code: public static int getCapacity(int capacity) {
          int c = 1;
          if(capacity >= MAX_POWER2) {
              c = MAX_POWER2;
          } else {
              while(c < capacity) c <<= 1;
          }
  
          if(isPowerOf2(c)) {
              return c;
          } else {
              throw new RuntimeException("Capacity is not a power of 2.");
          }
      }
docs: return the next power of two after @param capacity or
  capacity if it is already