覆盖构造函数类

Question

以下是我的代码。我没有得到什么错误。任何人都可以引导。

class State { 
    static String country; 
    static String capital; 

    State() // Constructor 
    { 
     country = "America's"; 
     capital = "Washington D.C"; 

    } 

    static void display() { 
     System.out.println(capital + " " + "is" + " " + country + " " + "capital."); 

    } 
} 

class Place extends State // Method Overriding 
{ 
    static void display() { 
     System.out.println("Capital is Washington D.C."); 
    } 

    public static void main(String[] args) { 

     State st = new State(); 
     Place pl = new Place(); 
     st.display(); 
     pl.display(); 
     st = pl; 

    } 
} 

在运行它显示为“错误：无法找到或加载主类国家$地方”

As the output needs: "Capital is Washington D.C." instead of (capital + " " + "is" + " " + country + " " +"capital.")

Answer 1

你Place类需要被定义为public。

编辑：该文件还必须命名为Place.java

Answer 2

class State 
{ 
    static String country; 
    static String capital; 


    State()  //Constructor 
    { 
     country = "America's"; 
     capital = "Washington D.C"; 

    } 

    static void display() 
    { 
     System.out.println(capital + " " + "is" + " " + country + " " +"capital."); 

    } 
} 

---

主类

class Place extends State // Inheritance 
    static void display() 
    { 
     System.out.println("Capital is Washington D.C."); 
    } 
    public static void main(String[] args) 
    { 

     State st = new State(); 
     st.display(); // to print sub class method 
     display(); // to print same class method 
     //st = pl; No idea of this point .. 

    } 
} 

Answer 3

Overriding取决于有一个类的实例。 polymorphism的要点是您可以继承一个类，并且实现这些subclasses的对象将具有与superclass（并在subclasses中重写）中定义的相同方法不同的行为。静态方法不与任何类的任何实例关联，因此这个概念不适用。

推动Java设计的两个因素影响了这一点。其中一个问题是性能问题：Smalltalk对它的速度太慢了（垃圾回收和多态调用是其中的一部分），Java的创建者决心避免这种情况。另一个决定是Java的目标受众是C++开发人员。使静态方法按照他们的方式工作的好处是程序员熟悉C++，并且速度也非常快，因为不需要等到运行时才能确定要调用哪种方法。

I give you a small example : // and it is not overriding method.May be this is you actually expect

public class HelloWorld { 

    public static void main(String args[]) { 

     Company st = new Company(); 
     eBay pl = new eBay(); 
     st=pl; 
     //st.address(); 
     pl.address(); 

    } 
} 

class Company { 
    static String country; 
    static String capital; 

    Company() // Constructor 
    { 
     country = "America's"; 
     capital = "Washington D.C"; 

    } 

    static void address() { 
     System.out.println(capital + " " + "is" + " " + country + " " + "capital."); 

    } 

} 

class eBay extends Company { 

    public static void address() { 
     System.out.println("Capital is Washington D.C"); 
    } 
}