在使用ArrayList 其核心的实现的原理就是基于数组的扩展,其中主要的方法是add(E element)和 get(int index)
这2个方法,看源码
1 add(E element)
public boolean add(E e) {
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}
接着看
private void ensureCapacityInternal(int minCapacity) {
if (elementData == EMPTY_ELEMENTDATA) {
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
}
ensureExplicitCapacity(minCapacity);
}
接着看
private void ensureExplicitCapacity(int minCapacity) {
modCount++;
// overflow-conscious code
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}
看到这个地方,当数组的容量不够的时候,我们会调用
grow(minCapacity);
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);
}
我们看到最终 数组的扩容是调的
Arrays.copyOf(elementData, newCapacity)
初始的数组的大小是10
2 get(int index)
public E get(int index) {
if (index >= size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
return (E) elementData[index];
}