hex conversion...

This works for me:
 

 char    cc = 'G';

        if(cc < '0' || cc > 'F')   System.out.println("bigger");

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There is a method hex to int you don't need to write it

http://download.oracle.com/javase/6/docs/api/java/lang/Integer.html#valueOf%28java.lang.String,%20int%29

  (Integere.valueOf(String, 16)).intVlaue()

That is becaiue you have additional catch loop there

 public static int hexToDecimal(char ch) throws NumberFormatException {

        int temp=0;
        

            if (ch<'0' || ch>'F')
                throw new NumberFormatException("not a hex number.");

            if (ch>='A' && ch <='F')
                temp = 10+ch-'A';
            else
                temp = ch-'0';

            return temp;
    }

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You were catching it in the additional catch loop and even not printing the stackTrace - so uyou just caught the exception and
didn't inform anyoje about it - and you didn't see that it was throwing it - the above post should work

That's how it works:

public class TryHexToDecimal {
 public static int hexToDecimal(char ch) throws NumberFormatException {

           int temp=0;
           

               if (ch<'0' || ch>'F')
                   throw new NumberFormatException("not a hex number.");

               if (ch>='A' && ch <='F')
                   temp = 10+ch-'A';
               else
                   temp = ch-'0';


           return temp;
       }



    public static void main(String[] args) {

        System.out.println(hexToDecimal('G'));

}
}

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Exception in thread "main" java.lang.NumberFormatException: not a hex number.
	at Miscellaneous.hexToDecimal(Miscellaneous.java:24)
	at Miscellaneous.main(Miscellaneous.java:39)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:597)
	at com.intellij.rt.execution.application.AppMain.main(AppMain.java:110)

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lowercase is bigger than upper case
so

'a' > 'F'

so it will cause exception also

so you want to do something like that:

   boolean good = false;
        if((ch >= '0' && ch <= 'F')  || (ch >= 'a' && ch <= 'f'))good = true;
        
               if (!good) 
                   throw new NumberFormatException("not a hex number.");

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But then you should take care of another oart where uyou
should assign values to a-f lowere case

Instead of doing all that just use the method
which makes int out of hex string which I posted above

and maybe even better method:
http://download.oracle.com/javase/6/docs/api/java/lang/Integer.html#parseInt%28java.lang.String,%20int%29

int i = Integer.parseInt(String s, int radix)

This is how it parseInt() method works:

    int inum = Integer.parseInt("12Fc10",16);

        System.out.println("inum: " + inum);
        

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Output:

inum: 1244176

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          int inum1 = Integer.parseInt("12Fc10g",16);

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Output:

Exception in thread "main" java.lang.NumberFormatException: For input string: "12Fc10g"
	at java.lang.NumberFormatException.forInputString(NumberFormatException.java:48)
	at java.lang.Integer.parseInt(Integer.java:458)
	at Miscellaneous.main(Miscellaneous.java:47)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:597)
	at com.intellij.rt.execution.application.AppMain.main(AppMain.java:110)

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On the original issue, I am getting error message like:

Enter a hex number: k
Exception in thread "main" java.lang.NumberFormatException: K is not a hex number.
      at Exercise13_6.hexToDecimal(Exercise13_6.java:31)
      at Exercise13_6.hexToDecimal(Exercise13_6.java:21)
      at Exercise13_6.main(Exercise13_6.java:13)
Java Result: 1
BUILD SUCCESSFUL (total time: 4 seconds)

Where I just want:

K is not a hex

import java.util.Scanner;

public class Exercise13_6{

    public static void main(String[] args) {

        Scanner input = new Scanner(System.in);


        System.out.print("Enter a hex number: ");
        String hex=input.nextLine();

        System.out.println("The decimal value for hex number "
                + hex + " is " + hexToDecimal(hex.toUpperCase()));
    }

    public static int hexToDecimal(String hex) {
        int decimalValue=0;
        for(int i=0; i<hex.length();i++) {
            char hexChar=hex.charAt(i);
            decimalValue=decimalValue*16+hexToDecimal(hexChar);
        }
            return decimalValue;
    }

