following clients visited this site </h1><br>");
for (int i = 0; i < v.size(); i++) {
aux = (Node)v.elementAt(i);
out.print("<hr> "+ aux.ip+" has been "+aux.count+" times here");
out.close();
Finally, retrieve each elemento of the
Vector printing the ip and count variables
Fina

16. Exercise
Modify the ShowCountsServlet in order to show only the IP and Counts from the client which has contacted the servlet most

17. Solution Most of the program remains the same. Only the output changes
public class ShowCountsServlet extends HttpServlet {
Vector v = new Vector();
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws IOException, ServletException { ….
Node nmax = (Node)v.elementAt(0);
for (int i = 1; i < v.size(); i++) {
Node aux = (Node)v.elementAt(i);
if (aux.count > nmax.count)
nmax = aux; }
out.print(“ the client with highest number of visits is “+nmax.ip
out.println(“and has been "+nmax.count+" times here");
out.close();
Most of the program remains the same.
Only the output changes
Instead of printing the content
of all elemnts, we search for that
one having the highest count …
… and we print it

18. Lets do the same but with a Dictionary (Hashtable)
A hashtable is like a dictionary: it has an object serving as an index and another with the information associated. Examples:
Word - explanation, RUT - name, IP - count
As the vector, it does not have a fixed number of elements
Most important methods of the class are
Hashtable h = new Hashtable()
creates a new (empty) dictionary
h.put(“hola”, “hello”)
adds the pair (“hola”,”hello”)being “hola” the key and “hello” the info associated
String s = (String)h.get(“hola”)
retrieves the information associated to the key “hola” (“hello” in this case) or null if it does not exits
Enumeration e = h.keys();
Retrieves all the keys contained in the hashtable in an object of the class Enumeration. This object can be used to retrieve each key one-by-one
h.remove(“hola”)
removes the pair (key, info) which has “hola” as a key

19. Create a new empty hastable
public class HashExampleServlet extends HttpServlet {
Hashtable h = new Hashtable();
public void doGet(HttpServletRequest request,
HttpServletResponse response) throws
IOException, ServletException {
response.setContentType("text/html");
PrintWriter out = response.getWriter();
String ip = request.getRemoteHost();
Integer i = (Integer)h.get(ip);
if (i == null) {
i = new Integer(0);
h.put(ip,i); }
int ival = i.intValue();
h.put(ip,new Integer(ival+1)); //replaces the older pair
out.println("<h1> Han visitado este sitio </h1><br>");
Enumeration e = h.keys();
while (e.hasMoreElements()) {
ip = (String)e.nextElement();
i = (Integer)h.get(ip);
ival = i.intValue();
out.print("<hr> "+ ip+" has been "+ival+" times here");
out.close();
Create a new empty hastable

20. Get client’s IP and use it to retrieve the associated
public class HashExampleServlet extends HttpServlet {
Hashtable h = new Hashtable();
public void doGet(HttpServletRequest request,
HttpServletResponse response) throws
IOException, ServletException {
response.setContentType("text/html");
PrintWriter out = response.getWriter();
String ip = request.getRemoteHost();
Integer i = (Integer)h.get(ip);
if (i == null) {
i = new Integer(0);
h.put(ip,i); }
int ival = i.intValue();
h.put(ip,new Integer(ival+1)); //replaces the older pair
out.println("<h1> Han visitado este sitio </h1><br>");
Enumeration e = h.keys();
while (e.hasMoreElements()) {
ip = (String)e.nextElement();
i = (Integer)h.get(ip);
ival = i.intValue();
out.print("<hr> "+ ip+" has been "+ival+" times here");
out.close();
Get client’s IP and use it
to retrieve the associated
information (count)

21. If there was no pair (ip, count) having this
public class HashExampleServlet extends HttpServlet {
Hashtable h = new Hashtable();
public void doGet(HttpServletRequest request,
HttpServletResponse response) throws
IOException, ServletException {
response.setContentType("text/html");
PrintWriter out = response.getWriter();
String ip = request.getRemoteHost();
Integer i = (Integer)h.get(ip);
if (i == null) {
i = new Integer(0);
h.put(ip,i); }
int ival = i.intValue();
h.put(ip,new Integer(ival+1)); //replaces the older pair
out.println("<h1> Han visitado este sitio </h1><br>");
Enumeration e = h.keys();
while (e.hasMoreElements()) {
ip = (String)e.nextElement();
i = (Integer)h.get(ip);
ival = i.intValue();
out.print("<hr> "+ ip+" has been "+ival+" times here");
out.close();
If there was no pair (ip, count) having this
IP has key then we put a new one

22. At this point, i will contain the
public class HashExampleServlet extends HttpServlet {
Hashtable h = new Hashtable();
public void doGet(HttpServletRequest request,
HttpServletResponse response) throws
IOException, ServletException {
response.setContentType("text/html");
PrintWriter out = response.getWriter();
String ip = request.getRemoteHost();
Integer i = (Integer)h.get(ip);
if (i == null) {
i = new Integer(0);
h.put(ip,i); }
int ival = i.intValue();
h.put(ip,new Integer(ival+1)); //replaces the older pair
out.println("<h1> Han visitado este sitio </h1><br>");
Enumeration e = h.keys();
while (e.hasMoreElements()) {
ip = (String)e.nextElement();
i = (Integer)h.get(ip);
ival = i.intValue();
out.print("<hr> "+ ip+" has been "+ival+" times here");
out.close();
At this point, i will contain the
Integer containing the count associated
to this ip. We get the int value
We use it for incrementing the count
value and replacing the older pair
whith this new one

23. Get Enumeration object containing keys
public class HashExampleServlet extends HttpServlet {
Hashtable h = new Hashtable();
public void doGet(HttpServletRequest request,
HttpServletResponse response) throws
IOException, ServletException {
response.setContentType("text/html");
PrintWriter out = response.getWriter();
String ip = request.getRemoteHost();
Integer i = (Integer)h.get(ip);
if (i == null) {
i = new Integer(0);
h.put(ip,i); }
int ival = i.intValue();
h.put(ip,new Integer(ival+1)); //replaces the older pair
out.println("<h1> Han visitado este sitio </h1><br>");
Enumeration e = h.keys();
while (e.hasMoreElements()) {
ip = (String)e.nextElement();
i = (Integer)h.get(ip);
ival = i.intValue();
out.print("<hr> "+ ip+" has been "+ival+" times here");
out.close();
Get Enumeration object containing keys
As long as there are elements
Get the next key element (IP)
Get the associates info
Get int value in order to print it

24. Get Enumeration object containing keys
public class HashExampleServlet extends HttpServlet {
Hashtable h = new Hashtable();
public void doGet(HttpServletRequest request,
HttpServletResponse response) throws
IOException, ServletException {
response.setContentType("text/html");
PrintWriter out = response.getWriter();
String ip = request.getRemoteHost();
Integer i = (Integer)h.get(ip);
if (i == null) {
i = new Integer(0);
h.put(ip,i); }
int ival = i.intValue();
h.put(ip,new Integer(ival+1)); //replaces the older pair
out.println("<h1> Han visitado este sitio </h1><br>");
Enumeration e = h.keys();
while (e.hasMoreElements()) {
ip = (String)e.nextElement();
i = (Integer)h.get(ip);
ival = i.intValue();
out.print("<hr> "+ ip+" has been "+ival+" times here");
out.close();
Get Enumeration object containing keys
As long as there are elements
Get the next key element (IP)
Get the associates info
Get int value in order to print it