Encryption

[Shelly Biswal]

Hi all,
i urgently need help....

i need to write a program which will encrypt a string using a character mapping provided in a file.
The input to your program will be a file which contains mapping for characters.
For E.g. A D
B R
S H
with this mapping if the string has an 'A' then it should be replaced by 'D' etc...
I have to use java exception handling to handle any exceptions like "File not found","File format not correct", "No mapping Present" etc....

[Ernest Friedman-Hill]

OK, so you've told us your assignment. Now you have to show us what you've done so far. If you haven't been able to start: reading in the data file would be a good first step!

As with all programming: take little steps. Write the code to open a file, make sure it compiles, test it. Then add code to read a line and print it out. Make sure it compiles, and then test it. Then write a loop to read all the lines, and test that. Take it one step at a time, and it will be easy.

[Shelly Biswal]

I wrote the code everyting wrks fine except line /*1*/ - the output is always null dont know why.. pls explain

import java.util.Arrays;
import java.lang.StringBuffer;
import java.io.IOException;
import java.util.HashMap;
 
public class ReverseAndEncrypt {
String s1;
public ReverseAndEncrypt(String str) {
s1 = str;
}
public String reverse () {
int length = 0;
StringBuffer s2 = new StringBuffer();
System.out.println(" String: "+s1);
System.out.println(" String length: "+s1.length());
length = s1.length();
s1 =  s1.toUpperCase();
for (int i = length-1;i>=0;i--){
s1.charAt(i);
s2.append(s1.charAt(i));
System.out.println(" String: "+s2.length());
}
return s2.toString();
}
public String encrypt() throws IOException{
 
ReadCharacterMapping readEncryptMap = new ReadCharacterMapping();
HashMap h1 = readEncryptMap.processMyFile();
int length = 0;
length = s1.length();
s1 =  s1.toUpperCase();
StringBuffer s2 = new StringBuffer();
System.out.println("Your String" +s1);
for (int i = length-1;i>=0;i--){
System.out.println("Your String characters" +s1.charAt(i) );
System.out.println(h1.get(s1.charAt(i))); /*1*/
s2.append(h1.get(s1.charAt(i)));
}
 
return s2.toString();
 
}
}

ReadCharacterMapping.java

import java.io.FileReader;
import java.io.FileWriter;
import java.io.BufferedReader;
import java.io.PrintWriter;
import java.io.IOException;
import java.util.StringTokenizer;
import java.util.HashMap;
 
public class ReadCharacterMapping{

public HashMap processMyFile() throws IOException {
BufferedReader inputStream = null;
        try {
inputStream = 
                new BufferedReader(new FileReader("xanadu.txt"));
String l = null;
String key = null;
StringTokenizer st = null;
    String value = null;
HashMap hEncryptData = new HashMap();
            while ((l = inputStream.readLine()) != null) {
st = new StringTokenizer(l);
key = st.nextToken().trim();
value = st.nextToken().trim();
//System.out.println(key +" KEY   VALUE "+value);
hEncryptData.put(key,value);
            }
return hEncryptData;
        } finally {
            if (inputStream != null) {
                inputStream.close();
            }
        }
    }
}

[Ernest Friedman-Hill]

You're storing the data using String objects as keys in the map, but you're trying to look it up using Character objects -- different kinds of objects, so they aren't interchangeable. Instead of get(s.charAt(i)), use get(String.valueOf(s.charAt(i))) .