Java Program to Convert Hex String to Byte Array - GeeksforGeeks

24 Sep, 2021
Hex String – A Hex String is a combination of the digits 0-9 and characters A-F, just like how a binary string comprises only 0’s and 1’s. Eg: “245FC” is a hexadecimal string.
Byte Array – A Java Byte Array is an array used to store byte data types only. The default value of each element of the byte array is 0.
Given a byte array, the task is to convert the Hex String to Byte Array. 
Example: 
Input - Hex String : "2f4a33"
Output - Byte Array : 47 74 51

Input - Hex String : "3b4a11"
Output - Byte Array : 59 74 17
There are numerous approaches for converting a Hex String to Byte Array, and a few of them are listed below.
Approaches:
Using parseInt() method of Integer class in Java
Using Byte Array Representation of BigInteger in Java
Using Bitwise Shift Operators
The integer.parseInt() method in Java converts a given string to an integer. According to the given problem statement, since we have to convert a hex string to a byte array, we will first convert the characters of the hex string in pairs to the specific byte formation and insert that number into the byte array. In this, the byte array is initialized with a size of half the length of the hex string. 
Following is the implementation of the foregoing approach –
Java
// Java Program to convert hex// string to byte array // Approach 1 - Using parseInt() method of// Integer class in Java import java.io.*; public class GFG {   public static void main(String[] args)   {           // Initializing the hex string and byte array     String s = "2f4a33";     byte[] ans = new byte[s.length() / 2];            System.out.println("Hex String : "+s);      for (int i = 0; i < ans.length; i++) {         int index = i * 2;                   // Using parseInt() method of Integer class         int val = Integer.parseInt(s.substring(index, index + 2), 16);         ans[i] = (byte)val;     }              // Printing the required Byte Array     System.out.print("Byte Array : ");     for (int i = 0; i < ans.length; i++) {         System.out.print(ans[i] + " ");     }   }}
Hex String : 2f4a33
Byte Array : 47 74 51 
In this approach, we will use the toByteArray() method of BigInteger class. After converting the hexadecimal number to an integer value using the parseInt() method, we are left to convert integers to a byte array. Here comes the role of toByteArray() method of BigInteger class which will transform the integer values to a byte array and return it.
Following is the implementation of the foregoing approach –
Java
// Java Program to convert hex// string to byte array // Approach 1 - Using Byte Array Representation// of BigInteger in Java import java.io.*;// importing BigInteger classimport java.math.BigInteger; class GFG {    public static void main(String[] args)    {        String s = "3b4a11";                     // converting string to integer value        int val = Integer.parseInt(s, 16);        System.out.println("Hexadecimal String : " + s);                     // converting integer value to Byte Array        BigInteger big = BigInteger.valueOf(val);        byte[] ans = (big.toByteArray());                 // printing the byte array        System.out.print("Byte Array : ");        for (int i = 0; i < ans.length; i++)            System.out.print(ans[i] + " ");    }}
Hexadecimal String : 3b4a11
Byte Array : 59 74 17 
Another way to convert a hex string to a byte array is to use the Binary shift operators of Java. Here “<<” bitwise left shift operator is used. In order to get the numeric value of the character in hexadecimal order, the Character.digit() method in Java is used.
Following is the implementation of the foregoing approach –
Java
// Java program to convert hex// string to byte array // Approach 3 - Using Bitwise Shift Operators import java.io.*; class GFG {    public static void main (String[] args) {                    // initializing hex string and byte array        String s ="1f3d44";         System.out.println("Hex String : " + s);                 int len = s.length();        byte[] ans = new byte[len / 2];               for (int i = 0; i < len; i += 2) {             // using left shift operator on every character            ans[i / 2] = (byte) ((Character.digit(s.charAt(i), 16) << 4)                    + Character.digit(s.charAt(i+1), 16));        }                     // printing the required result          System.out.print("Byte Array : ");                     for(int i=0;i<ans.length;i++){             System.out.print(ans[i]+" ");        }    }}
Hex String : 1f3d44
Byte Array : 31 61 68 
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