結論:

`StringBuffer、StringBuilder預設初始化是16個字元，預設增容為（原長度+1）*2，程式碼如下：



//預設初始化大小

public StringBuilder() {

super(16);

}

// 預設擴容

int newCapacity = (value.length + 1) * 2;

ArrayList、Vector、HashMap、HashTable是如何擴容的



1. ArrayList，預設初始10個大小，每次擴容是原容量的一半，具體程式碼如下:

public ArrayList() {

this(10);

} 

int newCapacity = (oldCapacity * 3)/2 + 1;



public static native void arraycopy(Object src, int  srcPos, Object dest, int destPos, int length);

2. Vector，預設初始10個大小，每次擴容是原容量的兩倍，具體程式碼如下:

public Vector() {

this(10);

} 

int newCapacity = (capacityIncrement > 0) ? (oldCapacity + capacityIncrement) : (oldCapacity * 2);

3. HashMap預設初始16個大小（必須是2的次方），預設每次擴容的因子為0.75，具體程式碼如下:

static final int DEFAULT_INITIAL_CAPACITY = 16; 

static final float DEFAULT_LOAD_FACTOR = 0.75f;

public HashMap() {

  this.loadFactor = DEFAULT_LOAD_FACTOR;

  threshold = (int)(DEFAULT_INITIAL_CAPACITY * DEFAULT_LOAD_FACTOR);

  table = new Entry[DEFAULT_INITIAL_CAPACITY];

  init();

}



resize(2 * table.length);

4. HashTable預設初始11個大小，預設每次擴容的因子為0.75，具體程式碼如下:

public Hashtable() {

this(11, 0.75f);

}

int newCapacity = oldCapacity * 2 + 1;`