ArrayList、Vector、HashMap、HashTable是如何扩容

a) ArrayList，默认初始10个大小，每次扩容是原容量的一半，具体代码如下

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);
 

  
1
  
2
  
3
  
4
  
5
  
6
  
7
  
8
 

b) Vector，默认初始10个大小，每次扩容是原容量的一倍，具体代码如下：

public Vector() {
this(10);
} 
int newCapacity = (capacityIncrement > 0) ? (oldCapacity + capacityIncrement) : (oldCapacity * 2);
 

  
1
  
2
  
3
  
4
 

c) HashMap默认初始16个大小（必须是2的次方），默认每次扩容的因子为0.75，具体代码如下：

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);
 

  
1
  
2
  
3
  
4
  
5
  
6
  
7
  
8
  
9
  
10
 

d) HashTable默认初始11个大小，默认每次扩容的因子为0.75

public Hashtable() {
this(11, 0.75f);
}
int newCapacity = oldCapacity * 2 + 1;