Week 8 Answers

1. Write a program using a "Stack" object that allows a user to type in numbers until the value -1 is seen, after which the numbers are displayed back in reverse order

   
   import java.io.*;
   public class Reverse 
   {
       public static void main(String[] args) 
       {
           BufferedReader stdin =
               new BufferedReader(new InputStreamReader(System.in));      
   
           java.util.Stack sp = new java.util.Stack();
           int answer = 0;
           do {
               System.out.print("Number ? ");
               try {
                   String val = stdin.readLine();
                   answer = Integer.parseInt(val);
                   if (answer != -1) {
                       sp.push(new Integer(answer));
                   }
               }
               catch (Exception e) {
                   // Any error, assume end of list
                   answer = -1;
               }
               
           }
           while (answer != -1);
           
           System.out.print("In reverse order, the numbers are : ");
           while (!(sp.empty())) {
               Integer i = (Integer)sp.pop();
               System.out.print(i + " ");
           }
           System.out.println();
       }
   }
   
   

2. Write a program that will convert values from Hex to Decimal and vice versa.

   public class Convert {
       public static void main(String[] args) 
       {
           try {
               
               String val = args[0].trim();
               if (val.startsWith("0x")) {
                   // Assume it's hex
                   int intVal = Integer.parseInt(val.substring(2),16);
                   System.out.println(val + " == " + intVal);
               }
               else {
                   // Assume it's decimal
                   Integer x = new Integer(val);
                   System.out.println(val + " == 0x" 
                                      + Integer.toHexString(x.intValue()));
               }
               
           }
           catch (Exception e) {
               System.out.println("Please give a meaningful argument");
           }
       }
   }
   
               
   

3. Write a program that will multiply two arbitrarily large integers

   import java.math.*;
   public class Multiply
   {
       public static void main(String[] args) 
       {
           BigInteger b1, b2;
           try {
               b1 = new BigInteger(args[0]);
               b2 = new BigInteger(args[1]);
           }
           catch (Exception e) {
               System.out.println("Usage : java Multiply [int_1] [int_2]");
               return;
           }
           BigInteger result = b1.multiply(b2);
           System.out.println(b1 + " * " + b2 + " = " + result);
       }
   }
   

4. Write a program that will tell you what day of the week your birthday is

   import java.util.*;
   import java.text.*;
   
   public class Birthday 
   {
       public static void main(String[] args) 
       {
           // This will hold our birthday
           GregorianCalendar birthday = new GregorianCalendar();
   
           // Start off with today's date, so we've got the current year
           birthday.setTime(new Date());
           
           // Set the day and month from the caller's arguments
   
           birthday.set(Calendar.DAY_OF_MONTH, Integer.parseInt(args[0]));
           
           // Months are numbered 0-11 so far as Calendar is concerned
           int calendarMonth = (Integer.parseInt(args[1])) - 1;
           birthday.set(Calendar.MONTH, calendarMonth);
   
   
           // Display the day name using "SimpleDateFormat"
   
           // "E" means day of week, make it more than 4 characters to get
           // the long form of the name
           final SimpleDateFormat sdf = new SimpleDateFormat("EEEEE");
   
   
           System.out.println("This year your birthday is on a "
                              + sdf.format(birthday.getTime()));
       }
   }
   
   

5. Write a program that lists the files in a zipfile

   import java.util.zip.*;
   public class ListZipContents 
   {
       public static void main(String[] args) 
       {
           try {
               
               ZipFile z = new ZipFile(args[0]);
               
               java.util.Enumeration e = z.entries();
               while (e.hasMoreElements()) {
                   ZipEntry ze = (ZipEntry)e.nextElement();
                   System.out.println(ze);
               }
           }
           catch (Exception e) {
               System.out.println(e);
           }
           
       }
   }
   

6. Write a program that will extract the host name and path from a www address

   import java.net.*;
   public class WWW 
   {
       public static void main(String [] args) 
       {
           try {
               URL url = new URL(args[0]);
               System.out.println("host is " + url.getHost());
               System.out.println("path is " + url.getPath());
           }
   
           catch (Exception e) {
               System.out.println(e);
           }
       }
   }
   

7. Write a Java program that accepts an IP address on the command line and translates that address into a proper nodename

   import java.net.*;
   class NodeInfo 
   {
       public static void main(String[] args) 
       {
           try {
               InetAddress i = InetAddress.getByName(args[0]);
               System.out.println(args[0] + " : " + i.getHostName() + "/" 
                                  + i.getHostAddress());
           }
           catch (Exception e) {
               System.out.println(e);
           }
           
       }
   }