上篇回顾
上一篇讲解了ArrayList的实现， 讲述了ArrayList中最重要的两个实现： 移位和扩容。 那么本篇Vector同ArrayList相似，因此不懂这两个概念的可以先去看一下上一篇中关于这部分的讲解。传送
顺序表之ArrayList
本篇东西很少，因为Vector的实现与ArrayList极其相似。因此本篇的内容
- Vector与ArrayList的区别
- Vector实现讲解
为什么一直强调Vector与ArrayList相似呢？ 我们来看一下
Vector的实现
/**
* The array buffer into which the components of the vector are
* stored. The capacity of the vector is the length of this array buffer,
* and is at least large enough to contain all the vector's elements.
*
* <p>Any array elements following the last element in the Vector are null.
*
* @serial
*/
protected Object[] elementData;
/**
* Constructs an empty vector so that its internal data array
* has size {@code 10} and its standard capacity increment is
* zero.
*/
public Vector() {
this(10);
}
同样的实现机制-内部数组，同样的初始容量10。
Vector的增删改查
/**
* Returns the element at the specified position in this Vector.
*
* @param index index of the element to return
* @return object at the specified index
* @throws ArrayIndexOutOfBoundsException if the index is out of range
* ({@code index < 0 || index >= size()})
* @since 1.2
*/
public synchronized E get(int index) {
if (index >= elementCount)
throw new ArrayIndexOutOfBoundsException(index);
return elementData(index);
}
/**
* Replaces the element at the specified position in this Vector with the
* specified element.
*
* @param index index of the element to replace
* @param element element to be stored at the specified position
* @return the element previously at the specified position
* @throws ArrayIndexOutOfBoundsException if the index is out of range
* ({@code index < 0 || index >= size()})
* @since 1.2
*/
public synchronized E set(int index, E element) {
if (index >= elementCount)
throw new ArrayIndexOutOfBoundsException(index);
E oldValue = elementData(index);
elementData[index] = element;
return oldValue;
}
/**
* Appends the specified element to the end of this Vector.
*
* @param e element to be appended to this Vector
* @return {@code true} (as specified by {@link Collection#add})
* @since 1.2
*/
public synchronized boolean add(E e) {
modCount++;
ensureCapacityHelper(elementCount + 1);
elementData[elementCount++] = e;
return true;
}
/**
* Deletes the component at the specified index. Each component in
* this vector with an index greater or equal to the specified
* {@code index} is shifted downward to have an index one
* smaller than the value it had previously. The size of this vector
* is decreased by {@code 1}.
*
* <p>The index must be a value greater than or equal to {@code 0}
* and less than the current size of the vector.
*
* <p>This method is identical in functionality to the {@link #remove(int)}
* method (which is part of the {@link List} interface). Note that the
* {@code remove} method returns the old value that was stored at the
* specified position.
*
* @param index the index of the object to remove
* @throws ArrayIndexOutOfBoundsException if the index is out of range
* ({@code index < 0 || index >= size()})
*/
public synchronized void removeElementAt(int index) {
modCount++;
if (index >= elementCount) {
throw new ArrayIndexOutOfBoundsException(index + " >= " +
elementCount);
}
else if (index < 0) {
throw new ArrayIndexOutOfBoundsException(index);
}
int j = elementCount - index - 1;
if (j > 0) {
System.arraycopy(elementData, index + 1, elementData, index, j);
}
elementCount--;
elementData[elementCount] = null; /* to let gc do its work */
}
如果你认真看，你会发现两个问题：
1） 增删改查的实现同ArrayList真的几乎一模一样
2） 所有的方法都带锁， synchronized
当然它也提供了自己的方法，而且很详尽，如elementAt(), firstElement(), lastElement()等。而且Vector是Stack的父类，因此提供的这些方法可能是为了给Stack提供足够的便利。
腻了吧！现在来说点不一样的。
1.Vector是什么？
说了这么多，还不知道Vector是什么，先引入百度百科的定义
Vector 是在 java 中可以实现自动增长的[对象数组]
它也是一个线性数组，而且可以实现自动增长。好的，那么我们来追踪一下自增长的实现。
自增长
如果你看它的源码，你会发现这么一个变量
/**
* The amount by which the capacity of the vector is automatically
* incremented when its size becomes greater than its capacity. If
* the capacity increment is less than or equal to zero, the capacity
* of the vector is doubled each time it needs to grow.
*
* @serial
*/
protected int capacityIncrement;
这是一个记录自增长的变量，你可以设置它来设置自增长的幅度。如
/**
* Constructs an empty vector with the specified initial capacity and
* capacity increment.
