Java多态方法无法解析

Question

我是一个编程新手，请耐心等待。我搜索并找不到可以回答此问题的现有主题。我写了以下代码，根据安全对象是否被用户标识为股票或债券，应该吐出stock.toString（）或bond.toString（）罐头短语。但是，我收到“安全无法解决”编译器错误。我想这是一个问题，因为安全对象的类没有在编译时定义。真的吗？如果是这样，是否有任何解决方法，而不诉诸反思方法？谢谢！

public static void main(String[] args) { 

    double thePrice; 
    double theShares; 
    double theEarnings; 
    double theRate; 
    String securityType; 

    Scanner in = new Scanner(System.in); 

    System.out.println("Is it a stock or a bond?"); 
    securityType = in.nextLine(); 

    if (securityType.compareToIgnoreCase("stock") == 0) { 
     System.out.println("Successfully set to STOCK"); 
     System.out.println("What are the earnings?"); 
     theEarnings = in.nextDouble(); 
     Stock security = new Stock(); 
     security.setEarnings(theEarnings); 
    } 

    else if (securityType.compareToIgnoreCase("bond") == 0) { 
     System.out.println("Successfully set to BOND"); 
     System.out.println("What is the rate?"); 
     theRate = in.nextDouble(); 
     Bond security = new Bond(); 
     security.setRate(theRate); 
    } 

    System.out.println("What is the price"); 
    thePrice = in.nextDouble();  

    System.out.println("How many shares are there?"); 
    theShares = in.nextDouble(); 

    security.setPrice(thePrice); 
    security.setShares(theShares); 

    System.out.println(security); 
} 

感谢@Jigur Joshi，@penartur和其他人。这是我们提出的解决方案，但让我知道是否有更好的选择。如果securityType既不是“股票”，也不是“债券”，我要添加一个else语句来清理:)

public static void main(String[] args) { 

       ... 
    Security security = null; 
    String securityType; 

    Scanner in = new Scanner(System.in); 

    System.out.println("Is it a stock or a bond?"); 
    securityType = in.nextLine(); 

    System.out.println("What is the price"); 
    thePrice = in.nextDouble();  

    System.out.println("How many shares are there?"); 
    theShares = in.nextDouble(); 

    if (securityType.compareToIgnoreCase("stock") == 0) { 
     System.out.println("Successfully registered STOCK"); 
     security = new Stock(); 
     System.out.println("What are the earnings?"); 
     theEarnings = in.nextDouble(); 
     ((Stock) security).setEarnings(theEarnings); 
    } 

    if (securityType.compareToIgnoreCase("bond") == 0) { 
     System.out.println("Successfully registered BOND"); 
     security = new Bond(); 
     System.out.println("What is the rate?"); 
     theRate = in.nextDouble(); 
     ((Bond) security).setRate(theRate); 
    } 

    security.setPrice(thePrice); 
    security.setShares(theShares); 

     System.out.println(security); 

} 

Answer 1

问题不在于类型。

你必须在内部范围内定义security，它只是在那里你做security.setPrice不存在外（。更糟的是，代码不能很容易地固定，如security不会在所有的定义（无论是内部范围或没有）时securityType既不是“羁绊”，也不是“股票”

不过我猜那是你想做的事是这样的：

public static void main(String[] args) { 

    double thePrice; 
    double theShares; 
    double theEarnings; 
    double theRate; 
    String securityType; 

    Scanner in = new Scanner(System.in); 

    System.out.println("Is it a stock or a bond?"); 
    securityType = in.nextLine(); 

    System.out.println("What is the price"); 
    thePrice = in.nextDouble();  

    System.out.println("How many shares are there?"); 
    theShares = in.nextDouble(); 

    if (securityType.compareToIgnoreCase("stock") == 0) { 
     System.out.println("Successfully set to STOCK"); 
     System.out.println("What are the earnings?"); 
     theEarnings = in.nextDouble(); 
     Stock security = new Stock(); 
     security.setEarnings(theEarnings); 
     security.setPrice(thePrice); 
     security.setShares(theShares); 
     System.out.println(security); 
    } 

    else if (securityType.compareToIgnoreCase("bond") == 0) { 
     System.out.println("Successfully set to BOND"); 
     System.out.println("What is the rate?"); 
     theRate = in.nextDouble(); 
     Bond security = new Bond(); 
     security.setRate(theRate); 
     security.setPrice(thePrice); 
     security.setShares(theShares); 
     System.out.println(security); 
    } 

} 

当然，这是一个可怕的解决方案，但你必须先明确自己的任务。

Answer 2

声明出来的，如果其他人，使其可用后，也可以侧面的if else

假设Stock是超一流的Bond，如果不申报security像

Object security = null; 

让它

Stock security = null; 
if (securityType.compareToIgnoreCase("stock") == 0) { 
     System.out.println("Successfully set to STOCK"); 
     System.out.println("What are the earnings?"); 
     theEarnings = in.nextDouble(); 
     security = new Stock(); 
     security.setEarnings(theEarnings); 
    } 

    else if (securityType.compareToIgnoreCase("bond") == 0) { 
     System.out.println("Successfully set to BOND"); 
     System.out.println("What is the rate?"); 
     theRate = in.nextDouble(); 
     security = new Bond(); 
     security.setRate(theRate); 
    } 

---

请参阅

• § 14.4.2 Scope of Local Variable Declarations

Answer 3

的问题是在你的变量的作用域。

security仅在if/else块内部定义。

您可以更改代码以这样的事：

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