棧是先存進去的數據只能最后被取出來，是FILO（First In Last Out，先進后出）。

棧結構.png

用鏈表實現棧：

class Node<E>{
    Node<E> next = null;
    E data;
    public Node(E data) {this.data = data;}
}
public class Stack<E>{
    Node<E> top = null;
    public boolean isEmpty(){
      return top ==  null;
    }
    public void push(E data){
        Node<E> newNode = new Node<E>(data);
        newNode.next = top;
        top = newNode;
    }
    public E pop(){
        if(this.isEmpty()){
            return null;
        }
        E data = top.data;
        top = top.next;
        return data;
    }
    public E peek(){
        if(isEmpty()) {
            return null;
        }
        return top.data;
    }
}

隊列是FIFO（First In First Out，先進先出），它保持進出順序一致。

隊列結構.png

class Node<E> {
    Node<E> next =null;
    E data;
    public Node(E data){
        this.data = data;
    }
}
public class MyQueue<E> {
    private Node<E> head = null;
    private Node<E> tail = null;
    public boolean isEmpty(){
      return head = tail;
    }
    public void put(E data){
        Node<E> newNode = new Node<E>(data);
        if(head == null && tail == null){
            head = tail = newNode;
        }else{
            tail.next = newNode;
            taile = newNode;
        }
    }
    public E pop(){
        if(this.isEmpty()){
            return null;
        }
        E data = head.data;
        head = head.next;
        return data;
    }
    public int size(){
        Node<E> tmp = head;
        int n = 0;
        while(tmp != null) {
            n++;
            tmp = tmp.next;
        }
        return n;
    }
    public static void main(String []args){
      MyQueue<Integer> q = new MyQueue<Integer>();
      q.put(1);
      q.put(2);
      q.put(3);
      System.out.println("隊列長度：" + q.size());
      System.out.println("隊列首元素：" + q.pop());
      System.out.println("隊列首元素：" + q.pop());
    }
}

輸出結果：

隊列長度：3
隊列首元素：1
隊列首元素：2

注：
如果需要實現多線程安全，要對操作方法進行同步，用synchronized修飾方法