Vector源码疑惑

Question

1.在看Vector源码是有2个小知识点比较疑惑
2.疑惑代码如下
/**

• 
  

• This implements the unsynchronized semantics of ensureCapacity.

• Synchronized methods in this class can internally call this

• method for ensuring capacity without incurring the cost of an

• extra synchronization.
  *

• @see #ensureCapacity(int)
  */

1. void ensureCapacityHelper(int minCapacity) {

      // overflow-conscious code
      if (minCapacity - elementData.length > 0)
          grow(minCapacity);

   }

   /**

• The maximum size of array to allocate.

• Some VMs reserve some header words in an array.

• Attempts to allocate larger arrays may result in

• OutOfMemoryError: Requested array size exceeds VM limit
  */

2. static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
   3.疑惑问题点如下：

问题点1：if (minCapacity - elementData.length > 0)，为啥不这样写
if(minCapacity>elementData.length),效率高在哪？

问题点2：MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;为啥是减8而不是其他，注释没看明白是啥意思