Question

我是一名编程新手，所以请耐心等待。我搜索并找不到可以回答这个问题的现有主题。我写了下面的代码，它应该根据用户是将安全对象识别为股票还是债券来吐出stock.toString（）或bond.toString（）罐头短语。但是，我得到“安全无法解决”的编译错误。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，有没有办法解决这个问题而不采用反思方法？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，但如果有更好的选择，请告诉我。我正在添加一个else语句来清理，以防securityType既不是“stock”也不是“bond”：）

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

在if else声明它的一面，以便在if else之后它可用

假设Stock是Bond的超类，如果没有声明security

 Object security = null;

成功

 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);
    }

查看

§ 14.4.2 Scope of Local Variable Declarations

Answer 2

问题不在于类型。

你在内部范围内定义了security，它在外部（你在security.setPrice处不存在。更糟糕的是，代码无法轻易修复，如{{当security既不是“债券”也不是“股票”时，根本不会被定义（无论是否是内部范围）。

但是我想你想要做的是：

securityType

当然这是一个可怕的解决方案，但你必须首先澄清自己的任务。

Answer 3

问题出在你的变量范围内。

security仅在if / else块中定义。

您可以将代码更改为以下内容：

Object security; // you can use common super class of Bond and Stock (maybe Securiy?)

if (securityType.compareToIgnoreCase("stock") == 0) {
    /* ... */
    Stock stock = new Stock();
    stock.setEarnings(theEarnings);
    security = stock;
}

else if (securityType.compareToIgnoreCase("bond") == 0) {
    /* ... */
    Bond bond = new Bond();
    bond.setRate(theRate);
    security = bond;
}

/* ... */
System.out.println(security);

Java多态方法无法解决

3 个答案: