Stack是一個後進先出（last in first out，LIFO）的堆棧，在Vector類的基礎上擴展5個方法而來

Deque（雙端隊列）比起Stack具有更好的完整性和一致性，應該被優先使用

1. E push(E item) 把項壓入堆棧頂部。
2. E pop() 移除堆棧頂部的對象，並作為此函數的值返回該對象。
3. E peek() 查看堆棧頂部的對象，但不從堆棧中移除它。
4. boolean empty() 測試堆棧是否為空。
5. int search(Object o) 返回對象在堆棧中的位置，以 1 為基數。

Stack本身通過擴展Vector而來，而Vector本身是一個可增長的對象數組（ a growable array of objects）那麽這個數組的哪裏作為Stack的棧頂，哪裏作為Stack的棧底？

答案只能從源代碼中尋找，jdk1.6：

1. public class Stack<E> extends Vector<E> {
2. /**

• * 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;
• }
• 通過peek()方法註釋The object at the top of this stack (the last item of the Vector object，可以發現數組（Vector）的最後一位即為Stack的棧頂

• pop、peek以及search方法本身進行了同步

  push方法調用了父類的addElement方法

  empty方法調用了父類的size方法

  Vector類為線程安全類

  綜上，Stack類為線程安全類(多個方法調用而產生的數據不一致問題屬於原子性問題的範疇)

• • public class Test {
  • public static void main(String[] args) {
  • Stack<String> s = new Stack<String>();
• System.out.println("------isEmpty");
• • System.out.println(s.isEmpty());
  • System.out.println("------push");
  • s.push("1");
  • s.push("2");
  • s.push("3");
  • Test.it(s);
  • System.out.println("------pop");
  • String str = s.pop();
  • System.out.println(str);
  • Test.it(s);
  • System.out.println("------peek");
  • str = s.peek();
  • System.out.println(str);
  • Test.it(s);
  • System.out.println("------search");
  • int i = s.search("2");
  • System.out.println(i);
  • i = s.search("1");
  • System.out.println(i);
  • i = s.search("none");
  • System.out.println(i);
  • }
  • public static void it(Stack<String> s){
  • System.out.print("iterator:");
  • Iterator<String> it = s.iterator();
  • while(it.hasNext()){
  • System.out.print(it.next()+";");
  • }
  • System.out.print("\n");
  • }
  • }
    • 結果：
    • • ------isEmpty
      • true
      • ------push
      • iterator:1;2;3;
      • ------pop
      • 3 --棧頂是數組最後一個
      • iterator:1;2;
      • ------peek
      • 2 --pop取後刪掉，peek只取不刪
      • iterator:1;2;
      • ------search
      • 1 --以1為基數，即棧頂為1
      • 2 --和棧頂見的距離為2-1=1
      • -1 --不存在於棧中
        

      • Stack並不要求其中保存數據的唯一性，當Stack中有多個相同的item時，調用search方法，只返回與查找對象equal並且離棧頂最近的item與棧頂間距離（見源碼中search方法說明）

java.util.Stack類簡介