Java繼承：爲什麼這個程序給出了0.0輸出

Question

我已經寫了這段代碼。產出應計算銀行的利息，但它給出0.0作爲產出。我創建了一個名爲Bank的類，並將其擴展到ICICI類。

import java.util.Scanner; 

public class Bank 
{ 
    static double rate; 
    // n = number of years 
    public double calculateInterest(double PrincipalAmount, double n) 
    { 
     double interest; 
     interest = (PrincipalAmount * n*rate) /100; // interest formula 
     return interest; 
    } 

    public static void main(String[] args) 
    { 
     Scanner s1 = new Scanner(System.in); 
     System.out.print("Enter PrincipalAmount :"); 
     double PrincipalAmount = s1.nextDouble(); 

     Scanner s2 = new Scanner(System.in); 
     System.out.print("Enter Number of Years :"); 
     double n = s2.nextDouble(); 

     ICICI ic; 
     ic = new ICICI();// new object created of ICICI Class 
     ic.rate = rate; // object call by reference 
     System.out.print("Interest of ICICI is " + ic.calculateInterest(PrincipalAmount,n)); 
    } 
} 

public class ICICI extends Bank 
{ 
    double rate = 4; 
} 

Answer 1

你正在做以下幾點：

ic.rate = rate; 

和速率未初始化....

所以ic.rate = 0.0;

你應該順便看看這個：

• https://docs.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html

• What does the 'static' keyword do in a class? 這

• http://www.oracle.com/technetwork/java/codeconventions-135099.html

Answer 2

calculateInterest方法是使用靜態速率變量不是實例率，繼承不會對變量應用，而不會覆蓋變量。所以靜態速率的默認值將爲0.0，因此calculateInterest將給出0.0（因爲它是雙倍的）答案。

Answer 3

如果意外改變了賦值語句：

ic.rate =率;

相反，它應該是： rate = ic.rate;

謝謝！