src/main/java/com/googlecode/javaewah/datastructure/ImmutableBitSet.java
public int cardinality() {
      int sum = 0;
      int length = this.data.limit();
      for(int k = 0; k < length; ++k)
          sum += Long.bitCount(this.data.get(k));
      return sum;
  }
public int cardinality() {
      int sum = 0;
      int length = this.data.limit();
      for(int k = 0; k < length; ++k)
          sum += Long.bitCount(this.data.get(k));
      return sum;
  }
Compute the number of bits set to 1

@return the number of bits