Java Extends
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