Converting binary string to a hexadecimal string JAVA

Use this for any binary string length:

String hexString = new BigInteger(binaryString, 2).toString(16);

If you don't have to implement that conversion yourself, you can use existing code :

int decimal = Integer.parseInt(binaryStr,2);
String hexStr = Integer.toString(decimal,16);

If you must implement it yourself, there are several problems in your code :

1. The loop should iterate from 0 to binary.length()-1 (assuming the first character of the String represents the most significant bit).
2. You implicitly assume that your binary String has 4*x charcters for some integer x. If that's not true, your algorithm breaks. You should left pad your String with zeroes to get a String of such length.
3. sum must be reset to 0 after each hex digit you output.
4. System.out.print(digitNumber); - here you should print sum, not digitNumber.

Here's how the mostly fixed code looks :

    int digitNumber = 1;
    int sum = 0;
    String binary = "011110101010";
    for(int i = 0; i < binary.length(); i++){
        if(digitNumber == 1)
            sum+=Integer.parseInt(binary.charAt(i) + "")*8;
        else if(digitNumber == 2)
            sum+=Integer.parseInt(binary.charAt(i) + "")*4;
        else if(digitNumber == 3)
            sum+=Integer.parseInt(binary.charAt(i) + "")*2;
        else if(digitNumber == 4 || i < binary.length()+1){
            sum+=Integer.parseInt(binary.charAt(i) + "")*1;
            digitNumber = 0;
            if(sum < 10)
                System.out.print(sum);
            else if(sum == 10)
                System.out.print("A");
            else if(sum == 11)
                System.out.print("B");
            else if(sum == 12)
                System.out.print("C");
            else if(sum == 13)
                System.out.print("D");
            else if(sum == 14)
                System.out.print("E");
            else if(sum == 15)
                System.out.print("F");
            sum=0;
        }
        digitNumber++;  
    }

Output :

7AA

This will work only if the number of binary digits is divisable by 4, so you must add left 0 padding as a preliminray step.

You can try something like this.

private void bitsToHexConversion(String bitStream){

    int byteLength = 4;
    int bitStartPos = 0, bitPos = 0;
    String hexString = "";
    int sum = 0;

    // pad '0' to make input bit stream multiple of 4 

    if(bitStream.length()%4 !=0){
         int tempCnt = 0;
         int tempBit = bitStream.length() % 4;           
         while(tempCnt < (byteLength - tempBit)){
             bitStream = "0" + bitStream;
             tempCnt++;
         }
    }

   // Group 4 bits, and find Hex equivalent 

    while(bitStartPos < bitStream.length()){
        while(bitPos < byteLength){
            sum = (int) (sum + Integer.parseInt("" + bitStream.charAt(bitStream.length()- bitStartPos -1)) * Math.pow(2, bitPos)) ;
            bitPos++;
            bitStartPos++;
        }
        if(sum < 10)
             hexString = Integer.toString(sum) + hexString;
        else 
             hexString = (char) (sum + 55) + hexString;

        bitPos = 0;
        sum = 0;
    }
    System.out.println("Hex String > "+ hexString);
}

Hope this helps :D