for converting hex to binary, to check the digits and letters which one is better switch or regular exp as regular exp not cover

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:

1. We check the length of the string received in the input parameter. It must be a multiple of 4.
2. If this is not the case, then add the required number of 0s to the beginning of the string.
3. 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.