使用抽象类应该注意的几个要点:

1. 包含一个或者多个抽象方法的类必须被声明为抽象类. 
2. 将类声明为抽象类,不一定含有抽象方法.
3. 通常认为,在抽象类中不应该包括具体方法,建议尽量将通用的域和方法放在超类中.
4. 抽象类不可以被实例化.即不能创建这个类的对象 

import java.util.*;

/**
 * This program demonstrates abstract classes.
 * @version 1.01 2004-02-21
 * @author Cay Horstmann
 */
public class PersonTest
{
   public static void main(String[] args)
   {
      Person[] people = new Person[2];

      // fill the people array with Student and Employee objects
      people[0] = new Employee("Harry Hacker", 50000, 1989, 10, 1);
      people[1] = new Student("Maria Morris", "computer science");

      // print out names and descriptions of all Person objects
      for (Person p : people)
         System.out.println(p.getName() + ", " + p.getDescription());
   }
}

abstract class Person
{
   public Person(String n)
   {
      name = n;
   }

   public abstract String getDescription();

   public String getName()
   {
      return name;
   }

   private String name;
}

class Employee extends Person
{
   public Employee(String n, double s, int year, int month, int day)
   {
      super(n);
      salary = s;
      GregorianCalendar calendar = new GregorianCalendar(year, month - 1, day);
      hireDay = calendar.getTime();
   }

   public double getSalary()
   {
      return salary;
   }

   public Date getHireDay()
   {
      return hireDay;
   }

   public String getDescription()
   {
      return String.format("an employee with a salary of $%.2f", salary);
   }

   public void raiseSalary(double byPercent)
   {
      double raise = salary * byPercent / 100;
      salary += raise;
   }

   private double salary;
   private Date hireDay;
}

class Student extends Person
{
   /**
    * @param n the student‘s name
    * @param m the student‘s major
    */
   public Student(String n, String m)
   {
      // pass n to superclass constructor
      super(n);
      major = m;
   }

   public String getDescription()
   {
      return "a student majoring in " + major;
   }

   private String major;
}

在代码块:

 for (Person p : people)
         System.out.println(p.getName() + ", " + p.getDescription());

中p.getDescription(),将引用具体子类的子类对象的方法.

不可以省略Person类中的getDescription(),原因是编译器只允许调用在类中声明的方法. 

Java抽象类简单学习