ArrayList扩容

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

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

JDK1.7+

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

 

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。