//    public static int hexToDecimal(char ch)  {
public static int hexToDecimal(char ch) throws NumberFormatException {
        int temp=0;
        
               if (ch<'0' || ch>'F')
                   throw new NumberFormatException(ch + " is not a hex number.");

               if (ch>='A' && ch <='F')
                   temp = 10+ch-'A';
               else
                   temp = ch-'0';


           return temp;        

    }
}

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You need to define your own custom Exception and
after you catcgh NumberFormat throw your Custom exception - look
at our previous exrcises  where you were doing calculator
 

Or you can do it like that - catch it
but not print the exception with toString() or with ex.printStackTrace()
but rather print your own message:

 public static int hexToDecimal(char ch)  {

           int temp=0;
           
        try{
        boolean good = false;
        if((ch >= '0' && ch <= 'F')  || (ch >= 'a' && ch <= 'f'))good = true;

               if (!good)
                   throw new NumberFormatException("not a hex number.");

               if (ch>='A' && ch <='F')
                   temp = 10+ch-'A';
               else
                   temp = ch-'0';
            
        } catch(Exception ex){
            System.out.println("Not a hex");
            
        }


           return temp;
       }

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Thanks I guess I can handle this.

Say, like that:

pubblic class HandlingExcept {  
public static int hexToDecimal(char ch)  {

           int temp=0;

        try{
        boolean good = false;
        if((ch >= '0' && ch <= 'F')  || (ch >= 'a' && ch <= 'f'))good = true;

               if (!good)
                   throw new NumberFormatException("not a hex number.");

               if (ch>='A' && ch <='F')
                   temp = 10+ch-'A';
               else
                   temp = ch-'0';

        } catch(Exception ex){
            System.out.println(ch + " is not a hex character");

        }


           return temp;
       }



    public static void main(String[] args) {

       System.out.println(hexToDecimal('g'));
}
}

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g is not a hex character

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I still need to read some of your last emails...

This is what I have so far, but it has one error:

import java.util.Scanner;

public class Exercise13_6{

    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        System.out.print("Enter a hex number: ");
        String hex=input.nextLine();
        System.out.println("The decimal value for hex number "
                + hex + " is " + hexToDecimal(hex.toUpperCase()));
    }

    public static int hexToDecimal(String hex) {
        int decimalValue=0;
        for(int i=0; i<hex.length();i++) {
            char hexChar=hex.charAt(i);
            decimalValue=decimalValue*16+hexToDecimal(hexChar);
        }
            return decimalValue;
    }

    public static int hexToDecimal(char ch) throws
        InvalidDataException13_6, NumberFormatException {
        int temp=0;
        boolean good = false;
        
        try{
            if((ch >= '0' && ch <= 'F')  || 
               (ch >= 'a' && ch <= 'f'))good = true;

               if (!good) //{
                   throw new InvalidDataException13_6(ch+" is not a hex number.");
//               }else{
                   if (ch>='A' && ch <='F')
                       temp = 10+ch-'A';
                   else
                       temp = ch-'0';
//               }
        } catch(Exception ex){
//            System.out.println("Not a hex");
            throw new InvalidDataException13_6 (ch+" is not a hex number.");
        }
           return temp;
    }
}

class InvalidDataException13_6 extends Exception { 
    private String mistake;
    public  InvalidDataException13_6(String err)
     {
       super(err);     // call super class constructor
       mistake = err;  // save message
     }

    public String toString(){
        return mistake;
    }
}

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I guess I have it now

import java.util.Scanner;

public class Exercise13_6{

    public static void main(String[] args) throws
        InvalidDataException13_6, NumberFormatException {
        Scanner input = new Scanner(System.in);
        System.out.print("Enter a hex number: ");
        String hex=input.nextLine();
        System.out.println("The decimal value for hex number "
                + hex + " is " + hexToDecimal(hex.toUpperCase()));
    }

