ArrayList

1.动态数组

2.线程不安全

3.存储空间连续

4.查询快，添加删除慢

• 构造方法

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

这个构造方法很简单，初始化了一个空的elementData,并没有赋予数组长度

• 元素添加

 /**  + 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  /**  + The size of the ArrayList (the number of elements it contains).  *  + @serial  */ private int size;  /**  + 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) {     // 首先进行扩充     ensureCapacityInternal(size + 1);  // Increments modCount!!      // 将元素追加到最后     elementData[size++] = e;      return true; }  // 扩充 private void ensureCapacityInternal(int minCapacity) {     ensureExplicitCapacity(calculateCapacity(elementData, minCapacity)); }  // 计算数组大小 第一次调用此处的elementData={}，所以返回值为DEFAULT_CAPACITY=10，也就是默认的数组长度是10 private static int calculateCapacity(Object[] elementData, int minCapacity) {     if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {         return Math.max(DEFAULT_CAPACITY, minCapacity);     }     return minCapacity; }  private void ensureExplicitCapacity(int minCapacity) {     modCount++;      // overflow-conscious code     if (minCapacity - elementData.length > 0) // 当加上当前元素后的集合长度（size）大于现在数组长度（elementData.length）在进行扩充         grow(minCapacity); }  // 真正的扩充操作 private void grow(int minCapacity) {     // overflow-conscious code     int oldCapacity = elementData.length; // 此处oldCapacity=0     int newCapacity = oldCapacity + (oldCapacity >> 1); // 此处newCapacity=0      if (newCapacity - minCapacity < 0) // 此处minCapacity=10         newCapacity = minCapacity; // 此处newCapacity=10     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); //数组拷贝 } 

真正的数组长度是在第一次添加的时候进行初始化的,默认为10
最主要的消耗是在扩容(数组拷贝)
当集合长度大于数组长度的时候进行扩充，扩充的标准是1.5倍[oldCapacity + (oldCapacity >> 1)]

• 查询

public E get(int index) {     rangeCheck(index);// 校验      return elementData(index); }  E elementData(int index) {     return (E) elementData[index]; }

Vector

1.动态数组，类似于ArrayList

2.线程安全

3.消耗大

• 构造方法

public Vector
                        
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