以下代码中，可以被gc回收的对象有 ,多选题 class A { } public class Test { private static A staticA = new A(); public static final String CONSTANT = "i am a string"; public static void main(String[] args) { A innerA = new A(); } } A. staticA B. CONSTANT C. “i am a string” D. innerA

在给定的代码中，以下对象可以被垃圾回收（GC）回收：

A. innerA：在 main 方法执行完毕后，innerA 引用的对象将成为不可达对象，因为其作用域仅限于 main 方法内部，没有其他引用指向它。

注意：选项 B 和 C 中的对象 "CONSTANT" 和 "i am a string" 是字符串常量，在 Java 中，字符串常量会被放入常量池中，而常量池中的对象不会被垃圾回收。选项 A 中的 staticA 是一个静态变量，其生命周期与整个程序运行期间相同，因此不会被垃圾回收。所以这两个选项都不属于可以被垃圾回收的对象。

所以答案是：D. innerA。

相关问题

Also create a ControllerCreate class that extends Controller.The create method takes as arguments the name of a new library user, a number of books (as a string), and an integer representing the role of user to create (where the integer 0 means a lender and the integer 1 means a borrower). The create method of the controller then transforms the book number from a string to an integer (using the Integer.parseInt static method), creates an object from the correct class (based on the role specified by the user input: lender or borrower) and calls the addUser method of the library to add the new user object to the library. • If no exception occurs then the create method of the controller returns the empty string. • If the constructor of the Borrower class throws a NotALenderException then the create method of the controller must catch this exception and return as result the error message from the exception object. • If the parseInt method of the Integer class throws a NumberFormatException (because the user typed something which is not an integer) then the create method of the controller must catch this exception and return as result the error message from the exception object. Modify the run method of the GUI class to add a ViewCreate view that uses a ControllerCreate controller and the same model as before (not a new model!) Do not delete the previous views. Note: if at the end of Question 7 you had manually added to your library (model object) some users for testing, then you must now remove those users from the run method of the anonymous class inside the GUI class. You do not need these test users anymore because you have now a graphical user interface to create new users! Run your GUI and check that you can correctly use the new view to create different users for your library, with different types of roles. • Check that, when you create a new user, the simple view is automatically correctly updated to show the new total number of books borrowed by all users. • Also use the “get book” view to check that the users are correctly created with the correct names and correct number of books. • Also check that trying to create a borrower with a negative number of books correctly shows an error message. Also check that trying to create a user with a number of books which is not an integer correctly shows an error message (do not worry about the content of the error message). After you created a new user, you can also check whether it is a lender or a borrower using the “more book” view to increase the number of books of the user by a big negative number: • if the new user you created is a lender, then increasing the number of books by a big negative value will work and the number of books borrowed by the user will just become a larger value (you can then check that using the “get book” view); • if the new user you created is a borrower, then increasing the number of books by a big negative value will fail with an error message and the number of books borrowed by the user will not change (you can then check that using the “get book” view). 完成符合以上要求的java代码

Model class:

import java.util.ArrayList;

public class Library {
    private ArrayList<User> users;
    private int totalBooksBorrowed;

    public Library() {
        users = new ArrayList<>();
        totalBooksBorrowed = 0;
    }

    public void addUser(User user) {
        users.add(user);
        totalBooksBorrowed += user.getNumberOfBooks();
    }

    public void removeUser(User user) {
        users.remove(user);
        totalBooksBorrowed -= user.getNumberOfBooks();
    }

    public ArrayList<User> getUsers() {
        return users;
    }

    public int getTotalBooksBorrowed() {
        return totalBooksBorrowed;
    }
}

User class:

public abstract class User {
    private String name;
    private int numberOfBooks;

    public User(String name, int numberOfBooks) {
        this.name = name;
        this.numberOfBooks = numberOfBooks;
    }

    public String getName() {
        return name;
    }

    public int getNumberOfBooks() {
        return numberOfBooks;
    }

    public void setNumberOfBooks(int numberOfBooks) {
        this.numberOfBooks = numberOfBooks;
    }

    public abstract boolean isLender();
}

Lender class:

public class Lender extends User {

    public Lender(String name, int numberOfBooks) {
        super(name, numberOfBooks);
    }

    @Override
    public boolean isLender() {
        return true;
    }
}

Borrower class:

public class Borrower extends User {

    public Borrower(String name, int numberOfBooks) throws NotALenderException {
        super(name, numberOfBooks);
        if (numberOfBooks < 0) {
            throw new NotALenderException("A borrower can't have a negative number of books!");
        }
    }

