Java-Vector-01Vector底层实现原理及扩容方式

一、底层实现原理：数组（查询快，增删慢，线程安全）

1.成员变量

    /**
     * 可以看出实现原理是数组
     * @serial
     */
    protected Object[] elementData;

    /**
     * 
     * @serial
     */
    protected int elementCount;

    /**
     * 扩容长度，在有参构造时可以给该参数赋值，扩容时会将该参数和原来容量进行对比，取较大值再加上原                 
     * 来容量；无参构造，次参数值为赋值为0
     */
    protected int capacityIncrement;

2.构造方法

public Vector() {
        this(10);
    }
public Vector(int initialCapacity) {
        this(initialCapacity, 0);
    }
public Vector(int initialCapacity, int capacityIncrement) {
        super();
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        this.elementData = new Object[initialCapacity];
        this.capacityIncrement = capacityIncrement;
    }

3.扩容机制：

  扩容长度，在有参构造时可以给capacityIncrement赋值，扩容时会将capacityIncrement和原来容量进行对比，取较大值再加上原来容量（扩容机制相比ArrayList更灵活，开发者可以根据实际业务决定扩容，减少扩容的次数来提升性能）；无参构造，次参数值为赋值为0

 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);
    }