1、下面的代碼是定義一個抽象類Shape，然后Circel和Square類繼承Shape類，實現Shape中的方法MianJi();??

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public?class?ShapeDemo?{??

????public?static?void?main(String[]?args)?{??

??????

????//向上轉型，創建一個父類Shape的引用，讓引用指向子類??

????????Shape?shape?=??new?Square(2);??

??????????

????????//如果shape引用的對象是Square類型的對象，返回true,否則為false??

????????if(shape?instanceof?Square)?{??

??????????????

????????????//將shape對象想下轉型為子類的對象??

????????????Square?square?=?(Square)shape;??

??????????????

????????????//在show方法中，以子類對象sqaure為參數，求出求出子類的面積??

????????????show(square);??

????????}??

???????????

????????//向上轉型，創建父類的引用，指向子類Circel的對象??

????????Shape?shape1?=?new?Circle(2);??

????????//在show方法中，以父類Shape1參數調用方法，求出子類的面積??

????????show(shape1);??

??????????

??????????

??????????

??????????

??????????

}??

??????

//定義靜態方法show來調用父類或則子類的MianJi()方法??

public?static?void?show(Shape?s)?{??

????????s.MianJi();??

}??

??

}??

??

//定義一個抽象類Shpe，給出抽象方法MianJi，不給出具體實現??

abstract?class?Shape{??

????float?r;??

??????

????abstract?void?MianJi();??

}??

??

??

??????

//定義一個Square正方形類，繼承父類Shape類，給出父類MainJi方法的具體實現??

class?Square?extends?Shape{??

????//定義參數邊長??

????float?r;??

??????

????//默認參數構造方法來初始化Square，??

????Square(float?r){??

????????super();??

????????this.r?=?r;??

????}??

??????

????//求出正方形的面積??

????void??MianJi()?{??

????????System.out.println(?"我是正方形，面積是："?+r?*?r);??

????}??

}??

??

//定義圓類，繼承父類Shape??

class?Circle?extends?Shape{??

????//定義參數半徑??

????float?r;??

??????

????//構造方法初始化??

????Circle(float?r){??

????????super();??

????????this.r?=?r;??

????}??

??????

????//求出圓的面積??

????void?MianJi(){??

????????System.out.println("我是圓形，面積是："?+3.14?*?r?*r);??

????} ?

}

運行結果：??

我是正方形，面積是：4.0??

我是圓形，面積是：12.56 ?

2.定義一個Animal父類，子類Cat和Dog繼承Animal類??

public?class?Demo{??

????public?static?void?main(String[]?args)?{??

??????show(new?Cat());??//?以?Cat?對象調用?show?方法??

??????show(new?Dog());??//?以?Dog?對象調用?show?方法??

??????????????????

??????Animal?a?=?new?Cat();??//?向上轉型????

??????a.eat();???????????????//?調用的是?Cat?的?eat??

??????Cat?c?=?(Cat)a;????????//?向下轉型????

??????c.work();????????//?調用的是?Cat?的?work??

??????????

??}????

??????????????

????public?static?void?show(Animal?a)??{??

??????a.eat();????

????????//?類型判斷??

????????if?(a?instanceof?Cat)??{??//?貓做的事情???

????????????Cat?c?=?(Cat)a;????

????????????c.work();????

????????}???

????else?if?(a?instanceof?Dog)?{?//?狗做的事情???

????????????Dog?c?=?(Dog)a;????

????????????c.work();????

????????}????

????}????

}??

???

//定義一個抽象Animal類，為父類??

abstract?class?Animal?{????

????//定義一個抽象的父類eat方法，子類必須具體實現這個方法??

????abstract?void?eat();????

}????

????

//繼承父類Animal??

class?Cat?extends?Animal?{????

????//實現父類的抽象方法??

????public?void?eat()?{????

????????System.out.println("我是貓，?吃魚");????

????}????

????//定義子類自己的方法??

????public?void?work()?{????

????????System.out.println("我是貓，抓老鼠");????

????}????

}????

????

//繼承父類Animal，實現自己的方法??

class?Dog?extends?Animal?{????

????//實現父類的方法??

????public?void?eat()?{????

????????System.out.println("我是狗，吃骨頭");????

????}????

????//定義自己的方法??

????public?void?work()?{????

????????System.out.println("我是狗，看家");????

????}????

}

我是貓，?吃魚??

我是貓，抓老鼠??

我是狗，吃骨頭??

我是狗，看家??

我是貓，?吃魚??

我是貓，抓老鼠 ?

總結

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