Explain the use of a bit vector for determining if all characters are unique

Question

I am confused about how a bit vector would work to do this (not too familiar with bit vectors). Here is the code given. Could someone please walk me through this?

Answer 1

Line 1:   public static boolean isUniqueChars(String str) {
Line 2:      int checker = 0;
Line 3:      for (int i = 0; i < str.length(); ++i) {
Line 4:          int val = str.charAt(i) - 'a';
Line 5:          if ((checker & (1 << val)) > 0) return false;
Line 6:         checker |= (1 << val);
Line 7:      }
Line 8:      return true;
Line 9:   }

The way I understood using Javascript. Assuming input var inputChar = "abca"; //find if inputChar has all unique characters

Lets Start

Line 4: int val = str.charAt(i) - 'a';

Above line Finds Binary value of first character in inputChar which is a, a = 97 in ascii, then convert 97 to binary becomes 1100001.

In Javascript Eg: "a".charCodeAt().toString(2) returns 1100001

checker = 0 // binary 32 bit representation = 0000000000000000000000000

checker = 1100001 | checker; //checker becomes 1100001 (In 32 bit representation it becomes 000000000.....00001100001)

But i want my bitmask (int checker) to set only one bit, but checker is 1100001

Line 4:          int val = str.charAt(i) - 'a';

Now above code comes handy. I just subtract 97 always (ASCII val of a)

val = 0; // 97 - 97  Which is  a - a
val = 1; // 98 - 97 Which is b - a
val = 1;  // 99 - 97 Which is c - a

Lets use val which is resetted

Line 5 and Line 6 is well explained @Ivan answer