使用抽象类应该注意的几个要点:

包含一个或者多个抽象方法的类必须被声明为抽象类.
将类声明为抽象类,不一定含有抽象方法.
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.
抽象类不可以被实例化.即不能创建这个类的对象
实例代码:
复制代码 代码如下:
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:
复制代码 代码如下:
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.