code: void checkIndex(int index, int fieldLength) {
  		if (isOutOfBounds(index, fieldLength, capacity())) {
  			throw new IndexOutOfBoundsException(
  					String.format("index: %d, length: %d (expected: range(0, %d))", index, fieldLength, capacity()));
  		}
  	}
docs: 检查当前数组某区块是否超出范围
  
  @param index
  @param fieldLength