Am I missing something?

Question about the task

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What's wrong with my code? The output seems correct but the task is not completed. Can someone help me, please?

The public static toHex(String) method must convert the string representation of a binary number, received as an input parameter, from the binary numeral system to hexadecimal and return its string representation.
And conversely, the public static toBinary(String) method converts from the string representation of a hexadecimal number to the string representation of a binary number.

The methods only work with non-empty strings.
If the input parameter is an empty string or null, then both methods return an empty string.
If the input parameter of the toHex(String) method contains any character other than 0 or 1, then the method returns an empty string.
If the input parameter of the toBinary(String) method contains any character other than digits from 0 to 9 or lowercase Latin letters from a to f, then the method returns an empty string.

Your task is to implement these methods.

One algorithm for converting the string representation of a binary number to the string representation of a hexadecimal number is as follows:

We check the length of the string received in the input parameter. It must be a multiple of 4.
If this is not the case, then add the required number of 0s to the beginning of the string.
We take every four characters (bits) and check which hexadecimal character they correspond to.

For example:

the binary representation is "100111010000", where "1001" is "9", "1101" is "d", and "0000" - "0"
the hexadecimal representation is "9d0".

One algorithm for converting the string representation of a hexadecimal number to the string representationof a binary number is as follows:
We take each character and check which binary number (4 bits) it corresponds to.

For example:

the hexadecimal representation is "9d0", where "9" is "1001", "d" is "1101", and "0" is "0000"
the binary representation is "100111010000".

The main() method is not involved in testing.

Requirements:

The toHex(String) method needs to be implemented as outlined in the task conditions.
The toBinary(String) method needs to be implemented as outlined in the task conditions.
The Integer.toBinaryString(int) and Long.toBinaryString(int) methods cannot be used.
The Integer.toHexString(int) and Long.toHexString(int) methods cannot be used.
The Integer.parseInt(String, int) and Long.parseLong(String, int) methods cannot be used.
The Integer.toString(int, int) and Long.toString(long, int) methods cannot be used.
You cannot use a BigInteger object.
You cannot use a BigDecimal object.

package en.codegym.task.pro.task09.task0908; /* Binary to hexadecimal converter */ public class Solution { private static final String HEX = "0123456789abcdef"; public static void main(String[] args) { String binaryNumber = "100111010000"; System.out.println("Binary number " + binaryNumber + " is equal to hexadecimal number " + toHex(binaryNumber)); String hexNumber = "9d0"; System.out.println("Hexadecimal number " + hexNumber + " is equal to binary number " + toBinary(hexNumber)); } public static String toHex(String binaryNumber) { if (binaryNumber == null) return ""; String h = ""; int len = binaryNumber.length(); while (binaryNumber.length() % 4 != 0) { binaryNumber = "0" + binaryNumber; } for (int i = 0; i < len; i += 4) { int index0 = binaryNumber.charAt(len - i - 1) == '0' ? 0 : 1; int index1 = binaryNumber.charAt(len - i - 1 - 1) == '0' ? 0 : 1; int index2 = binaryNumber.charAt(len - i - 2 - 1) == '0' ? 0 : 1; int index3 = binaryNumber.charAt(len - i - 3 - 1) == '0' ? 0 : 1; int index = index0 + index1 * 2 + index2 * 4 + index3 * 8; h = HEX.charAt(index) + h; } return h; } public static String toBinary(String hexNumber) { if (hexNumber == null) return ""; int len = hexNumber.length(); String b = ""; for (int i = 0; i < len; i++) { int decimalNumber = HEX.indexOf(hexNumber.charAt(i)); String aux = ""; if (decimalNumber == 0) { aux = "0000"; } while (decimalNumber != 0) { aux = decimalNumber % 2 + aux; decimalNumber = decimalNumber / 2; } b += aux; } return b; } }

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Aldo Luna Bueno

Level 28 , Peru

22 February 2022, 00:18

I think you are missing to check if the input string is not empty. For example:

if (binaryNumber == null || binaryNumber.isEmpty())
            return "";

A String variable can have a null reference or reference to an empty string (""). They are different things. In the conditions of the task says: "If the input parameter is an empty string or null, then both methods return an empty string."

Lisa

Level 41

22 February 2022, 00:12

the bin to hex... test that method with a binary number length % 4 != 0 the hex to bin... puh... it gives wrong results. Tested with ff01 and got as result 1111111100001 Without a calculator I can say that's wrong, f is 1111 and 1 is 0001 -> 1111111100000001 So it looks as if you forgot to fill up the result of the 1 -> 1 conversion with leading zeros

Jesus Bárbara

Level 22 , Portugal

22 February 2022, 09:52

yeah, you're completely right, I'm gonna fix it. Thank you!

Jesus Bárbara

Level 22 , Portugal

22 February 2022, 10:13

I fixed that, still the task is incomplete... Here's the method public static String toBinary(String hexNumber) { if (hexNumber == null || hexNumber.isEmpty()) return ""; int len = hexNumber.length(); String b = ""; for (int i = 0; i < len; i++) { int decimalNumber = HEX.indexOf(hexNumber.charAt(i)); String aux = ""; for (int j = 0; j < 4; j++) { if (decimalNumber != 0) { aux = (decimalNumber % 2) + aux; decimalNumber = decimalNumber / 2; }else{ aux = '0' + aux; } } b += aux; } return b; }

Lisa

Level 41

22 February 2022, 11:34

'If the input parameter of the toHex(String) method contains any character other than 0 or 1, then the method returns an empty string. If the input parameter of the toBinary(String) method contains any character other than digits from 0 to 9 or lowercase Latin letters from a to f, then the method returns an empty string.' Try to implement these requirements, too.

Jesus Bárbara

Level 22 , Portugal

22 February 2022, 13:31

It worked!! Thank you. I created an auxiliary method. public static boolean isHex(String hex){ for (int i = 0; i < hex.length(); i++) { hex.toLowerCase(); char c = hex.charAt(i); if((c < '0' || c > '9') && (c < 'a' || c > 'f')) return false; } return true; }

Lisa

Level 41

22 February 2022, 14:14

excellent idea and well done ;) that wasn't easy at all. just a fyi, the toLowerCase has no effent at all as you do not save the result in a variable (hex itself is immutable, and you call that method repeatedly which is unnecessary). You could combine it with toCharArray like

public static boolean isHex(String hex){
    for (char c : hex.toLowerCase().toCharArray())
        if((c < '0' || c > '9') && (c < 'a' || c > 'f'))
            return false;
    return true;
}