一 栈简介
栈是一种常用的线性抽象数据结构,在计算机中应用极为广泛。栈有两个为人熟知的特性,一是进出栈的顺序是先进后出(FILO),即最先进栈的数据最后出栈,当然相对的也有后进先出(LIFO),最后进栈的数据先出栈。另一个是支持两个通用操作:Push:将元素推进栈;Pull:从栈中取出数据。
栈的操作演示见下图,这里需要注意的是1的位置对应的是栈底,后面再Push的元素所在位置为栈顶,所有对栈的操作,都
是在栈顶操作。
图1 栈操作示意图
二 Java栈实现
Java栈继承自Vector,关系图如下:
图2 Java Stack继承关系图
在前面的文章Java Vector源码解析解析中,我们已经对Vector做了介绍,这是一种加锁实现的List。Stack是从Vector中扩展而来, 功能相对要简单许多,下面看看具体实现。
图3 Java Stack实现
/**
*
* <p>A more complete and consistent set of LIFO stack operations is
* provided by the {@link Deque} interface and its implementations, which
* should be used in preference to this class. For example:
* <pre> {@code
* Deque<Integer> stack = new ArrayDeque<Integer>();}</pre>
*
* @author Jonathan Payne
* @since 1.0
*/
public
class Stack<E> extends Vector<E> {
/**
* Creates an empty Stack.
*/
public Stack() {
}
/**
* Pushes an item onto the top of this stack.
*/
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.
*/
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.
*/
public synchronized E peek() {
int len = size();
if (len == 0)
throw new EmptyStackException();
return elementAt(len - 1);
}
/**
* Tests if this stack is empty.
*
*/
public boolean empty() {
return size() == 0;
}
/**
* Returns the 1-based position where an object is on this stack.
* If the object {@code o} 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 {@code 1}. The {@code equals}
* method is used to compare {@code o} 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}
* 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;
}
}
由于代码比较少,这里把基本都贴出来了。从继承关系可以看出,Stack就是一个阉割了的Vector。Vector本质上是一个数组,所以Stack也是由数组实现。方法实现的方式如下:
- Push:将元素添加到栈的最后,也即数组的末尾;
- Pop:将栈末尾的元素取出并删除;
- Peek:取出最后一个元素,但不从栈中删除;
- Search:查找元素距离栈顶的位置;
这里需要注意的是,上面的操作都是加锁了的,也即是线程安全的,因此Stack本身是一个线程安全的数据类型,当然这也导致了它的性能会受到影响。
三 小结
请关注类开头的注释:more complete and consistent set of LIFO stack operations is provided by the {@link Deque} interface and its implementations, which should be used in preference to this class. For example:
Deque<Integer> stack = new ArrayDeque<Integer>();
官方推荐使用双向队列来实现栈,而不是再继续使用Stack这个数据结构。当然Stack的实现方式还是有一定参考价值的。