Java 实现队列

import java.util.LinkedList;

class GenQueue<E> {
   private LinkedList<E> list = new LinkedList<E>();
   public void enqueue(E item) {
      list.addLast(item);
   }

   public E dequeue() {
      return list.poll();
   }

   public boolean hasItems() {
      return !list.isEmpty();
   }

   public int size() {
      return list.size();
   }

   public void addItems(GenQueue<? extends E> q) {
      while (q.hasItems()) list.addLast(q.dequeue());
   }
}

public class GenQueueTest {
   public static void main(String[] args) {
      GenQueue<Employee> empList;
      empList = new GenQueue<Employee>();
      GenQueue<HourlyEmployee> hList;

      hList = new GenQueue<HourlyEmployee>();
      hList.enqueue(new HourlyEmployee("T", "D"));
      hList.enqueue(new HourlyEmployee("G", "B"));
      hList.enqueue(new HourlyEmployee("F", "S"));
      empList.addItems(hList);
      System.out.println("The employees' names are:");

      while (empList.hasItems()) {
         Employee emp = empList.dequeue();
         System.out.println(emp.firstName + " " + emp.lastName);
      }
   }
}

class Employee {
   public String lastName;
   public String firstName;
   public Employee() {
   }

   public Employee(String last, String first) {
      this.lastName = last;
      this.firstName = first;
   }

   public String toString() {
      return firstName + " " + lastName;
   }
}

class HourlyEmployee extends Employee {
   public double hourlyRate;
   public HourlyEmployee(String last, String first) {
      super(last, first);
   }
}