Java List 扩容机制探究（ArrayList VectorLinkedList）

Posted 2022-11-03 流楚丶格念

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文章目录

List扩容
其他问题
- 扩容后的size是多少？还是实际元素大小
- 给了初始值，它会给我自动扩充为10、15、22......吗？不会

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 code
       if (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 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);

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