Converting from Base 10 to 16 (Decimal to Hex) in Java

The challenge

Convert an integer which is base 10, to a hexadecimal base 16 string.

Java provides various built-in options such as Integer.toString(a, 16), or String.format("0x%X", a), or Integer.toHexString(a)

The solution in Java code

Option 1:

public class Hexadecimal{
    public static String convertToHex(int a){
        return "0x"+Integer.toHexString(a).toUpperCase();
    }
}

Option 2:

public class Hexadecimal{

private static final String hexDigits = "0123456789ABCDEF";
    public static String convertToHex(int a) {
        String hexadecimal = "";

        while (a > 0) {
            int digit = a % 16;
            hexadecimal = hexDigits.charAt(digit) + hexadecimal;
            a = a / 16;
        }
        return "0x" + hexadecimal;
    }
}

Option 3:

public class Hexadecimal{

    private static final String HEX = "0123456789ABCDEF";
    
    public static String convertToHex(int a){
        final StringBuilder sb = new StringBuilder("");
        while (a != 0) {
          sb.append(HEX.charAt((a & 0xf)));
          a = a >> 4;
        }
        return "0x" + sb.reverse().toString();
    }

}

Option 4:

public class Hexadecimal {
    private static final char[] numbers = {'0' , '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B', 'C', 'D', 'E', 'F'};
    public static String convertToHex(int a) {
      if (a < 16)
        return "0x" + numbers[a];
      return convertToHex(a/16) + numbers[a%16];
    }
}

Test cases to validate our solution

import org.junit.Test;
import static org.junit.Assert.assertEquals;

public class HexadecimalShould {
    @Test
    public void test (){
        assertEquals("0x100",Hexadecimal.convertToHex(256));
    }
}

Additional test cases

import org.junit.Test;
import java.util.HashMap;
import static org.junit.Assert.assertEquals;

public class HexadecimalShould {


    @Test
    public void test (){
        assertEquals("0x100",Hexadecimal.convertToHex(256));
        
        int numbers[] = new int[1000];
        for (int i = 0; i < numbers.length; i++) {
            numbers[i] = (int)Math.floor(Math.random()*4294967295.);
            assertEquals((new Hex()).convertToHex(numbers[i]),Hexadecimal.convertToHex(numbers[i]));
        }

    }
    
    private class Hex {
    public String convertToHex(int a) {
        String num = "";
        HashMap<String, String> mapeo = new HashMap<>();
        for (int i = 0; i < 10; i++) mapeo.put(i + "", i + "");
        mapeo.put("10", "A");
        mapeo.put("11", "B");
        mapeo.put("12", "C");
        mapeo.put("13", "D");
        mapeo.put("14", "E");
        mapeo.put("15", "F");

        while (a >= 16) {
            num += mapeo.get(a % 16 + "") + "";
            a /= 16;
        }
        num += mapeo.get(a + "");
        
        return "0x"+(new StringBuilder(num)).reverse().toString();

    }

}
}