基于JDK1.8
只列出關(guān)鍵方法，主要關(guān)注默認情況、初始化、擴容、add、remove。

private static final int DEFAULT_CAPACITY = 10;
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
public ArrayList() { //默認是個空數(shù)組，也有重載方法可以設置初始數(shù)組大小
    this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
public boolean add(E e) {
    ensureCapacityInternal(size + 1); //關(guān)注這個方法
    elementData[size++] = e;
    return true;
}
private void ensureCapacityInternal(int minCapacity) {
    if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
        minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity); //默認就是10
    }
    ensureExplicitCapacity(minCapacity);
}
private void ensureExplicitCapacity(int minCapacity) {
    modCount++;
    if (minCapacity - elementData.length > 0) //超過數(shù)據(jù)長度
        grow(minCapacity);
}
private void grow(int minCapacity) {
    int oldCapacity = elementData.length;
    int newCapacity = oldCapacity + (oldCapacity >> 1); //按1.5倍擴容
    if (newCapacity - minCapacity < 0)
        newCapacity = minCapacity;
    if (newCapacity - MAX_ARRAY_SIZE > 0)
        newCapacity = hugeCapacity(minCapacity);
    elementData = Arrays.copyOf(elementData, newCapacity);
}

get(int index)比較簡單，直接返回elementData[index]。
ArrayList比較簡單，默認情況下是空數(shù)據(jù)，第一次add默認初始化長度10，每次按1.5倍擴容。再看下remove

public E remove(int index) {
    rangeCheck(index);
    modCount++;
    E oldValue = elementData(index);
    int numMoved = size - index - 1; //判斷是不是最后一項，如果不是最后一項，就把該index后面的數(shù)據(jù)項都賦值給前一位，然后再把最后一項設為null
    if (numMoved > 0)
        System.arraycopy(elementData, index+1, elementData, index,
                         numMoved);
    elementData[--size] = null; //這行代碼值得很多人學習，數(shù)組集合里沒用的項要及時釋放
    return oldValue;
}

附圖