How can I convert a hexadecimal number into binary without using parse or automatic conversion

This code is based on some that came from here but that link no longer seems to be active. Anyway, from a hex string you can get an int like this:

int hexToDecimal(String s){
  int result = 0;
  int digit = 0;
  for (int i = 0; i < s.length(); i++) {
    char c = s.charAt(i);
    if (c >= '0' && c <= '9')
       digit = c - '0';
    else 
      if (c >= 'A' && c <= 'F')
        digit = 10 + c - 'A';
      else 
        inputError(s);
    result = 16 * result + digit;
  }
  return result
}

I modified your code a little.

a. In your code only the first hex was printed.

Change:

• call hexToBinary for every hex String.

b. the binary value was discarded after printing, so it couldn't be reused.

Change:

• Changed returntype of hexToBinary from void to String and returned the binary value calculated.

• To be able to return a String I add the peaces(nibbles) of the hex/binary to a String in every switch(case) clause.(a Stringbuilder might be better than a String - you can additionally improve that)

• in the main: additionally collect all the returned binary values in a arraylist called "binaryValues" in order to have them for the next step.

With the above (little) changes I now have all the binary values that had already been calculated.

So I am able to simply use them in a binaryToDecimal method which just sums up the binary values weighted by their position.

Why not do it again? Because youd need to convert the A-F to numbers what your hexToBinary already did. So storing the values saves you doing that step again. I have a feeling that is what your teacher had in mind when he/she combined the tasks like this.

The resulting code is:

import java.io.*;
import java.util.Scanner;
import java.util.ArrayList;

public class Main
{
     static String hexToBinary(char hexdec[]) {
            String hex = "";
            for (char c : hexdec) {
                switch (c) {
                    case '0':
                        System.out.print("0000");
                        hex += "0000";
                        break;
                    case '1':
                        System.out.print("0001");
                        hex += "0001";
                        break;
                    case '2':
                        System.out.print("0010");
                        hex += "0010";
                        break;
                    case '3':
                        System.out.print("0011");
                        hex += "0011";
                        break;
                    case '4':
                        System.out.print("0100");
                        hex += "0100";
                        break;
                    case '5':
                        System.out.print("0101");
                        hex += "0101";
                        break;
                    case '6':
                        System.out.print("0110");
                        hex += "0110";
                        break;
                    case '7':
                        System.out.print("0111");
                        hex += "0111";
                        break;
                    case '8':
                        System.out.print("1000");
                        hex += "1000";
                        break;
                    case '9':
                        System.out.print("1001");
                        hex += "1001";
                        break;
                    case 'A':
                        System.out.print("1010");
                        hex += "1110";
                        break;
                    case 'B':
                        System.out.print("1011");
                        hex += "1111";
                        break;
                    case 'C':
                        System.out.print("1100");
                        hex += "1100";
                        break;
                    case 'D':
                        System.out.print("1101");
                        hex += "1110";
                        break;
                    case 'E':
                        System.out.print("1110");
                        hex += "1110";
                        break;
                    case 'F':
                        hex += "1111";
                        System.out.print("1111");
                        break;
                    default:
                        System.out.print("\nInvalid hexadecimal digit " + hexdec[c]);
                }
            }
            System.out.println();
            return hex;
        }

        public static int binaryToDecimal(String binary) {
            int decimal = 0;
            for (int i = 1; i < binary.length()-1; i++) {
                    decimal += Math.pow(2, i-1) * (binary.charAt(binary.length()-i) - '0');
            }
            return decimal;
        }

        public static void main(String[] args) throws IOException {
            Scanner sc = new Scanner(new File("RAMerrors8x4c"));
            ArrayList<String> hexValues = new ArrayList<>();
            ArrayList<String> binaryValues = new ArrayList<>();
            while (sc.hasNext()) {
                hexValues.add(sc.nextLine());
            }
            for (String hex : hexValues) {
                String binary = hexToBinary(hex.toCharArray());
                binaryValues.add(binary);
                System.out.println(binary);
            }
            for (String binary : binaryValues) {
                int decimal = binaryToDecimal(binary);
                System.out.println(decimal);
            }
        }
    }
}

Besides using a Stringbuilder another idea could be to do all the printing of the binary values in the main. The hexToBinary returns the String - so you can print it in the loop - if you want.