Here's my code that I've written :

```
public String binary(String s)
{
    String[] a = {
      "0000","0001","0010","0011","0100","0101","0110","0111",
      "1000","1001","1010","1011","1100","1101","1110","1111"
    };

    String k = "";
    for(int i = 0; i <= s.length() - 1; i++)
    {
        if      (s.charAt(i) == 'a') { k += a[10]; }
        else if (s.charAt(i) == 'b') { k += a[11]; }
        else if (s.charAt(i) == 'c') { k += a[12]; }
        else if (s.charAt(i) == 'd') { k += a[13]; }
        else if (s.charAt(i) == 'e') { k += a[14]; }
        else if (s.charAt(i) == 'f') { k += a[15]; }
        else { k += a[i]; }
    }
    return k;
}

```

I am getting output as a[0-9] = 0000. How can I fix this? What am I doing wrong?