覆蓋構造函數類

Question

以下是我的代碼。我沒有得到什麼錯誤。任何人都可以引導。

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; 

    } 
} 

在運行它顯示爲「錯誤：無法找到或加載主類國家$地方」

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

Answer 1

你Place類需要被定義爲public。

編輯：該文件還必須命名爲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."); 

    } 
} 

---

主類

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取決於有一個類的實例。 polymorphism的要點是您可以繼承一個類，並且實現這些subclasses的對象將具有與superclass（並在subclasses中重寫）中定義的相同方法不同的行爲。靜態方法不與任何類的任何實例關聯，因此這個概念不適用。

推動Java設計的兩個因素影響了這一點。其中一個問題是性能問題：Smalltalk對它的速度太慢了（垃圾回收和多態調用是其中的一部分），Java的創建者決心避免這種情況。另一個決定是Java的目標受衆是C++開發人員。使靜態方法按照他們的方式工作的好處是程序員熟悉C++，並且速度也非常快，因爲不需要等到運行時才能確定要調用哪種方法。

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"); 
    } 
}