java之ArrayList.add
ArrayList添加
public boolean add(E e) { ensureCapacityInternal(size + 1); // Increments modCount!! elementData[size++] = e; return true; }
elementData[size++] = e :e为传入的需要存储的元素,elementData 是ArrayList中存放元素的数组缓存区,当ArrayList初始化时长度为0,当存放第一个元素时,长度为10
/** * The array buffer into which the elements of the ArrayList are stored. * The capacity of the ArrayList is the length of this array buffer. Any * empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA * will be expanded to DEFAULT_CAPACITY when the first element is added. */ transient Object[] elementData; // non-private to simplify nested class access
ensureCapacityInternal(size + 1) :size为ArrayList的长度,表示当前集合中的元素数量
private void ensureCapacityInternal(int minCapacity) { ensureExplicitCapacity(calculateCapacity(elementData, minCapacity)); }
private static int calculateCapacity(Object[] elementData, int minCapacity) { if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) { return Math.max(DEFAULT_CAPACITY, minCapacity); } return minCapacity; }
minCapacity:表示最小容量,calculateCapacity()方法中逻辑:如果向集合中添加元素时elementData为长度为零的数组,则设置为初始容量为DEFAULT_CAPACITY=10
private void ensureExplicitCapacity(int minCapacity) { modCount++; // overflow-conscious code if (minCapacity - elementData.length > 0) grow(minCapacity); }
modCount:修改次数,判断使用最小容量减去当前数组的长度是否大于零,如果大于零则表示需要扩容,反之当前容量不需要扩容
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); }
grow()为扩容方法,传入当前容量值,获取当前数组缓存区的长度,根据当前缓冲区长度进行计算扩容,扩容的数量是当前缓存区长度的1.5倍。最后使用Arrays.copyOf方法进行数组扩容