错误：找不到或加载主类应用程序

Question

Could any one show me how do I run the main method on this code, please? I would like to check if there is any compile time error or run-time error, but I got this error message "Could not find or load main class Application".

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

You're file should be ReviewPolymorp.java and class ReviewPolymorp{ should be public , since it the lauching point of the program with the main method

You may also want to check that the Main class in the project is ReviewPolymorp . I only know how to do this in Netbeans

• Right-click on the Project
• Select Properties
• Click Run
• Make sure your ReviewPolymorp i the Main Class with fully qualified name eg mypackage.ReviewPolymorp
• Then rebuild your project

Answer 2

When I copy your code to my IDE(It's eclipse),a compile error show up on this line:

netbook = csbook;

You can't cast an instance of super class to subclass.

After disable this line,I get output from console once run the code:

<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)

It's the same problem with the compile error above.

After disable these three lines

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

Code is work well without any modification.