*
* @param initialCapacity the initial capacity of the vector
* @param capacityIncrement the amount by which the capacity is
* increased when the vector overflows
* @throws IllegalArgumentException if the specified initial capacity
* is negative
*/
public Vector(int initialCapacity, int capacityIncrement) {
super();
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
this.elementData = new Object[initialCapacity];
this.capacityIncrement = capacityIncrement;
}
既然是自增长，肯定是在扩容策略的一种实现，那么我们再来看一下扩容
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);
}
相比于ArrayList, 我们的第一次系统扩容因子不同了，换为了以下代码
int newCapacity = oldCapacity + ((capacityIncrement > 0) ?
capacityIncrement : oldCapacity);
有设置扩容幅度，则按幅度进行扩容。否则以原数组长度等比扩容。这时的扩容因子是1，记住，不是0.5了。后面的实现同ArrayList就一致了，不再赘述。看，就多了这么点东西。
那么现在来总结一下ArrayList与Vector的区别
1） ArrayList不是线程安全的， Vector是线程安全的
2） Vector的扩容策略不同于ArrayList, Vector是按扩容幅度进行扩容
其实差别真的很小，不过Vector是线程安全的，切记。
顺带一提
Queue队列是FIFO（先进先出）的，表现在它不会给你提供插入和删除的功能，只提供进和出两个方法。意味着你只能调用入(从队列尾部进入)，出（从队列头部取出）；而不会允许你对中间的元素进行操作。如
image.png
Stack栈是LIFO(后进先出)的，它也不会给你提供操作中间元素的机会，只有入和出。不过它是后入先出的。如
image.png
顺便看一下Stack的源码，它是继承自Vector的
public
class Stack<E> extends Vector<E> {
/**
* Creates an empty Stack.
*/
public Stack() {
}
/**
* Pushes an item onto the top of this stack. This has exactly
* the same effect as:
* <blockquote><pre>
* addElement(item)</pre></blockquote>
*
* @param item the item to be pushed onto this stack.
* @return the <code>item</code> argument.
* @see java.util.Vector#addElement
*/
public E push(E item) {
addElement(item);
return item;
}
/**
* Removes the object at the top of this stack and returns that
* object as the value of this function.
*
* @return The object at the top of this stack (the last item
* of the <tt>Vector</tt> object).
* @throws EmptyStackException if this stack is empty.
*/
public synchronized E pop() {
E obj;
int len = size();
obj = peek();
removeElementAt(len - 1);
return obj;
}
/**
* Looks at the object at the top of this stack without removing it
* from the stack.
*
* @return the object at the top of this stack (the last item
* of the <tt>Vector</tt> object).
* @throws EmptyStackException if this stack is empty.
*/
public synchronized E peek() {
int len = size();
if (len == 0)
throw new EmptyStackException();
return elementAt(len - 1);
}
/**
* Tests if this stack is empty.
*
* @return <code>true</code> if and only if this stack contains
* no items; <code>false</code> otherwise.
*/
public boolean empty() {
return size() == 0;
}
/**
* Returns the 1-based position where an object is on this stack.
* If the object <tt>o</tt> occurs as an item in this stack, this
* method returns the distance from the top of the stack of the
* occurrence nearest the top of the stack; the topmost item on the
* stack is considered to be at distance <tt>1</tt>. The <tt>equals</tt>
* method is used to compare <tt>o</tt> to the
* items in this stack.
*
* @param o the desired object.
* @return the 1-based position from the top of the stack where
* the object is located; the return value <code>-1</code>
* indicates that the object is not on the stack.
*/
public synchronized int search(Object o) {
int i = lastIndexOf(o);
if (i >= 0) {
return size() - i;
}
return -1;
}
/** use serialVersionUID from JDK 1.0.2 for interoperability */
private static final long serialVersionUID = 1224463164541339165L;
}
主要看一下它提供的核心方法。
/**
* Pushes an item onto the top of this stack. This has exactly
* the same effect as:
* <blockquote><pre>
* addElement(item)</pre></blockquote>
*
* @param item the item to be pushed onto this stack.
* @return the <code>item</code> argument.
* @see java.util.Vector#addElement
*/
public E push(E item) {
addElement(item);
return item;
}
Push是入的方法，类似ArrayList的add()。
/**
* Removes the object at the top of this stack and returns that
* object as the value of this function.
*
* @return The object at the top of this stack (the last item
* of the <tt>Vector</tt> object).
* @throws EmptyStackException if this stack is empty.
*/
public synchronized E pop() {
E obj;
int len = size();
obj = peek();
removeElementAt(len - 1);
return obj;
}
public synchronized E peek() {
int len = size();
if (len == 0)
throw new EmptyStackException();
return elementAt(len - 1);
}
pop是出的方法， 先执行了peek,peek是取最后一个元素；然后删除最后一个元素。在数组中，越往后加入的元素，会存储于数组底部。
认真看的话，你会发现，Stack是线程安全的。Activity的保存与取出也是采用栈策略。
好了，关于Vector和Stack的讲解结束了，你明白了吗？
网友评论