java - 如何在调用 GUI 时修复 NullPointerException

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 6 年前。

我的异常(exception)是在我调用 GUI 的行中:XmasGUI tableGUI = new XmasGUI (xmasDataModel);我的另一个异常(exception)是我在 XmasGUI.java 代码中将内容面板设置为 (rootPanel) 的行。我不明白什么可能为空。有人能指出我正确的方向吗？看来我会被标记为反对票。似乎我的问题总是(太模糊)，但对我来说它们非常简单且切题。

import java.sql.*;

public class XmasDB {
    static final String DB_CONNECTION_URL = "jdbc:mysql://localhost:3306/";
    static final String USER = "root";   //TODO replace with your username
    static final String PASS = "****";   //TODO replace with your password
    static private final String DB_NAME = "xmas";


    public final static String WANT_TABLE_NAME = "want_list";
    public final static String NEED_TABLE_NAME = "need_list";
    // Each solver will have a unique ID
    public final static String PK_COLUMN = "id";
    // A primary key is needed to allow updates to the database on modifications to ResultSet
    public final static String NAME_COLUMN = "name";
    public final static String PRICE_COLUMN = "price";
    public final static String PRIORITY_COLUMN = "riority";

    public final static int MOVIE_MIN_RATING = 1;
    public final static int MOVIE_MAX_RATING = 5;



    // Name our database

    static Statement statement = null;
    static Connection conn = null;
    static ResultSet rs = null;
    // Create out data model
    private static XmasDataModel xmasDataModel;

    public static void main(String args[]) {


        //setup creates database (if it doesn't exist), opens connection, and adds sample data

        if (!setup()) {
            System.exit(-1);
        }

        if (!loadAllMovies()) {
            System.exit(-1);
        }

        //If no errors, then start GUI
        XmasGUI tableGUI = new XmasGUI (xmasDataModel);

    }

    //Create or recreate a ResultSet containing the whole database, and give it to movieDataModel
    public static boolean loadAllMovies(){

        try{

            if (rs!=null) {
                rs.close();
            }

            String getSomeData = "SELECT * FROM " + WANT_TABLE_NAME;
            String getRestData = "SELECT * FROM " + NEED_TABLE_NAME;

            rs = statement.executeQuery (getSomeData);
            rs = statement.executeQuery (getRestData);


            if (xmasDataModel == null) {
                //If no current movieDataModel, then make one
                xmasDataModel = new XmasDataModel (rs);
            } else {
                //Or, if one already exists, update its ResultSet
                xmasDataModel.updateResultSet(rs);
            }

            return true;

        } catch (Exception e) {
            System.out.println("Error loading or reloading lists");
            System.out.println(e);
            e.printStackTrace();
            return false;
        }

    }

    public static boolean setup(){
        try {

            //Load driver class
            try {
                String Driver = "com.mysql.jdbc.Driver";
                Class.forName(Driver);
            } catch (ClassNotFoundException cnfe) {
                System.out.println("No database drivers found. Quitting");
                return false;
            }

            conn = DriverManager.getConnection(DB_CONNECTION_URL + DB_NAME, USER, PASS);

            statement = conn.createStatement(ResultSet.TYPE_SCROLL_INSENSITIVE, ResultSet.CONCUR_UPDATABLE);

            //Does the table exist? If not, create it.
            if (! xmasTableOneExists ()) {

                //Create a table
                String createNeedTableSQL = "CREATE TABLE " + NEED_TABLE_NAME + " (" + PK_COLUMN + " int NOT NULL AUTO_INCREMENT, " + NAME_COLUMN + " varchar(50), " + PRICE_COLUMN + " int, " + PRIORITY_COLUMN + " int, PRIMARY KEY(" + PK_COLUMN + "))";
                System.out.println ( createNeedTableSQL );


                statement.executeUpdate ( createNeedTableSQL );

                System.out.println ( "Created Need table..." );

                String addDataSQL = "INSERT INTO " + NEED_TABLE_NAME + "(" + NAME_COLUMN + ", " + PRICE_COLUMN + ", " + PRIORITY_COLUMN + ")" + " VALUES ('New Winter Boots', 85, 3)";
                statement.executeUpdate ( addDataSQL );
                addDataSQL = "INSERT INTO " + NEED_TABLE_NAME + "(" + NAME_COLUMN + ", " + PRICE_COLUMN + ", " + PRIORITY_COLUMN + ")" + " VALUES('New Car', 3000, 2)";
                statement.executeUpdate ( addDataSQL );
                addDataSQL = "INSERT INTO " + NEED_TABLE_NAME + "(" + NAME_COLUMN + ", " + PRICE_COLUMN + ", " + PRIORITY_COLUMN + ")" + " VALUES ('Gift Card to Grocery Store', 300, 1)";
                statement.executeUpdate ( addDataSQL );

            }else if (! xmasTableTwoExists ()) {
                String createWantTableSQL = "CREATE TABLE " + WANT_TABLE_NAME + " (" + PK_COLUMN + " int NOT NULL AUTO_INCREMENT, " + NAME_COLUMN + " varchar(50), " + PRICE_COLUMN + " int, " + PRIORITY_COLUMN + " int, PRIMARY KEY(" + PK_COLUMN + "))";
                System.out.println (createWantTableSQL);
                statement.executeUpdate(createWantTableSQL);
                System.out.println("Created Want Table...");

