ArrayList和Vector异同点

ArrayList和Vector异同点

相同点：

• 底层都是数组

  //ArrayList
  transient Object[] elementData;
  
  //Vector
  protected Object[] elementData;
  

• 默认大小都是10

  //Vector
  public Vector() {
      this(10);
  }
  //ArrayList
  private static final int DEFAULT_CAPACITY = 10;
  public ArrayList(int initialCapacity) {
          if (initialCapacity > 0) {
              this.elementData = new Object[initialCapacity];
          } else if (initialCapacity == 0) {
              this.elementData = EMPTY_ELEMENTDATA;
          } else {
              throw new IllegalArgumentException("Illegal Capacity: "+
                                                 initialCapacity);
          }
  }
  

不同点

• 线程安全

  //ArrayList线程不安全，例如
  public boolean add(E e) {
          ensureCapacityInternal(size + 1);  // Increments modCount!!
          elementData[size++] = e;
          return true;
  }
  //Vector线程安全，例如，加了synchronized关键字
  public synchronized boolean add(E e) {
          modCount++;
          ensureCapacityHelper(elementCount + 1);
          elementData[elementCount++] = e;
          return true;
  }
  

• 扩容倍数

  //ArrayList  1.5倍
  private void grow(int minCapacity) {
          // overflow-conscious code
          int oldCapacity = elementData.length;
          int newCapacity = oldCapacity + (oldCapacity >> 1);//1.5倍扩容
          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 2倍扩容
  private void grow(int minCapacity) {
          // overflow-conscious code
          int oldCapacity = elementData.length;
          int newCapacity = oldCapacity + ((capacityIncrement > 0) ?
                                           capacityIncrement : oldCapacity);//2倍扩容
          if (newCapacity - minCapacity < 0)
              newCapacity = minCapacity;
          if (newCapacity - MAX_ARRAY_SIZE > 0)
              newCapacity = hugeCapacity(minCapacity);
          elementData = Arrays.copyOf(elementData, newCapacity);
  }