Reverse Bits

Reverse bits of a given 32 bits unsigned integer.

For example, given input 43261596 (represented in binary as 00000010100101000001111010011100), return 964176192 (represented in binary as00111001011110000010100101000000).

Follow up:
If this function is called many times, how would you optimize it?

Related problem: Reverse Integer

My code:
 1 public class ReverseBits {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        ReverseBits test = new ReverseBits();
        
        System.out.println(test.reverseBits(43261597));
    }

    // Accepted
      public long reverseBits(int n) {
          
          String str = Integer.toBinaryString(n);
          StringBuilder sb = new StringBuilder();
          int len = str.length();
         
              for(int i = len -1; i >=0 ; i--)
              {
                  sb.append(str.charAt(i));
              }
             for(int i = len; i < 32; i++)
              {
                  sb.append("0");
              }
             
             long  base = 1;
             long re = 0;
          for(int i = 31 ; i >=0; i--)
          {
              if( sb.charAt(i) == '1')
              {
                  re += base;
              }
              base *=2;
          }
          return re;
          }
}