栈是一种比较经典的数据结构，遵循LIFO原则,先进栈的元素总是要等到后进栈的元素出栈以后才能出栈，JVM内存划分中其中就有栈区，每个线程包含一个栈区，栈中只保存基础数据类型的对象和自定义对象的引用(不是对象)，对象都存放在堆区中，而且每个栈中的数据(原始类型和对象引用)都是私有的，其他栈不能访问。在Android开发中，如果不使用特殊的Activity启动模式，每次开启一个Activity都会在栈顶加入Activity，点击返回时，之前的Activity就会弹出栈，简单了解下栈的应用后，下面来看Java中的栈:
Java中的栈(Stack)
如下图所示Stack类的继承关系和实现
1.jpeg
Stack继承于Vector,Stack本身通过扩展Vector而来，而Vector本身是一个可增长的对象数组（ a growable array of objects）那么这个数组的哪里作为Stack的栈顶，哪里作为Stack的栈底?
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).
* @exception 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).
* @exception 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;
}
通过peek()方法注释The object at the top of this stack (the last item of the Vector object)，可以发现数组（Vector）的最后一位即为Stack的栈顶
由上边源码可以看出:
pop、peek以及search方法本身进行了同步
push方法调用了父类的addElement方法
empty方法调用了父类的size方法
Vector类为线程安全类
综上，Stack类为线程安全类
Stack简单Demo
public class StackDemo {
public static void main(String[] args){
Stack<Integer> stack = new Stack<>();
push(stack);
peek(stack);
search(stack,1);
pop(stack);
pop(stack);
pop(stack);
pop(stack);
}
static void push(Stack stack){
System.out.println("stack:" + stack);
for(int i = 0; i < 3; i++){
stack.push(i);
System.out.println("push_" + i);
}
System.out.println("stack:" + stack);
}
static void pop(Stack stack){
System.out.println("-----------以下是pop操作-----------");
if (stack.empty()) {
System.out.println("Stack is empty.");
} else {
Integer a = (Integer) stack.pop();
System.out.println("pop_" + a);
System.out.println("stack: " + stack);
}
}
static void peek(Stack stack) {
System.out.println("---------以下是peek操作------------");
if (stack.empty()) {
System.out.println("Stack is empty.");
} else {
Integer a = (Integer) stack.peek();
System.out.println("peek_" + a);
System.out.println("stack: " + stack);
}
}
static void search(Stack stack, int i) {
System.out.println("---------以下是search操作------------");
Integer index = (Integer) stack.search(i);
System.out.println("index_" + index);
System.out.println("stack: " + stack);
}
}
运行结果如下：
stack:[]
push_0
push_1
push_2
stack:[0, 1, 2]
---------以下是peek操作------------
peek_2
stack: [0, 1, 2]
---------以下是search操作------------
index_2
stack: [0, 1, 2]
-----------以下是pop操作-----------
pop_2
stack: [0, 1]
-----------以下是pop操作-----------
pop_1
stack: [0]
-----------以下是pop操作-----------
pop_0
stack: []
-----------以下是pop操作-----------
Stack is empty.
Stack是线程安全的，所以其性能必然受到影响，如果需要使用一个非线程安全的Stack，可以直接使用LinkedList，LinkedList本身提供的方法就包含了Stack的操作。
