行动创造奇迹

理解继承是理解面向对象程序设计的关键.在Java中,通过关键字extends继承一个已有的类,被继承的类称为父类(超类，基类),新的类称为子类(派生类).在Java中不允许多继承.code:
class Animal
{
 int height,weight;
 void eat()
 {
  System.out.println("Animal eat!");
 }
 void sleep()
 {
  System.out.println("Animal sleep!");
 }
 void breathe()
 {
  System.out.println("Animal breathe!");
 }
}
class Fish extends Animal
{
 
}
class DoMain
{
 public static void main(String[] args)
 {
  Animal an=new Animal();
  Fish fn=new Fish();
 
  an.breathe();
  fn.breathe();
  fn.height=30;
  fn.weight=20;
 }
}
Result:
F:\Java Develop>javac Animal.java

F:\Java Develop>java DoMain
Animal breathe!
Animal breathe!
(这说明派生类继承了父类的所有方法和成员变量.)

方法的覆盖(override)
在子类中定义一个与父类同名,返回类型,参数类型均相同的一个方法,称为方法的覆盖,方法的覆盖发生在子类与父类之间.
code:
class Animal
{
 int height,weight;
 void eat()
 {
  System.out.println("Animal eat!");
 }
 void sleep()
 {
  System.out.println("Animal sleep!");
 }
 void breathe()
 {
  System.out.println("Animal breathe!");
 }
}
class Fish extends Animal
{
 int weight,height;   //隐藏了父类的weight,height;
 void breathe()  //override method breathe()
 {
  super.breathe();  //用super调用父类的构造方法
  System.out.println("Fish bubble");
 }
}
class DoMain
{
 public static void main(String[] args)
 {
 // Animal an=new Animal();
  Fish fn=new Fish();
 
  an.breathe();
  fn.breathe();
  fn.height=30;
  fn.weight=20;
 }
}
输出结果:
F:\Java Develop>javac Animal.java

F:\Java Develop>java DoMain
Animal breathe!
Fish bubble

特殊变量super
* 使用特殊变量super提供对父类的访问
* 可以使用super访问父类被子类隐藏的变量或覆盖的方法
* 每个子类构造方法的第一条语句都是隐含的调用super,如果父类没有这种形式的构造函数就会报错.
code:
class Animal
{
 int height,weight;
 Animal()
 {
  System.out.println("Animal construct");
 }
 void eat()
 {
  System.out.println("Animal eat!");
 }
 void sleep()
 {
  System.out.println("Animal sleep!");
 }
 void breathe()
 {
  System.out.println("Animal breathe!");
 }
}

class Fish extends Animal
{
 Fish()
 {
  System.out.println("Fish construct");
 }
 void breathe()  //override method breathe()
 {
  System.out.println("Fish bubble");
 }
}
class DoMain
{
 public static void main(String[] args)
 {
  //Animal an=new Animal();
  Fish fn=new Fish();
 
  //an.breathe();
  //fn.breathe();
  //fn.height=30;
  //fn.weight=20;
 }
}
输出结果:
F:\Java Develop>javac Animal.java

F:\Java Develop>java DoMain
Animal construct
Fish construct



 通过覆盖父类的方法来实现,在运行时根据传递对象的引用,来调用相应的方法.
code:
class Animal
{
 int height,weight;
 Animal()
 {
  System.out.println("Animal construct");
 }
 void eat()
 {
  System.out.println("Animal eat!");
 }
 void sleep()
 {
  System.out.println("Animal sleep!");
 }
 void breathe()
 {
  System.out.println("Animal breathe!");
 }
}

class Fish extends Animal
{
 Fish()
 {
  System.out.println("Fish construct");
 }
 void breathe()  //override method breathe()
 {
  System.out.println("Fish bubble");
 }
}
class DoMain
{
 static void fn(Animal an)
 {
  an.breathe();
 }
 public static void main(String[] args)
 {
  //Animal an=new Animal();
  Fish fh=new Fish();
  Animal an;
  an=fh;
  DoMain.fn(an);
 }
}