is perfect square

IsPerfectSquare.java

public class IsPerfectSquare {
  // Taken from http://stackoverflow.com/a/18686659/180719
  private static long goodMask = computeGoodMask(); // 0xC840C04048404040 computed below
  private static long computeGoodMask() {
    long goodMask = 0L;
    for (int i = 0; i < 64; i++) {
      goodMask |= Long.MIN_VALUE >>> (i * i);
    }
    return goodMask;
  }
  
  public static boolean isPerfectSquare(long x) {
      // This tests if the 6 least significant bits are right.
      // Moving the to be tested bit to the highest position saves us masking.
      if (goodMask << x >= 0) {
        return false;
      }
      final int numberOfTrailingZeros = Long.numberOfTrailingZeros(x);
      // Each square ends with an even number of zeros.
      if ((numberOfTrailingZeros & 1) != 0) {
        return false;
      }
      x >>= numberOfTrailingZeros;
      // Now x is either 0 or odd.
      // In binary each odd square ends with 001.
      // Postpone the sign test until now; handle zero in the branch.
      if ((x & 7) != 1 || x <= 0) {
        return x == 0;
      }
      // Do it in the classical way.
      // The correctness is not trivial as the conversion from long to double is lossy!
      final long tst = (long) Math.sqrt(x);
      return tst * tst == x;
  }
}