List扩容

ArrayList 扩容机制

结论：无参构造创建的ArrayList的初始空间为0，在添加第一个元素的时候空间会默认为10，之后扩容会为当前容量的1.5倍。

0->10->15->22->33->49

演示代码

public static void main(String[] args) {ArrayList list = new ArrayList();for (int i = 0; i <= 9; i++) {list.add(i);}list.add(10);list.add(11);list.add(12);
}

第一次调试截图：此时ArrayList内未添加任何元素，默认为0；

第二次调试截图：当向ArrayList中添加第一个元素的时候，空间变为10。

10条插入完后，再插入就会变成15

这是第二次扩容为，扩容为15

再继续插入16个，扩容为22

第三次扩容22

源码分析

1.ArrayList list = new ArrayList();

会调用无参构造器，新建一个ArrayList。将elementData设置为DEFAULTCAPACITY_EMPTY_ELEMENTDATA

   /*** Constructs an empty list with an initial capacity of ten.*/public ArrayList() {this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;}

通过源码，我们知道DEFAULTCAPACITY_EMPTY_ELEMENTDATA是一个空的数组。

  /*** Shared empty array instance used for default sized empty instances. We* distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when* first element is added.*/private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

2. list.add(1);

将1进行Integer包装

   public static Integer valueOf(int i) {if (i >= IntegerCache.low && i <= IntegerCache.high)return IntegerCache.cache[i + (-IntegerCache.low)];return new Integer(i);}

通过add类进行元素添加

/*** 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;}

通过ensureCapacityInternal方法来确保容量足够，此处的Math.max(DEFAULT_CAPACITY, minCapacity);就是确保第一次扩容空间为10

   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;}

ensureCapacityInternal方法会调用ensureExplicitCapacity方法来确保空间，modCount用来多线程的判断，此处不赘述

   private void ensureExplicitCapacity(int minCapacity) {modCount++;// overflow-conscious codeif (minCapacity - elementData.length > 0)grow(minCapacity);}

ensureExplicitCapacity方法会调用grow方法，扩容1.5倍，就是在此发生的。 int newCapacity = oldCapacity + (oldCapacity >> 1);

/*** 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 codeint 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);}

Vector 扩容机制

结论：无参构造默认空间为10，每次扩大一倍。

如下代码

public class ArrayExercise {public static void main(String[] args) {Vector vector = new Vector();for (int i = 0; i < 10; i++) {vector.add(i);}vector.add(10);vector.add(11);vector.add(12);}
}

调试过程略。

源码分析

初始化会默认调用无参，再调用有参数构造器，空间默认10。

   /*** Constructs an empty vector so that its internal data array* has size {@code 10} and its standard capacity increment is* zero.*/public Vector() {this(10);}

扩容的主要操作在grow中的int newCapacity = oldCapacity + ((capacityIncrement > 0) ? capacityIncrement : oldCapacity);方法，capacityIncrement默认是0，所以等价newCapacity=oldCapacity+oldCapacity；

   /*** Appends the specified element to the end of this Vector.** @param e element to be appended to this Vector* @return {@code true} (as specified by {@link Collection#add})* @since 1.2*/public synchronized boolean add(E e) {modCount++;ensureCapacityHelper(elementCount + 1);elementData[elementCount++] = e;return true;}private void ensureCapacityHelper(int minCapacity) {// overflow-conscious codeif (minCapacity - elementData.length > 0)grow(minCapacity);}private void grow(int minCapacity) {// overflow-conscious codeint oldCapacity = elementData.length;int newCapacity = oldCapacity + ((capacityIncrement > 0) ?capacityIncrement : oldCapacity);if (newCapacity - minCapacity < 0)newCapacity = minCapacity;if (newCapacity - MAX_ARRAY_SIZE > 0)newCapacity = hugeCapacity(minCapacity);elementData = Arrays.copyOf(elementData, newCapacity);}

LinkedList 扩容机制

结论：LinkedList是一个双向链表,没有初始化大小,也没有扩容的机制,就是一直在前面或者后面新增就好。

其他问题

扩容后的size是多少？还是实际元素大小

调试下我们可以发现，虽然elementData进行了扩容，但是实际大小还是元素的个数，别被唬住了

给了初始值，它会给我自动扩充为10、15、22…吗？不会

别激动，给多少就是多少