Java 抽象类，

Java 抽象类，

抽象类

/* 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;
   }
}

继承抽象类

/* 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;
   }
}

尽管我们不能实例化一个Employee类的对象，但是如果我们实例化一个Salary类对象，该对象将从Employee类继承3个成员变量和7个成员方法。

/* 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();
    }
}

运行结果：

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.

---

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。
声明抽象方法会造成以下两个结果：

• 如果一个类包含抽象方法，那么该类必须是抽象类。
• 任何子类必须重写父类的抽象方法，或者声明自身为抽象类。

继承抽象方法的子类必须重载该方法。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。

例子：

public abstract class Employee {
   private String name;
   private String address;
   private int number;

   public abstract double computePay();

   //其余代码
}

/* 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
}