    @Override
    public boolean isLender() {
        return false;
    }
}

NotALenderException class:

public class NotALenderException extends Exception {
    public NotALenderException(String message) {
        super(message);
    }
}

ViewCreate class:

import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

public class ViewCreate extends JPanel implements ActionListener {
    private JTextField nameField = new JTextField(10);
    private JTextField numberOfBooksField = new JTextField(10);
    private JRadioButton lenderButton = new JRadioButton("Lender");
    private JRadioButton borrowerButton = new JRadioButton("Borrower");
    private ButtonGroup roleButtonGroup = new ButtonGroup();
    private JButton createButton = new JButton("Create");

    private ControllerCreate controller;

    public ViewCreate(Model model) {
        this.controller = new ControllerCreate(model, this);

        setLayout(new GridBagLayout());

        roleButtonGroup.add(lenderButton);
        roleButtonGroup.add(borrowerButton);
        lenderButton.setSelected(true);

        GridBagConstraints gc = new GridBagConstraints();
        gc.gridx = 0;
        gc.gridy = 0;
        gc.anchor = GridBagConstraints.LINE_END;
        gc.insets = new Insets(5, 5, 5, 5);
        add(new JLabel("Name: "), gc);

        gc.gridx++;
        gc.anchor = GridBagConstraints.LINE_START;
        add(nameField, gc);

        gc.gridx = 0;
        gc.gridy++;
        gc.anchor = GridBagConstraints.LINE_END;
        add(new JLabel("Number of books: "), gc);

        gc.gridx++;
        gc.anchor = GridBagConstraints.LINE_START;
        add(numberOfBooksField, gc);

        gc.gridx = 0;
        gc.gridy++;
        gc.anchor = GridBagConstraints.LINE_END;
        add(new JLabel("Role: "), gc);

        gc.gridx++;
        gc.anchor = GridBagConstraints.LINE_START;
        add(lenderButton, gc);

        gc.gridx++;
        add(borrowerButton, gc);

        gc.gridx = 1;
        gc.gridy++;
        gc.anchor = GridBagConstraints.LINE_END;
        add(createButton, gc);

        createButton.addActionListener(this);
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        String name = nameField.getText();
        String numberOfBooksString = numberOfBooksField.getText();
        int role = lenderButton.isSelected() ? 0 : 1;

        String result = controller.create(name, numberOfBooksString, role);
        JOptionPane.showMessageDialog(this, result, "Result", JOptionPane.INFORMATION_MESSAGE);
    }
}

ControllerCreate class:

public class ControllerCreate extends Controller {
    private ViewCreate view;

    public ControllerCreate(Model model, ViewCreate view) {
        super(model);
        this.view = view;
    }

    public String create(String name, String numberOfBooksString, int role) {
        try {
            int numberOfBooks = Integer.parseInt(numberOfBooksString);
            User user = null;
            if (role == 0) {
                user = new Lender(name, numberOfBooks);
            } else if (role == 1) {
                user = new Borrower(name, numberOfBooks);
            }
            model.addUser(user);
            view.updateTotalBooksBorrowed();
            return "";
        } catch (NumberFormatException e) {
            return "Invalid number of books!";
        } catch (NotALenderException e) {
            return e.getMessage();
        }
    }
}

GUI class:

import javax.swing.*;
import java.awt.*;

public class GUI {
    private JFrame frame = new JFrame("Library");
    private Model model = new Library();

    private ViewSimple viewSimple = new ViewSimple(model);
    private ViewGetBook viewGetBook = new ViewGetBook(model);
    private ViewMoreBook viewMoreBook = new ViewMoreBook(model);
    private ViewCreate viewCreate = new ViewCreate(model);

    public GUI() {
        Container cp = frame.getContentPane();
        cp.setLayout(new BorderLayout());
        JTabbedPane tp = new JTabbedPane();
        tp.addTab("Simple", viewSimple);
        tp.addTab("Get book", viewGetBook);
        tp.addTab("More book", viewMoreBook);
        tp.addTab("Create", viewCreate);
        cp.add(tp, BorderLayout.CENTER);
        frame.pack();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);

        // Remove test users from model
        model.removeUser(new Lender("John", 2));
        model.removeUser(new Borrower("Mary", 3));
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                new GUI();
            }
        });
    }
}