                String addMoreSQL = "INSERT INTO " + WANT_TABLE_NAME + "(" + NAME_COLUMN + ", " + PRICE_COLUMN + ", " + PRIORITY_COLUMN + ")" + " VALUES ('Gaming PC', 500, 3)";
                statement.executeUpdate(addMoreSQL);
                addMoreSQL = "INSERT INTO " + WANT_TABLE_NAME +  "(" + NAME_COLUMN + ", " + PRICE_COLUMN + ", " + PRIORITY_COLUMN + ")" + " VALUES('Astrology Book', 150, 2)";
                statement.executeUpdate(addMoreSQL);
                addMoreSQL = "INSERT INTO " + WANT_TABLE_NAME +  "(" + NAME_COLUMN + ", " + PRICE_COLUMN + ", " + PRIORITY_COLUMN + ")" + " VALUES ('Hover Board', 300, 1)";
                statement.executeUpdate(addMoreSQL);
            }
            return true;

        } catch (SQLException se) {
            System.out.println(se);
            se.printStackTrace();
            return false;
        }
    }

    private static boolean xmasTableOneExists () throws SQLException {

        String checkNeedTablePresentQuery = "SHOW TABLES LIKE '" + NEED_TABLE_NAME + "'";   //Can query the database schema
        ResultSet tablesRS = statement.executeQuery(checkNeedTablePresentQuery);

        if (tablesRS.next()) {    //If ResultSet has a next row, it has at least one row... that must be our table
            return true;
        }
        return false;

    }
    private static boolean xmasTableTwoExists () throws SQLException {

        String checkWantTablePresentQuery = "SHOW TABLES LIKE '" + WANT_TABLE_NAME + "'";

        ResultSet tableTwoRS = statement.executeQuery(checkWantTablePresentQuery);

        if (tableTwoRS.next()) {    //If ResultSet has a next row, it has at least one row... that must be our table
            return true;
        }
        return false;

    }

    //Close the ResultSet, statement and connection, in that order.
    public static void shutdown(){
        try {
            if (rs != null) {
                rs.close();
                System.out.println("Result set closed");
            }
        } catch (SQLException se) {
            se.printStackTrace();
        }

        try {
            if (statement != null) {
                statement.close();
                System.out.println("Statement closed");
            }
        } catch (SQLException se){
            //Closing the connection could throw an exception too
            se.printStackTrace();
        }

        try {
            if (conn != null) {
                conn.close();
                System.out.println("Database connection closed");
            }
        }
        catch (SQLException se) {
            se.printStackTrace();
        }
    }
}

//(This is my XmasGUI.java Class)
/


import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.WindowEvent;
import java.awt.event.WindowListener;
import java.util.Calendar;

/**
 * Created by Nnamdi on 11/29/2016.
 */
public class XmasGUI extends JFrame implements WindowListener{
    private JTable needTable;
    private JTextPane NEEDTABLETextPane1;
    private JTextPane WANTTABLETextPane;
    private JTable wantTable;
    private JButton addButton;
    private JButton exitButton;
    private JTextPane welcomeToXmasListTextPane;
    private JButton transferButton;
    private JRadioButton radioNeed;
    private JRadioButton radioWant;
    private JTextField txtNameInput;
    private JSpinner prioritySpinner;
    private JTextField txtPriceInput;
    private JPanel rootPanel;
    private JButton deleteButton;

    XmasGUI(final XmasDataModel xmasDataTableModel) {

        setContentPane(rootPanel);
        pack();
        setTitle("Christmas List Database Application");
        addWindowListener(this);
        setVisible(true);
        setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);


        //Set up JTables
        needTable.setGridColor(Color.BLACK);
        needTable.setModel(xmasDataTableModel);
        wantTable.setGridColor (Color.YELLOW);
        wantTable.setModel (xmasDataTableModel );

        //Set up the priority spinner.
        //SpinnerNumberModel constructor arguments: spinner's initial value, min, max, step.
        prioritySpinner.setModel(new SpinnerNumberModel(1, XmasDB.MOVIE_MIN_RATING, XmasDB.MOVIE_MAX_RATING, 1));

        exitButton.addActionListener ( new ActionListener ( ) {
            @Override
            public void actionPerformed ( ActionEvent e ) {
            XmasDB.shutdown();
            System.exit (0);
            }
        } );
        deleteButton.addActionListener ( new ActionListener ( ) {
            @Override
            public void actionPerformed ( ActionEvent e ) {
                int currentRow = needTable.getSelectedRow();
                int currentrow = wantTable.getSelectedRow();

                if (currentRow == -1) {      // -1 means no row is selected. Display error message.
                    JOptionPane.showMessageDialog(rootPane, "Please choose a item to delete");
                }
                boolean deleted = xmasDataTableModel.deleteRow(currentRow);
                if (deleted) {
                    XmasDB.loadAllMovies();
                } else {
                    JOptionPane.showMessageDialog(rootPane, "Error deleting item");
                }
            }
        } );
    }


    @Override
    public void windowOpened ( WindowEvent e ) {

    }

    @Override
    public void windowClosing ( WindowEvent e ) {
        System.out.println("closing");
        XmasDB.shutdown();
    }

    @Override
    public void windowClosed ( WindowEvent e ) {

    }

    @Override
    public void windowIconified ( WindowEvent e ) {

    }

    @Override
    public void windowDeiconified ( WindowEvent e ) {

    }

    @Override
    public void windowActivated ( WindowEvent e ) {

    }

    @Override
    public void windowDeactivated ( WindowEvent e ) {

    }

    private void createUIComponents () {
        // TODO: place custom component creation code here
    }
}

Answer 1

请首先确保您在使用 XmasGUI 类之前已导入它:
例如:import com.mypackage.XmasGUI;上课XmasDB 。

二、setContentPane(rootPanel);拼写错误。应该是setContentPanel(rootPanel);

第三，需要保证rootPanel的值将其传递给 setContentPanel(); 时不为 null
您可以在将其传递给 setContentPanel(); 之前对其进行初始化.