行動(dòng)創(chuàng)造奇跡

理解繼承是理解面向?qū)ο蟪绦蛟O(shè)計(jì)的關(guān)鍵.在Java中,通過關(guān)鍵字extends繼承一個(gè)已有的類,被繼承的類稱為父類(超類，基類),新的類稱為子類(派生類).在Java中不允許多繼承.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!
(這說明派生類繼承了父類的所有方法和成員變量.)

方法的覆蓋(override)
在子類中定義一個(gè)與父類同名,返回類型,參數(shù)類型均相同的一個(gè)方法,稱為方法的覆蓋,方法的覆蓋發(fā)生在子類與父類之間.
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;?? //隱藏了父類的weight,height;
?void breathe()? //override method breathe()
?{
? super.breathe();? //用super調(diào)用父類的構(gòu)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;
?}
}
輸出結(jié)果:
F:\Java Develop>javac Animal.java

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

特殊變量super
* 使用特殊變量super提供對(duì)父類的訪問
* 可以使用super訪問父類被子類隱藏的變量或覆蓋的方法
* 每個(gè)子類構(gòu)造方法的第一條語句都是隱含的調(diào)用super,如果父類沒有這種形式的構(gòu)造函數(shù)就會(huì)報(bào)錯(cuò).
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;
?}
}
輸出結(jié)果:
F:\Java Develop>javac Animal.java

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



?通過覆蓋父類的方法來實(shí)現(xiàn),在運(yùn)行時(shí)根據(jù)傳遞對(duì)象的引用,來調(diào)用相應(yīng)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);
?}
}