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為傳入的需要存儲的元素，elementData 是ArrayList中存放元素的數組緩存區，當ArrayList初始化時長度為0，當存放第一個元素時，長度為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為ArrayList的長度，表示當前集合中的元素數量

  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（）方法中邏輯：如果向集合中添加元素時elementData為長度為零的數組，則設置為初始容量為DEFAULT_CAPACITY=10

    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;

        // overflow-conscious code
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }

modCount：修改次數，判斷使用最小容量減去當前數組的長度是否大於零，如果大於零則表示需要擴容，反之當前容量不需要擴容

  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()為擴容方法，傳入當前容量值，獲取當前數組緩存區的長度，根據當前緩沖區長度進行計算擴容，擴容的數量是當前緩存區長度的1.5倍。最后使用Arrays.copyOf方法進行數組擴容