错误:无法找到或加载主类应用程序

Question

有人能告诉我如何在这段代码上运行main方法吗？我想检查是否有任何编译时错误或运行时错误，但我得到这个错误消息“无法找到或加载主类应用程序”。

class Book {
private static int internalID = 0;
private String isbn;
private int myID;

public Book(String isbnP) {
    if (isbnP == null) {
        throw new IllegalArgumentException("null ISBN not accepted");
    }
    isbn = isbnP;
    myID = internalID++;
}

public String getBookinfo() {
    return isbn;
}

public String toString() {
    return "<" + myID + "," + isbn + ">";
}
// To Do: Override Object.equals()
// Two objects are equal iff isbn of the two books are same

}

class ComSBook extends Book {
private String category;

public ComSBook(String isbnP, String catP) {
    super(isbnP);
    category = catP;
}

@override
public String getBookInfo() {
    return "ComS " + category + " " + super.getBookinfo();
}
}

class NetworkBook extends ComSBook {
private boolean isWithCD;

public NetworkBook (String isbnP, boolean withCD){
    super(isbnP,"Network");
    isWithCD = withCD;
}
@override
public String getBookInfo(){
    return super.getBookInfo() + " withCD: " + isWithCD;
}
}


class ReviewPolymorp{
public static void main(String[] args){
    Book abook = new Book("A-1");
    Book bbook = new Book("B-1");
    ComSBook csbook = new ComSBook("C-11", "General");
    NetworkBook netbook = new NetworkBook("N-11", true);

    System.out.println(abook);
    System.out.println(bbook);
    System.out.println(csbook);
    System.out.println(netbook);

    abook = csbook;
    System.out.println(abook.getBookinfo());

    bbook = netbook;
    System.out.println(bbook.getBookinfo());

    netbook = (NetworkBook) bbook;
    System.out.println(netbook.getBookinfo());

    netbook = (NetworkBook) csbook;
    System.out.println(netbook.getBookinfo());

    netbook = csbook;

}
}

Answer 1

您的文件应该是ReviewPolymorp.java，class ReviewPolymorp{应该是public，因为它是使用main方法的程序的加载点

您可能还希望检查项目中的主类是否为ReviewPolymorp。我只知道如何在Netbeans中执行此操作

在项目上单击鼠标右键并选择Properties

• Click

• 确保您的项目是具有完全限定名称的主类，例如mypackage.ReviewPolymorp

• Then rebuild
• ReviewPolymorp

Answer 2

当我将您的代码复制到我的IDE(它是eclipse)中时，下面这一行显示了一个编译错误：

netbook = csbook;

不能将超类的实例强制转换为子类。

禁用此行后，一旦运行以下代码，我就会从控制台获得输出：

<0,A-1>
<1,B-1>
Exception in thread "main" <2,C-11>
<3,N-11>
C-11
N-11
N-11
java.lang.ClassCastException: com.test.ComSBook cannot be cast to com.test.NetworkBook
at com.test.ReviewPolymorp.main(ReviewPolymorp.java:75)

上面的编译错误也有同样的问题。

禁用这三行后

netbook = (NetworkBook) csbook;
System.out.println(netbook.getBookinfo());
csbook = netbook;

代码在没有任何修改的情况下工作良好。