    public static int hexToDecimal(String hex) throws
        InvalidDataException13_6, NumberFormatException {
        int decimalValue=0;
        for(int i=0; i<hex.length();i++) {
            char hexChar=hex.charAt(i);
            decimalValue=decimalValue*16+hexToDecimal(hexChar);
        }
            return decimalValue;
    }

    public static int hexToDecimal(char ch) throws
        InvalidDataException13_6, NumberFormatException {
        int temp=0;
        try{
            if((ch >= '0' && ch <= 'F')  || (ch >= 'a' && ch <= 'f'))//good = true;
               throw new InvalidDataException13_6(ch+" is not a hex number.");
                   if (ch>='A' && ch <='F')
                       temp = 10+ch-'A';
                   else
                       temp = ch-'0';
        } catch(Exception ex){
            throw new InvalidDataException13_6 (ch+" is not a hex number.");
        }
           return temp;
    }
}

class InvalidDataException13_6 extends Exception { 
    private String mistake;
    public  InvalidDataException13_6(String err)
     {
       super(err);     // call super class constructor
       mistake = err;  // save message
     }

    public String toString(){
        return mistake;
    }
}

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not quite there yet.

Seems OK, but I'm not sure if it will be lower case, this part will work correctly:

 if (ch>='A' && ch <='F')
                       temp = 10+ch-'A';
                   else
                       temp = ch-'0';

and if ch = 'b' - ?

>>
         if (((ch >= '0') && (ch <= 'F')) || ((ch >= 'a') && (ch <= 'f'))) { //good = true;
                throw new InvalidDataException13_6(ch +
                    " is not a hex number.");
            }
>>

Your logic is the wrong way around there

I ran for k, this is whay I got:

run:
Enter a hex number: k
The decimal value for hex number k is 27
BUILD SUCCESSFUL (total time: 5 seconds)

which is wrong result.  and:

Enter a hex number: A22
Exception in thread "main" A is not a hex number.
      at Exercise13_6.hexToDecimal(Exercise13_6.java:35)
      at Exercise13_6.hexToDecimal(Exercise13_6.java:19)
      at Exercise13_6.main(Exercise13_6.java:10)
Java Result: 1
BUILD SUCCESSFUL (total time: 7 seconds)

re:> Your logic is the wrong way around there

I will revise and test it again

Use this logic :

  boolean good = false;
        if((ch >= '0' && ch <= 'F')  || (ch >= 'a' && ch <= 'f'))good = true;

               if (!good)
                   throw new NumberFormatException("not a hex number.");

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but rervise the part where you calculate the actuall value form lower case - it should be sdifferent

OK

I think this should work:

  public static int hexToDecimal(char ch)  {

           int temp=0;

        try{
        boolean good = false;
        if((ch >= '0' && ch <= 'F')  || (ch >= 'a' && ch <= 'f'))good = true;

               if (!good)
                   throw new NumberFormatException("not a hex number.");

               if (Character.toUpperCase(ch)>='A' && Character.toUpperCase(ch)<='F')
                   temp = 10+Character.toUpperCase(ch)-'A';
               else
                   temp = Character.toUpperCase(ch)-'0';

        } catch(Exception ex){
            System.out.println(ch + " is not a hex character");

        }


           return temp;
       }

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Or just convert your character to upper case form the very nebgging and that will make your code
mauch easier (baseically like it was in the beginning)

>>  if((ch >= '0' && ch <= 'F')  || (ch >= 'a' && ch <= 'f'))good = true;

Won't work unfortunately - it will set good to true for '@'

You'd be better off with

ch = Character.toLowerCase(ch);
good = "0123456789abcdef".indexOf(ch) > -1;

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>>if (Character.toUpperCase(ch)>='A' && Character.toUpperCase(ch)<='F')

Don't do that. Convert it straight away and call the method once, not multiple times

Well, in the end use, parseInt as I mentioned above - this is definitely something wahich you sdon't want to write yourself

>>this is definitely something wahich you sdon't want to write yourself

Well i think that goes without saying, outside an academic exercise, which is what i assume this is

>>I think this should also work:

It doesn't - for the reason i gave at http:#:36945129


Can you tell me why you accepted an incorrect answer eghtebas?