手撕Vector底层源码

public abstract class AbstractList<E> extends AbstractCollection<E> implements List<E> {
    //外部操作数
    protected transient int modCount = 0;
}

public class Vector<E> extends AbstractList<E> implements List<E>{
    //元素容器
    protected Object[] elementData;//new Object[10]{null,null,null,null,...}
    //元素个数
    protected int elementCount;
    //容量增量
    protected int capacityIncrement;//0
    
    public Vector() {
        this(10);
    }
    
    //initialCapacity - 10
    public Vector(int initialCapacity) {
        this(initialCapacity, 0);
    }
    
    //initialCapacity - 10
    //capacityIncrement - 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;
    }
    
    public synchronized boolean add(E e) {//线程安全的
        modCount++;
        ensureCapacityHelper(elementCount + 1);//扩容
        elementData[elementCount++] = e;
        return true;
    }
    
    private void ensureCapacityHelper(int minCapacity) {
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }
    
    private void grow(int minCapacity) {
        // oldCapacity - 10
        int oldCapacity = elementData.length;
        //newCapacity - 10 + 10
        int newCapacity = oldCapacity + ((capacityIncrement > 0) ?
                                         capacityIncrement : oldCapacity);
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        
        //扩容
        elementData = Arrays.copyOf(elementData, newCapacity);
    }
}

Vector<String> v = new Vector<>();

v.add("张三");
v.add("李四");
v.add("王五");

1. Vector底层数据结构是什么？
   Object类型的一维数组
2. Vector的默认初始化容量是多少？
   10
3. Vector的扩容机制是什么？
   如果容量增量小于等于0，扩容机制是数组原来长度的两倍
   如果容量增量大于0，扩容机制是数组原来长度+容量增量
4. Vector的特点是什么？
   Vector是线程安全的集合