List<String> list = new ArrayList<>();

?? ?/**
? ? ?* The maximum size of array to allocate.
? ? ?* Some VMs reserve some header words in an array.
? ? ?* Attempts to allocate larger arrays may result in
? ? ?* OutOfMemoryError: Requested array size exceeds VM limit
? ? ?*/
? ? private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
? ??
?? ?/**
? ? ?* Default initial capacity.
? ? ?*/
? ? private static final int DEFAULT_CAPACITY = 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

?? ?/**
? ? ?* Shared empty array instance used for empty instances.
? ? ?*/
? ? private static final Object[] EMPTY_ELEMENTDATA = {};?? ?

?? ?/**
? ? ?* Constructs an empty list with the specified initial capacity.
? ? ?*
? ? ?* @param ?initialCapacity ?the initial capacity of the list
? ? ?* @throws IllegalArgumentException if the specified initial capacity
? ? ?* ? ? ? ? is negative
? ? ?*/
? ? 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);
? ? ? ? }
? ? }

?? ?/**
? ? ?* Increases the capacity to ensure that it can hold at least the
? ? ?* number of elements specified by the minimum capacity argument.
? ? ?*
? ? ?* @param minCapacity the desired minimum capacity
? ? ?*/
? ? 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);
? ? }

public static void main(String[] args) {
? ? final int count = 200 * 10000;
? ? List<Integer> list = new ArrayList<>();
? ? long begin = System.currentTimeMillis();
? ? for(int i = 0; i < count ; i++) {
? ? ? ? list.add(i);
? ? }
? ? System.out.println("沒(méi)有設(shè)置ArrayList初始容量： " + (System.currentTimeMillis() - begin) + " ms");

? ? List<Integer> list2 = new ArrayList<>(10);
? ? long begin2 = System.currentTimeMillis();
? ? for(int i = 0; i < count ; i++) {
? ? ? ? list2.add(i);
? ? }
? ? System.out.println("設(shè)置了ArrayList初始容量： " + (System.currentTimeMillis() - begin2) + " ms");
}

/**
?* Appends the specified element to the end of this list.
?*
?* @param e element to be appended to this list
?* @return <tt>true</tt> (as specified by {@link Collection#add})
?*/
public boolean add(E e) {
? ? ensureCapacityInternal(size + 1); ?// Increments modCount!!
? ? elementData[size++] = e;
? ? return true;
}

private void ensureCapacityInternal(int minCapacity) {
? ? ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
}

private static int calculateCapacity(Object[] elementData, int minCapacity) {
? ? if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {// 第一次add的時(shí)候，都會(huì)走這一步
? ? ? ? return Math.max(DEFAULT_CAPACITY, minCapacity);//初始化容量小于默認(rèn)值10都會(huì)取10，反之取自定義的容量
? ? }
? ? return minCapacity;
}

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

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

/**
?* Increases the capacity to ensure that it can hold at least the
?* number of elements specified by the minimum capacity argument.
?*
?* @param minCapacity the desired minimum capacity
?*/
?private void grow(int minCapacity) {//minCapacity是當(dāng)前容量，比如，默認(rèn)容量下，add一次后就是10+1
? ? // 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);
}

public static void main(String[] args) {
? ? List<Integer> list = new ArrayList<>(10);
? ? list.set(0, 666);
}