private static final int DEFAULT_CAPACITY = 10;//數(shù)組默認初始容量

private static final Object[] EMPTY_ELEMENTDATA = {};//定義一個空的數(shù)組實例以供其他需要用到空數(shù)組的地方調用 

private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};//定義一個空數(shù)組，跟前面的區(qū)別就是這個空數(shù)組是用來判斷ArrayList第一添加數(shù)據的時候要擴容多少。默認的構造器情況下返回這個空數(shù)組 

transient Object[] elementData;//數(shù)據存的地方它的容量就是這個數(shù)組的長度，同時只要是使用默認構造器（DEFAULTCAPACITY_EMPTY_ELEMENTDATA ）第一次添加數(shù)據的時候容量擴容為DEFAULT_CAPACITY = 10 

private int size;//當前數(shù)組的長度

private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
public ArrayList() {
    this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}

public ArrayList(int initialCapacity) {
    if (initialCapacity > 0) {
        this.elementData = new Object[initialCapacity];
    } else if (initialCapacity == 0) {
        this.elementData = EMPTY_ELEMENTDATA;
    } else {
        throw new IllegalArgumentException("Illegal Capacity: "+
                                           initialCapacity);
    }
}

//1.add方法為添加元素在數(shù)組末尾
public boolean add(E e) {
    //確保數(shù)組容量 size指向數(shù)組的末尾
    ensureCapacityInternal(size + 1);
    //在完成添加之前要確保數(shù)組長度足夠
    elementData[size++] = e;
    return true;
}
//3.elementData為ArrayList底層維護的數(shù)組，minCapacity為此時數(shù)組的大小
private static int calculateCapacity(Object[] elementData, int minCapacity) {
    //如果數(shù)組為初始化的值，就初始化數(shù)組容量為10(空參的構造方法下首次添加)
    if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
        return Math.max(DEFAULT_CAPACITY, minCapacity);
    }
    return minCapacity;
}
//2.minCapacity表示此時數(shù)組的大小
private void ensureCapacityInternal(int minCapacity) {
    ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
}
//4.minCapacity表示此時數(shù)組的大小
private void ensureExplicitCapacity(int minCapacity) {
    modCount++;
    //如果此時數(shù)組容量的大小不夠就擴容
    if (minCapacity - elementData.length > 0)
        grow(minCapacity);
}

//擴容源碼
private void grow(int minCapacity) {
    //獲取當前數(shù)組的長度
    int oldCapacity = elementData.length;
    //>>右移相當于整除2，新容量相當于就舊容量的1.5倍
    int newCapacity = oldCapacity + (oldCapacity >> 1);
    //如果擴容后的容量還不夠那么就以需要的容量為新容量
    if (newCapacity - minCapacity < 0)
        newCapacity = minCapacity;
    //如果新容量已經超過最大容量了，那么就直接使用最大容量
    if (newCapacity - MAX_ARRAY_SIZE > 0)
        newCapacity = hugeCapacity(minCapacity);
    //講新容量的數(shù)組拷貝
    elementData = Arrays.copyOf(elementData, newCapacity);
}

ArrayList<Integer> arrayList = new ArrayList<Integer>(20);
for(int i=1;i<=50;i++) {
     arrayList.add(i);
}