Dive into Java Source

gistfile1.java

/* Bit Wtiddling Hacks http://graphics.stanford.edu/~seander/bithacks.html#RoundUpPowerOf2
java.util.ArrayDeque - find the nearest power of two */

private void allocateElements(int numElements) {
  int initialCapacity = MIN_INITIAL_CAPACITY;
  // Find the best power of two to hold elements.
  // Tests "<=" because arrays aren't kept full.
  if (numElements >= initialCapacity) {
    initialCapacity = numElements;
    initialCapacity |= (initialCapacity >>>  1);
    initialCapacity |= (initialCapacity >>>  2);
    initialCapacity |= (initialCapacity >>>  4);
    initialCapacity |= (initialCapacity >>>  8);
    initialCapacity |= (initialCapacity >>> 16);
    initialCapacity++;

    if (initialCapacity < 0)   // Too many elements, must back off
      initialCapacity >>>= 1;// Good luck allocating 2 ^ 30 elements
  }
  elements = new Object[initialCapacity];
}