在面向对象的概念中，所有的对象都是通过类来描绘的，但是反过来，并不是所有的类都是用来描绘对象的，如果一个类中没有包含足够的信息来描绘一个具体的对象，这样的类就是抽象类。
抽象类除了不能实例化对象之外，类的其它功能依然存在，成员变量、成员方法和构造方法的访问方式和普通类一样。
由于抽象类不能实例化对象，所以抽象类必须被继承，才能被使用。也是因为这个原因，通常在设计阶段决定要不要设计抽象类。
父类包含了子类集合的常见的方法，但是由于父类本身是抽象的，所以不能使用这些方法。
在Java语言中使用abstract class来定义抽象类。如下实例：
| /* 文件名 : Employee.java */ |
| public abstract class Employee |
| { |
| private String name; |
| private String address; |
| private int number; |
| public Employee(String name, String address, int number) |
| { |
| System.out.println("Constructing an Employee"); |
| this.name = name; |
| this.address = address; |
| this.number = number; |
| } |
| public double computePay() |
| { |
| System.out.println("Inside Employee computePay"); |
| return 0.0; |
| } |
| public void mailCheck() |
| { |
| System.out.println("Mailing a check to " + this.name |
| + " " + this.address); |
| } |
| public String toString() |
| { |
| return name + " " + address + " " + number; |
| } |
| public String getName() |
| { |
| return name; |
| } |
| public String getAddress() |
| { |
| return address; |
| } |
| public void setAddress(String newAddress) |
| { |
| address = newAddress; |
| } |
| public int getNumber() |
| { |
| return number; |
| } |
| } |
| Employee.java:46: Employee is abstract; cannot be instantiated |
| Employee e = new Employee("George W.", "Houston, TX", 43); |
| ^ |
| 1 error |
我们能通过一般的方法继承Employee类：
| /* 文件名 : Salary.java */ |
| public class Salary extends Employee |
| { |
| private double salary; //Annual salary |
| public Salary(String name, String address, int number, double |
| salary) |
| { |
| super(name, address, number); |
| setSalary(salary); |
| } |
| public void mailCheck() |
| { |
| System.out.println("Within mailCheck of Salary class "); |
| System.out.println("Mailing check to " + getName() |
| + " with salary " + salary); |
| } |
| public double getSalary() |
| { |
| return salary; |
| } |
| public void setSalary(double newSalary) |
| { |
| if(newSalary >= 0.0) |
| { |
| salary = newSalary; |
| } |
| } |
| public double computePay() |
| { |
| System.out.println("Computing salary pay for " + getName()); |
| return salary/52; |
| } |
| } |
| /* 文件名 : AbstractDemo.java */ |
| public class AbstractDemo |
| { |
| public static void main(String [] args) |
| { |
| Salary s = new Salary("Mohd Mohtashim", "Ambehta, UP", 3, 3600.00); |
| Employee e = new Salary("John Adams", "Boston, MA", 2, 2400.00); |
| System.out.println("Call mailCheck using Salary reference --"); |
| s.mailCheck(); |
| System.out.println("\n Call mailCheck using Employee reference--"); |
| e.mailCheck(); |
| } |
| } |
| Constructing an Employee |
| Constructing an Employee |
| Call mailCheck using Salary reference -- |
| Within mailCheck of Salary class |
| Mailing check to Mohd Mohtashim with salary 3600.0 |
| Call mailCheck using Employee reference-- |
| Within mailCheck of Salary class |
| Mailing check to John Adams with salary 2400. |
如果你想设计这样一个类，该类包含一个特别的成员方法，该方法的具体实现由它的子类确定，那么你可以在父类中声明该方法为抽象方法。
Abstract关键字同样可以用来声明抽象方法，抽象方法只包含一个方法名，而没有方法体。
抽象方法没有定义，方法名后面直接跟一个分号，而不是花括号。
| public abstract class Employee |
| { |
| private String name; |
| private String address; |
| private int number; |
| public abstract double computePay(); |
| //其余代码 |
| } |
声明抽象方法会造成以下两个结果：
继承抽象方法的子类必须重写该方法。否则，该子类也必须声明为抽象类。最终，必须有子类实现该抽象方法，否则，从最初的父类到最终的子类都不能用来实例化对象。
如果Salary类继承了Employee类，那么它必须实现computePay()方法：
| /* 文件名 : Salary.java */ |
| public class Salary extends Employee |
| { |
| private double salary; // Annual salary |
| public double computePay() |
| { |
| System.out.println("Computing salary pay for " + getName()); |
| return salary/52; |
| } |
| //其余代码 |
| } |
