线程“AWT-EventQueue-0”中的异常java.lang.ClassCastException：java.util.ArrayList

Question

I'm doing a library management project and when I register a member to the library is registers properly "the write method works fine." the Error message is "Exception in thread "AWT-EventQueue-0" java.lang.ClassCastException: java.util.ArrayList cannot be cast to comsatslibrary.Person strong text "

private class listener implements ActionListener {

    @Override
    public void actionPerformed(ActionEvent e) {


        String user = text.getText();
        String pass = text1.getText();
            boolean flag = true;
            ArrayList<Person> personlist = HelperClass3.readAllData("person.txt");
            if (e.getSource() == b) {
                for (int i = 0; i < personlist.size(); i++) {

                    if ((user.equals((personlist.get(i).getFirstname()))) && (pass.equals(personlist.get(i).getPassword()))) {
                        flag = false;
                    }
                }
                if (flag == false) {
                    librarian l = new librarian();
                }

        } else{
            JOptionPane.showMessageDialog(null, "Incorrect UserName or Password");
        }

    }
}

}

   if ((user.equals((personlist.get(i).getFirstname()))) && (pass.equals(personlist.get(i).getPassword())))

this is the line of error

I have created a generic helper Class

Read method

public class HelperClass3 {
    public static <T> ArrayList<T> readAllData(String path){
        ArrayList<T> List = new ArrayList<T>(0);
        ObjectInputStream inputStream = null;
        try {

            inputStream = new ObjectInputStream(new FileInputStream(path));
            boolean EOF = false;
            while(!EOF) {
                try {
                    T myObj = (T)inputStream.readObject();
                    List.add(myObj);

                    }catch (ClassNotFoundException e) {
                    System.out.println("Class not found");
                    }catch(EOFException e) {
                    EOF = true;
                    }
            }
        } catch(FileNotFoundException e) {
            System.out.println("Cannot find file");
        } catch(IOException e) {
            System.out.println("IO Exception while opening stream");
        } finally {
            try {
                if(inputStream!=null) {
                    inputStream.close();
                }
            }catch(IOException e) {
                System.out.println("IO Exception while closing file");
            }

        }
        return List;

    }

write method

    public static<T> void addArrayListToFile(T s , String path) {
            ArrayList<T> List = readAllData(path);
            List.add(s);
        ObjectOutputStream outputStream =null;
        try {
            outputStream = new ObjectOutputStream(new FileOutputStream(path));
            for(int i = 0 ; i < List.size() ; i++) {
                outputStream.writeObject(List.get(i));
            }

        } catch(IOException e) {
            System.out.println("IO Exception while opening file");
        }
        finally { 
            try {
                if(outputStream != null) {
                    outputStream.close();                               
                }

            } catch (IOException e) {
                    System.out.println("IO Exception while closing file");
              }

        }
    }

}

T myObj = (T)inputStream.readObject(); this is where the problem is.

Answer 1

Your action listener attempts to read Person objects from the file "person.txt" using an ObjectInputStream. As an aside, I would recommend a different file extension. ".txt" implies a text file, but this file contains Objects. It's a sort of binary file rather than pure text.

You didn't show us the contents of the file, or how you call the write method to populate the file; but judging by the name of the method, addArrayListToFile , I presume you pass an ArrayList to the method. You then read the contents of the file, add the whole ArrayList as a single element, and write it back. The problem is here:

List.add(s);

The argument s is an ArrayList, so you add this object as a single element to List . So your file doesn't contain Person objects; it in fact contains ArrayList objects. When you execute this statement:

if ((user.equals((personlist.get(i).getFirstname()))) && (pass.equals(personlist.get(i).getPassword()))) {

since personlist is a generic (parameterized) type, the compiler automatically tries to cast personlist.get(i) to a Person object, but since it's in fact an ArrayList, you get the ClassCastException.

The quickest way to fix this is to change the above-mentioned line in the write method to:

List.addAll(s);

Then the list will contain Person objects instead of ArrayList objects.