ArrayList原理扩容解析（jdk1.8）
1.先模拟数组扩容需要的条件
ArrayList2.当开始添加第11个元素时，会调用add(index,element)方法，开始进行下标判断，调用ensureCapacityInternal方法，参数值为11
public void add(int index, E element) {
rangeCheckForAdd(index);
ensureCapacityInternal(size + 1); // Increments modCount!!
System.arraycopy(elementData, index, elementData, index + 1,
size - index);
elementData[index] = element;
size++;
}3.判断用不用赋初值，调用ensureExplicitCapacity方法，参数值为11
private void ensureCapacityInternal(int minCapacity) {
if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
}
ensureExplicitCapacity(minCapacity);
}4.判断是否需要扩容，minCapacity=11， elementData.length=10，判断成立，所以进入grow方法
private void ensureExplicitCapacity(int minCapacity) {
modCount++;
// overflow-conscious code
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}5. minCapacity=11，oldCapacity =10，newCapacity =15,直接进行数组复制，elementData 数组中已有的元素, newCapacity=15
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);
}6.从下图得出copy=true，所以最后T[]copy为一个大小为15的数组
public static T[] copyOf(U[] original, int newLength, Class newType) {
@SuppressWarnings("unchecked")
T[] copy = ((Object)newType == (Object)Object[].class)
? (T[]) new Object[newLength]
: (T[]) Array.newInstance(newType.getComponentType(), newLength);
System.arraycopy(original, 0, copy, 0,
Math.min(original.length, newLength));
return copy;
} 7.进行数组的复制，original为原数组，copy为大小为15的空数组，Math.min(original.length, newLength)=10
那么意思就是，把原数组从下标0开始复制复制的大小是10.从copy中下标为0的位置开始
System.arraycopy(original, 0, copy, 0,Math.min(original.length, newLength));