无法解析Java多态方法

Question

我是编程新手，所以请多多包涵。我进行了搜索，但找不到可以回答此问题的现有线程。我编写了以下代码，该代码根据用户将安全对象识别为股票还是债券来吐出stock.toString（）或bond.toString（）固定短语。但是，我收到“无法解决的安全性”编译器错误。我认为这是一个问题，因为安全对象的类未在编译时定义。真的吗？如果是这样，有什么办法可以不依靠反射方法呢？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。这是我们想出的解决方案，但请告诉我是否有更好的选择。并且我添加了else语句来清理，以防万一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

将其声明为其他，以便在其他情况下可用

假设Stock是Bond的超类，如果没有像security那样声明

 Object security = null;

制作

 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);
    }

---

See

• § 14.4.2 Scope of Local Variable Declarations

Answer 2

问题不在于类型。

您在内部作用域中定义了security，而在外部范围中根本就不存在（您在security.setPrice处进行定义。更糟糕的是，由于security不能securityType既不是“债券”也不是“股票”时，则完全定义（无论是否为内部范围）。

但是我想您想做的是这样：

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，这是一个糟糕的解决方案，但是您必须首先对任务进行澄清。

Answer 3

问题出在您的变量范围内。 security仅在if / else块内定义。

您可以将代码更改为如下所示：

Object security; //you can use common superclass of Bond and Stock (maybe Security?)

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);