如何实现队列？（How to implement Queue?）

问题描述 (Problem Description)

如何实现队列？

解决方案 (Solution)

以下示例显示了如何在员工结构中实现队列。

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);
   }
}

结果 (Result)

上面的代码示例将产生以下结果。

The employees' names are:
D T
B G
S F