How do I say "return true if the string is a valid hexadecimal string, false otherwise" in java

/***************************************************

  Name: 
  Date: 
  Homework #7
  
  Program name:        HexConversion
  Program description: Accepts hexadecimal numbers as input and converts each number to binary
                       and to the decimal equivalent.
                       Valid input examples: F00D, 000a, 1010, FFFF, Goodbye, GOODBYE
                       Enter GOODBYE (case insensitive) to exit the program.
  
****************************************************/

package hw8;

import java.util.Scanner;

public class HexConversion {

  public static void main(String[] args) {
    	
    // Maximum length of input string
    final byte INPUT_LENGTH = 4;
    	
    String userInput = "";                   // Initialize to null string
    Scanner input = new Scanner(System.in);

    // Process the inputs until GOODBYE is entered
    do {
      // Input a 4 digit hex number
      System.out.print("\nEnter a hexadecimal string, or enter GOODBYE to quit:  ");
      userInput = input.next().toUpperCase();
		  
      // Process the input
      switch (userInput) {
		  
        case "GOODBYE": break;
            
        default:        if (isValidHex(userInput, INPUT_LENGTH)) {
                          // The input is a valid hexadecimal string
				
                          // Convert the hexadecimal string to binary and print the binary number as a string
                          String binVal = hex2Bin(userInput, INPUT_LENGTH);
		                      
                          // Convert the hexadecimal string to decimal and print the decimal number
                          long decVal = hex2Dec(userInput, INPUT_LENGTH);
                          System.out.printf("      0x%s = %s in binary = %d in decimal (unsigned).\n", userInput, binVal, decVal);
                        }
			
                        else {
                          // String is either the wrong length or is not a valid hexadecimal string
                          System.out.printf("      The string %s is not a valid input.\n", userInput);
                        }
                        break;
        }
    } while (!userInput.equals("GOODBYE"));
		
    // Exit the program
    System.out.println("\nGoodbye!");
    input.close();
  }
 
  // Method to validate the input
  public static boolean isValidHex(String userIn, byte inputLen) {
    boolean isValid = false;
    	
    // If length of the input string is equal to inputLen, continue with validation,
    // otherwise return false
    if (userIn.TBD == inputLen) {
        
      // The length is correct, now check that the characters are legal hexadecimal digits
      for (int i = 0; i < TBD; i++) {
        char thisChar = userIn.TBD;
          
        // Is the character a decimal digit (0..9)? If so, advance to the next character
        if (Character.isDigit(thisChar)) {
          isValid = true;
        }
	        
        else {	
          // Character is not a decimal digit (0..9), is it a valid hexadecimal digit (A..F)?
          TBD
        }
      }
    }
        
    // Return true if the string is a valid hexadecimal string, false otherwise
    TBD
  }

    
  // Method to convert the hex number to a binary string
  public static String hex2Bin(String hexString, byte inputLen) {
    String binString = "";     // Initialize binString to null string
    	
    // Convert each hexadecimal digit to its binary equivalent
    for (int i = 0; i < TBD; i++) {
      char thisChar = hexString.TBD;
	        
      // Convert hexString to a binary string, e.g. F00D = 1111000000001101
      TBD
    }
  }
    
  // Method to convert the hex number to decimal number
  public static long hex2Dec(String hexString, byte inputLen) {
    	
    // Convert hexadecimal string to decimal, e.g. F00D = 61453 in unsigned decimal
    TBD
  }
}