ArrayList扩容

Posted 2020-09-18 小明在努力

ArrayList是Array的扩展版,具有自动增长的特性,可以理解为动态数组.

其内部的实现关键是:用以存储任何对象的Object[] elementData, 并在增加元素的时候,检测目前数组的大小是否足以容纳新元素,不足则扩容-复制-添加,其扩容算法如下:

JDK1.6

 1 // JDK6的扩容机制
 2 public void ensureCapacity(int minCapacity) {  
 3 modCount++;  
 4 int oldCapacity = elementData.length;  
 5 if (minCapacity > oldCapacity) {  
 6     Object oldData[] = elementData;  
 7     int newCapacity = (oldCapacity * 3)/2 + 1;  
 8         if (newCapacity < minCapacity)  
 9             newCapacity = minCapacity;  
10   // minCapacity is usually close to size, so this is a win:  
11   elementData = Arrays.copyOf(elementData, newCapacity);  
12 }  
13 }

JDK1.7+

 1 private void grow(int minCapacity) {
 2         // overflow-conscious code
 3         int oldCapacity = elementData.length;
 4         int newCapacity = oldCapacity + (oldCapacity >> 1);
 5         if (newCapacity - minCapacity < 0)
 6             newCapacity = minCapacity;
 7         if (newCapacity - MAX_ARRAY_SIZE > 0)
 8             newCapacity = hugeCapacity(minCapacity);
 9         // minCapacity is usually close to size, so this is a win:
10         elementData = Arrays.copyOf(elementData, newCapacity);
11     }

 

在查资料时,发现各个博客对于增长的速率书写于我查看源码的速率不对，便猜想可能跟JDK版本有关。

也说明，写软件的软件要以当前JDK的源码为准，所以说查源码还是很有用的。