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。
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;
}
} public class AbstractClass
{
public static void main(String [] args)
{
/* 以下是不允许的，会引发错误 */
Employee e = new Employee("George W.", "Houston, TX", 43); System.out.println("\n Call mailCheck using Employee reference--");
e.mailCheck();
}
}
如上面代码，抽象类不能直接实例化对象，不然会发生编译错误！
使用抽象类
通过一般的方法继承Employee类：
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;
}
} 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);
System.out.println("Constructing a 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;
}
} public class AbstractClass
{
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();
}
}
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" 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