Java 抽象类 初学者笔记

我们为了使一个方法在继承中获得更高的层次，通常使用abstract关键字声明一个抽象类并将此方法声明为抽象方法放在抽象类中。

一个实例

public class PersonTest
{
   public static void main(String[] args)
   {
      var 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());
      //p分别引用Student类与Employee类中的getDescription方法
   }
}

 抽象类

package abstractClasses;

public abstract class Person//我是一个抽象的类
{
   public abstract String getDescription();//我是一个抽象的方法
   private String name;

   public Person(String name)//抽象类的构造器
   {
      this.name = name;
   }

   public String getName()//我也是所有子类最基础的方法
   {
      return name;
   }
}

 两个子类

public class Student extends Person
{
   private String major;

   /**
    * @param name the student's name
    * @param major the student's major
    */
   public Student(String name, String major)
   {
      super(name);//调用抽象类Person构造器，处理name
      this.major = major;//用Student的实例字段处理传入的参数变量
   }

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

 

package abstractClasses;

import java.time.*;

public class Employee extends Person
{
   private double salary;
   private LocalDate hireDay;

   public Employee(String name, double salary, int year, int month, int day)
   {
      super(name);//调用抽象类Person构造器，处理name
      this.salary = salary;
      hireDay = LocalDate.of(year, month, day);
   }

   public double getSalary()
   {
      return salary;
   }

   public LocalDate 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;
   }
}